I collected more than 17 lists of the greatest or most important scientific discoveries of all time and combined them into one list – here are the results.  The numbers in bold and underlined indicate the number of lists the scientific discovery was on. You may notice there is some overlap with the Best Inventions lists – it appears that the line between ‘invention’ and ‘discovery’ is often a blurry one. I have provided some information on the nature of the discovery and the identities of the discovers. As with inventions, the discovery is often one link in a chain of scientific work that extends before and after the discovery in time, or is a collaboration (sometimes rivalry) among multiple discoverers. Also, for some reason, history sometimes identifies the discoverer as the person who first hypothesized the correct answer to a question, while in other cases, the credit goes to the person who confirmed the hypothesis by experiments or observations. I have also provided images of the scientists or their discoveries where available and where the narrative for one discovery mentions another discovery, I have placed it in boldface.  This list includes every discovery on three or more of the 17+ lists.  For a chronological timeline of every discovery on two or more lists, go here.

17 Lists
ELECTRICITY (1600)
ELECTROMAGNETISM (1820)
Electricity is the name for a set of physical phenomena associated with the presence and flow of electric charge.  One of the first to examine the phenomenon was Thales of Miletus (Ancient Greece), who studied static electricity in 600 BCE.  It was not until the careful research of William Gilbert (England) in 1600 that electricity became a subject of scientific study.  Gilbert also coined the Latin term ‘electricus’ from the Greek word for amber, which he rubbed to produce static electricity.  The English words ‘electric’ and ‘electricity’ were derived by Thomas Browne in 1646. Otto von Guericke (Germany) made the first static electricity generator in 1660. Stephen Gray (England) discovered the conduction of electricity in 1729. The Leyden Jar, the first capacitor, was invented independently in 1745 in Germany and The Netherlands. Henry Cavendish (England) measured conductivity of materials in 1747. Benjamin Franklin (US) discovered that lightning is a form of electricity in 1752. Luigi Galvani (Italy) discovered the electrical basis of nerve impulses in 1786. Alessandro Volta (Italy) invented the electric battery in 1800. Hans Christian Ørsted (Denmark) noticed an interaction between electricity and magnetism in 1820, but it was French scientist André-Marie Ampère’s follow-up experiments that demonstrated the unity of electricity and magnetism. Beginning in 1831, Michael Faraday (England) discovered electromagnetic induction, diamagnetism and electrolysis and invented the first current-generating electric generator, or dynamo. Joseph Henry (US) discovered induction at about the same time. James Clerk Maxwell (England) linked electricity, magnetism and light in 1861-1862 in a series of mathematical equations. In 1866, Werner von Siemens (Germany) invented an industrial generator that did not need external magnetic power.  In 1882, Thomas Edison (US) built the first large-scale electrical supply network, which provided 110 volts of direct current (DC) to 59 homes in Manhattan.  In the late 1880s, George Westinghouse (US) set up a rival system using alternating current (AC), using an induction motor and transformer invented by Nikola Tesla (Serbia/US). AC eventually prevailed over DC.  Another key invention was Sir Charles Parsons’ steam turbine, from 1884, which provides the mechanical power for most of the world’s electric power.

A 19th Century painting by Arthur Ackland Hunt entitled, ‘William Gilbert Demonstrates His Experiment on Electricity to Queen Elizabeth I and Her Court.’

André-Marie Ampère (1775-1836).

15 Lists
THE LAW OF UNIVERSAL GRAVITATION (1686)
The law of universal gravitation states that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.  Sir Isaac Newton (England) articulated the law in the first book of his Philosophiae Naturalis Principia Mathematica, which was presented to the Royal Society in 1686. The law was based in part on Galileo Galilei’s law of falling bodies, the result of experiments in 1589-1590.  Newton was not the first to recognize the existence of gravity and the famous falling apple story is probably apocryphal. Previous theories of gravity contained some of the elements of Newton’s law, particularly the theory proposed by Robert Hooke (England) in 1674-1679, who accused Newton of stealing his idea. Newton’s law was amended (some would say superseded) by Einstein’s general theory of relativity in 1916.

Portrait of Sir Isaac Newton (1643-1727) painted in 1689, when Newton was 46, by Sir Godfrey Kneller.

THE STRUCTURE OF DNA (1953)
Friedrich Miescher (Switzerland) first isolated deoxyribonucleic acid (DNA) in 1869. In 1910, Thomas Hunt Morgan determined that genes are located on chromosomes.  In 1928, experiments by Frederick Griffith (UK) showed that traits could be transferred from one type of organism to another. In 1943, Oswald Avery (Canada/US), Colin MacLeod (Canada/US) and Maclyn McCarty (US) identified that genes are made of DNA. In 1952, Rosalind Franklin (UK) and Raymond Gosling (UK) created an x-ray diffraction image of DNA that was then used by James Watson (US) and Francis Crick (UK) to determine the double-helical structure of DNA in 1953. Experiments in 1953 by Maurice Wilkins (NZ/UK) confirmed the structure. The double helix structure, with paired nucleotide bases forming the rungs between the two strands, perfectly explains how DNA replicates during mitosis.

James Watson (b. 1928) (left) and Francis Crick (1916-2004) with a model of the DNA molecule.

14 Lists
THE HELIOCENTRIC MODEL (1543)
With some exceptions, man believed that he was the center of the universe for most of history.  (Archimedes suggested a heliocentric universe in 260 BCE.)  In the 16^th Century, it was a tenet of Christian doctrine that the Earth was a stationary globe, around which the sun, the planets and the stars revolved, and it was heresy to say otherwise.  As early as 1514, astronomer and mathematician Nicolaus Copernicus (Poland) became convinced through his observations and mathematical calculations that the Earth and other planets revolved around the sun, not the other way around. He held off publishing his results until just before his death in 1543, for fear of reprisals.  The Copernican model, which posited circular orbits, was revised by Johannes Kepler (Germany), who discovered that the orbits of the planets were ellipses, in his 1609 laws of planetary motion. Later in the 17^th Century, Galileo Galilei (Italy) publicized telescopic observations that confirmed the heliocentric model and popularized the new view in his 1632 book Dialogue Concerning the Two Chief World Systems, the book that led to Galileo’s arrest and imprisonment by the Roman Catholic Church.
Copernicus’s sun-centered model of the solar system.

13
THE PERIODIC TABLE (1869)
Dmitri Mendeleev (Russia) discovered in 1869 that the elements could be arranged according to their atomic weights and chemical properties into a table. It was then possible to derive relationships between the properties of the elements and to predict the existence, nature and properties of then-unknown elements. Mendeleev’s periodic table is essentially the same as the one in use today. Prior to Mendeleev’s discovery, other scientists made attempts to define the nature of an element, and to catalogue and categorize the known elements.  These scientists included: Robert Boyle (UK) who defined an element in 1661 as “a substance that cannot be broken down into a simpler substance by a chemical reaction”; Antoine-Laurent Lavoisier (France), who made a list of elements in 1789; Johann Wolfgang Döbereiner (Germany), who made one of the first attempts to classify the elements into groups in 1829; geologist Alexandre-Emile Béguyer de Chancourtois (France), who first noticed that similar elements occur at regular intervals when ordered by their atomic weights and made an early version of the periodic table in 1862-1863; and chemist John Newlands (UK), who classified the 56 known elements into 11 groups based on their physical properties in 1865.

Mendeleev’s 1871 version of the periodic table.

12
CLASSICAL MECHANICS (1687)
In his 1687 book Philosophiae Naturalis Principia Mathematica, Sir Isaac Newton (England) established the three laws of motion – (1) the law of inertia; (2) the law of acceleration; and (3) the law of action and reaction – and derived the mathematical basis for the laws. These laws and Newton’s law of universal gravitation formed the basis for the science of physics for more than 200 years. In the early 20^th Century, classical mechanics was displaced by relativity and quantum mechanics, but Newton’s laws still accurately explain the behavior of most objects in environments familiar to human life.
Newton’s laws of motion, in graphic form.

EVOLUTION BY NATURAL SELECTION (1858)
While most 19th Century scientists believed that biological organisms had undergone evolution over time, no one had been able to provide a convincing evolutionary mechanism. After returning from a voyage to South America in 1836, and reading Thomas Malthus’ works on population growth, Charles Darwin (UK) came to believe that (1) all species contained individual variations; (2) some of the variations were more advantageous than others and (3) given limits on population growth, those individuals with the more advantageous variations would be more likely to survive and reproduce. The result of such a system, over a long period of time, would be the generation of new species. Although Darwin first formulated the theory in 1839, he was afraid to publish, fearing the reaction to a theory based essentially on chance. Instead, he spent the next 20 years collecting evidence to support his conclusions. He drafted a comprehensive essay on the matter in 1844, but did not publish it. In 1858, Darwin learned that another biologist, Alfred Russell Wallace (UK), had reached nearly identical conclusions. Wallace’s paper was presented to the Royal Society in 1858 along with excerpts from Darwin’s 1844 essay. In 1859, Darwin published The Origin of Species, which set out the evidence behind his theory. The theory of evolution by means of natural selection is now the fundamental premise of the science of biology.

An 1857 photograph of Charles Darwin (1809-1882).

X-RAYS (1895)
Researchers first noticed unidentified rays emanating from experimental discharge tubes called Crookes tubes around 1875.  In 1886, Ivan Pulyui (Ukraine/Germany) discovered that sealed photographic plates darkened when exposed to Crookes tubes. Nikola Tesla (Serbia/US) began experimenting with the rays in 1887. Fernando Sanford (US) generated and detected the rays in 1891. Wilhelm Röntgen (Germany) began studying the rays in 1895 and announced their existence (coining the term ‘X-rays’) in a scientific paper. Röntgen was the first to recognize the medical use of X-rays when he X-rayed his wife’s hand. In 1896, Thomas Edison (US) invented the flouroscope for X-ray examinations.  In the same year, John Hall-Edwards (UK) was the first physician to use X-rays under clinical conditions. Problems with the cold cathode tubes used to generate X-rays led to the invention of the Coolidge tube by William D. Coolidge (US) in 1913.

A photograph of Wilhelm Röntgen (1845-1923).

THE SPECIAL THEORY OF RELATIVITY (1905)
Albert Einstein (Germany) developed the special theory of relativity in 1905 to correct Newton’s laws of classical mechanics, which do not accurately explain phenomena at velocities near the speed of light. The theory explains how objects behave when moving at a constant speed relative to each other. Einstein relied on the principles that (a) the law of physics remain the same despite your frame of reference; and (b) the speed of light is the same to all observers. Under the theory, space and time are two aspects of the same phenomenon, meaning that reality has four dimensions instead of three. A key implication of the special theory of relativity is that time slows down as acceleration increases, a fact that has been proven many times by experiment.

A photograph of Albert Einstein (1879-1955) in about 1905.

CONTINENTAL DRIFT (1912)
On January 6, 1912, Alfred Wegener (Germany) proposed that the continents had once formed a single landmass and had drifted to their current positions, a theory he called ‘continental drift’. The idea that the continents moved was not new and had been suggested by Abraham Ortelius (Flanders) in 1596, Theodor Christoph Lilienthal (Germany) in 1756; Alexander von Humboldt (Germany) in 1801, Antonio Snider-Pellegrini (France) in 1858; Franklin Coxworthy in (1848-1890); Roberto Mantovani (Italy) in 1889-1909; William Henry Pickering (US) in 1907; and Frank Taylor (US) in 1908. Most scientists rejected Wegener’s hypothesis because, although there was fossil and glacial evidence to support the idea, he proposed no mechanism to explain the movements. In 1956 the discovery by Keith Runcorn (UK) and Warren Carey (Australia) that paleomagnetic stripes on the seafloor emanated from the mid-ocean ridges provided a clue to a continental drift mechanism. In 1963, Lawrence Morley (Canada), Fred Vine (UK), and Drummond Matthews (UK) independently proposed that Runcorn’s and Carey’s discovery was evidence that the seafloor was spreading, as predicted by Harry Hess in 1960-1962, and that was itself the mechanism for continental drift.   Further support for the theory was found in the 1961 work of Allan Cox (US) on the magnetization of lava; W.C. Pitman’s discovery of similar patterns in the mid-Pacific ridge in 1966, and historical seismographic data analyzed by Jack Oliver (US) in 1968. Since the mid-1960s, continental drift has been subsumed within the more comprehensive plate tectonics theory.

A graphic depiction of some of the fossil evidence supporting the continental drift theory.

THE GENERAL THEORY OF RELATIVITY (1916)
Albert Einstein’s general theory of relativity amended Newton’s law of universal gravitation to explain that the gravitational ‘pull’ of an object is best understood not as a force but as a warp in the curvature of space-time caused by the object’s mass. In 1919, Arthur Eddington and Frank W. Dyson (UK) measured the bending of starlight by the gravitational pull of the sun, thus confirming Einstein’s general theory of relativity. The general theory of relativity makes many predictions, including the expanding universe, and the existence of black holes and gravitational waves.

A 1921 photograph of Albert Einstein (1879-1955).

PENICILLIN (1928)
In 1928, Alexander Fleming (UK) discovered that a mold, Penicillium notatum, destroyed bacterial colonies. After years of research following up on Fleming’s discovery, Howard Florey (Australia/UK), Norman Heatley (UK), Ernst Chain (Germany/UK) and Andrew J. Moyer (US) developed a method of manufacturing penicillin as a drug in 1942.  Dorothy Hodgkin (UK) discovered the structure of the penicillin molecule in 1943. The first antibiotic, penicillin proved to be effective against many serious diseases caused by bacterial infections.

A photograph of Alexander Fleming (1881-1955).

11
THE LAW OF FALLING BODIES (1589-1590)
In the mid-16^th Century, European scientists began to challenge Aristotle’s claim that heavier objects fall faster than light ones through experimentation. Simon Stevin (Flanders), for example, showed in 1586 that two balls – one ten times heavier than the other – hit the ground at the same time when dropped 30 feet from a Delft church tower. In 1589-1590, while teaching at the University of Pisa, Galileo Galilei (Italy) not only performed similar experiments, but he also derived the mathematical equations to explain the phenomenon, as well as the acceleration of falling bodies and the phenomena of inertia and friction. He elaborated on his theories in 1634 and 1638 publications. The story that Galileo proved the theory by dropping balls from the Leaning Tower of Pisa is told by his pupil Vincenzo Viviani but may not be true, as Galileo preferred to experiment by rolling balls down an inclined board to reduce air resistance and simplify measurements. Galileo’s findings led to Isaac Newton’s law of universal gravitation.

A 1636 portrait of Galileo Galilei (1564-1642) by Giusto Sustermans.

THE LAWS OF PLANETARY MOTION (1609, 1619)
After studying the detailed astronomical observations of astronomer Tycho Brahe (Denmark), Johannes Kepler (Germany) derived three laws that determine the motion of the planets. He devised the first two laws in 1609: (1) The orbit of every planet is an ellipse with the sun at one of the two foci; and (2) A line joining a planet and the sun sweeps out equal areas during equal time intervals. In 1619, Kepler discovered a third law: (3) The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. In 1687, Sir Isaac Newton showed that Kepler’s laws were consistent with classical mechanics.

A 1610 portrait of Johannes Kepler (1571-1630).

THE CIRCULATORY SYSTEM (1628)
After years of careful study, physician William Harvey (England) published De Motu Cordis (On the Motion of the Heart and Blood), a 1628 book in which he described the entire system by which the heart distributes the blood through the arteries and the blood returns to the heart via the veins as well as many other details of the circulatory system of humans and animals. Prior discoveries about the circulatory system had been made by Galen (Ancient Greece/Ancient Rome) in the 2^nd and 3^rd centuries CE and Ibn al-Nafis (Syria) in 1242. Michael Servetus (Spain) published important discoveries about pulmonary circulation in 1553.

A portrait of William Harvey (1578-1657).

OXYGEN (1772)
Carl Wilhelm Scheele (Sweden) was the first to create oxygen gas and identify it as a separate element in 1772, although he did not publish his discovery until 1777. Joseph Priestley (England) isolated oxygen in 1774; because he published his findings in 1775, he is generally acknowledged as the discoverer. Prior to Scheele and Priestley, 16^th Century scientist Robert Boyle (Ireland) determined that air was necessary for combustion and John Mayow (England) discovered that only a portion of the air was necessary for combustion and respiration. Research in the 17^th and 18^th centuries was slowed by the erroneous phlogiston theory, which held that when a substance burned it released phlogiston into the air, and the reason some substances burned more completely than others was that they consisted of a higher proportion of phlogiston. Although Antoine Laurent Lavoisier (France) claimed that he also discovered oxygen in 1774, most historians dispute it.  Lavoisier did discover the nature of combustion and conducted important experiments on oxidation. His work also definitively disproved the phlogiston theory.

An engraved portrait of Carl Wilhelm Scheele (1742-1786).

VACCINATION (1796)
The idea of infecting healthy individuals with some form of the same or a similar disease in order to create an immunity has a long history in China, Africa, and India. There is also evidence that inoculation was practiced in Turkey in the early 18th Century.  Although there is some evidence that vaccination for smallpox occurred in England in the 1770s, Dr. Edward Jenner’s 1796 experiments with cowpox are usually identified as the first vaccinations. Jenner took pus from the blisters of farm workers infected with cowpox, a disease similar to but less lethal than smallpox, and exposed uninfected patients to it, making them immune to smallpox. He published his results in 1796, coining the phrase ‘vaccine’ to describe the method. In 1885, Louis Pasteur weakened or killed anthrax and rabies pathogens and vaccinated French citizens with great success. Pasteur adopted Jenner’s term ‘vaccination’ to describe his treatments.  Vaccination became a common form of disease prevention, and vaccines have been developed for numerous other diseases, such as the polio vaccine. American microbiologist Maurice Hilleman developed 36 successful vaccines in the 1950s and 1960s for such diseases as measles, mumps, hepatitis A and B, chicken pox, meningitis and pneumonia.

An engraving taken from an 1833 portrait of Edward Jenner (1749-1823).

RADIOACTIVITY (1896)
Radioactivity, also known as radioactive decay or nuclear decay, occurs when unstable atoms emit either alpha particles, beta particles or gamma rays from their nuclei.  In the process of emitting radiation, the atom changes from one element to another.  Henri Becquerel (France) discovered the radioactivity of uranium in 1896; he recognized the phenomenon was different from the recently discovered X-rays. In 1898, Marie and Pierre Curie (France) identified radium and polonium, two more radioactive elements.  Ernest Rutherford (NZ/UK) identified two types of radiation – the alpha and beta rays – in 1899.  Pierre Curie classified alpha and beta particle radiation in 1900. Paul Ulrich Villard (France) discovered a third type of radiation in 1900, which Rutherford called gamma rays.  The dangerous effects of radiation exposure to humans were not identified until much later. Marie Curie herself died of an illness that was probably related to her frequent exposure to radioactivity.

A photograph of Henri Becquerel (1852-1908).

QUANTUM THEORY (1900)
Around the beginning of the 20^th Century, physicists began to explore certain phenomena that did not appear to follow the rules of Newton’s classical mechanics, leading to the development of quantum theory, also known as old quantum theory, which was superseded by the more systematic quantum mechanics in about 1925.  In 1900, German physicist Max Planck explained the results of his studies of light emission and absorption by theorizing that light and other forms of electromagnetic energy could only be emitted in quantized form, or quanta, which would later be renamed photons. In 1905, Albert Einstein (Germany) explained the photoelectric effect (identified by Heinrich Hertz in 1887) by postulating that light is made of individual quantum particles.  Einstein also used quantum principles to explain the specific heat of solids.  In 1913, Niels Bohr (Denmark) revised the model of atomic structure to explain the atomic spectra by incorporating quantum energy states into the electron orbits.  In the following years Arnold Sommerfeld (Germany) further developed quantum theory.

A 1915 photograph of Max Planck (1858-1947).

THE STRUCTURE OF THE ATOM (1911)
In 1911, Ernest Rutherford (NZ/UK) rejected J.J. Thompson’s ‘plum pudding’ model of the atom, and proposed instead what some referred to the solar system model, with a sun-like nucleus orbited by planet-like electrons.  The negatively-charged electrons, which had very low mass, orbited a very small positively-charged nucleus, which contained most of the atom’s mass. Rutherford’s proposal was based in part on the 1909 experiments by Hans Geiger (Germany) and E. Marsden (UK), who scattered alpha particles using thin films of heavy metals, providing evidence that atoms possessed a discrete nucleus.  Niels Bohr (Denmark) revised the model in 1913 to make it consistent with quantum theory.  His electrons had fixed orbits and could only jump from one orbit to another.  Arnold Sommerfeld (Germany) further revised the model to incorporate elliptical (instead of circular) electron orbits about 1920.

A photograph of Ernest Rutherford (1871-1937).

QUANTUM MECHANICS (1925)
In 1924, Louis de Broglie (France) used Einstein’s special theory of relativity as the basis for a theory that particles can exhibit the characteristics of waves, and vice versa.  De Broglie’s theory of matter waves set off a chain reaction of discoveries in 1925 setting out the principles of quantum mechanics: German physicists Werner Heisenberg, Max Born and Pascual Jordan created matrix mechanics; and Austrian physicist Erwin Schrödinger developed the Schrödinger equation, which allowed scientists to determine the likelihood that a particle would be in a particular place at a particular time, thus giving birth to wave mechanics. Further developments included Heisenberg’s uncertainty principle in 1927, and British physicist Paul Dirac’s 1928 equation, which describes the electorn’s wave function and predicted electron spin and the positron. John von Neumann (Hungary) formulated the mathematical basis for quantum mechanics in 1932.

A photograph of Erwin Schrödinger (1887-1962).

THE EXPANDING UNIVERSE (1929)
In 1912, Vesto Slipher (US) became the first astronomer to discover and measure the Doppler redshifts of nebulae (later found to be distant galaxies), which provided the observational basis for the theory that the universe is expanding. In a 1924 paper, Alexander Friedmann (USSR) developed the mathematical basis for a number of possible universes, including an expanding universe. Georges LeMaitre (Belgium) first proposed that the universe was expanding in 1927. Edwin Hubble (US) obtained the first direct evidence that the universe is expanding in 1929 by comparing the distances to other galaxies with their redshifts. Hubble also devised the Hubble constant – a measure of the rate at which the universe is expanding.  Recently, scientists have discovered that the expansion of the universe is accelerating.

A photograph of Edwin Hubble (1889-1953).

THE BIG BANG (1931)
Drawing on the findings of Slipher, Friedmann, Hubble and others, Georges Lemaître (Belgium) proposed in 1931 that the expanding universe, projected back in time, must have begun at a point when all the mass of the universe was concentrated at a single point, which he termed ‘the primeval atom.’  In the 1940s, George A. Gamow (USSR/US) was a stalwart proponent of Lemaître’s theory, which acquired the name ‘Big Bang’ in 1949 from steady state advocate and ‘Big Bang’ theory opponent Fred Hoyle (UK).  Gamow developed aspects of the Big Bang theory, including a 1948 paper with Ralph Alpher (US) showing how the Big Bang explained current levels of hydrogen and helium in the universe through Big Bang nucleosynthesis.  In addition, Alpher predicted in 1948 that cosmic microwave background radiation generated by the Big Bang should be detectable.  In 1964, Arno A. Penzias and Robert W. Wilson (US) accidentally discovered the cosmic microwave background radiation.  NASA’s Cosmic Microwave Background Explorer, launched in 1989, has since provided much more accurate and complex data which, as analyzed by John C. Mather and George Smoot (US) in 1992, showed fluctuations in the Big Bang that explain the large-scale structure of the universe.

A photograph of Georges Lemaître (1894-1966).

10
THE GERM THEORY OF DISEASE (1859)
Beginning in 1859, Louis Pasteur (France) conducted a series of experiments that proved the connection between disease and microorganisms, or germs, the results of which were published in 1862. This discovery revolutionized medicine and eventually had a significant impact on human mortality rates. Scientists whose prior work led to Pasteur’s discovery include: Girolamo Fracastoro (Italy), who proposed a germ theory in 1546; Agostino Bassi (Italy) who conducted crucial experiments in 1808-1813; Ignaz Semmelweis (Hungary) who conducted clinical studies of disease in 1847; and John Snow (UK), who studied public health response to disease outbreaks in 1854-1855.  Following up on Pasteur’s findings in 1884, Robert Koch (Germany) articulated a four-part test for determining if disease is caused by microorganisms and also identified the bacteria that cause cholera, tuberculosis and anthrax.

A photograph of Louis Pasteur (1822-1895) by Nadar.

THE LAWS OF HEREDITY (1865)
In the early 19th Century, a number of scientists working with pea plants noticed the segregation of a recessive trait, one of the key elements of the laws of heredity, but unfortunately none of these early scientists kept records of later generations, severely limiting the benefit of their work.  Augustinian friar Gregor Mendel (Silesia) performed a comprehensive series of experiments on numerous generations of pea plants between 1856 and 1863 that allowed him to develop the basic rules of heredity and inheritance, including the existence of dominant and recessive traits, which would form the basis of the modern science of genetics. Mendel presented the results of his work in a paper he read at meetings of the Natural History Society of Brno, Moravia in 1865, which was published in 1866. Because the work was perceived to be about hybridization and not inheritance, it did not receive wide distribution, and most scientists, including Charles Darwin (UK), never learned of it. It was only after 1900, when other scientists, particularly Hugo de Vries (The Netherlands), Carl Correns (Germany), Erich von Tschermak (Austria) and William Jasper Spillman (US), independently rediscovered Mendel’s work, that his importance to science was appreciated.

A photograph of Gregor Mendel (1822-1884).

THE TRANSISTOR (1947)
A transistor is a device made of semiconductor material that amplifies and switches electronic signals and electrical power. The precursor to the transistor was the vacuum-tube triode, or thermionic valve, first created in 1907 by Lee De Forest (US).  Julius Edgar Lilienfeld (Austria/Hungary) patented a field-effect transistor in 1925, but his work was ignored at the time.  (Years later, William Shockley and Gerald Pearson (US) at Bell Labs made a functional device using Lilienfeld’s design.) German physicist Oskar Heil patented a field-effect transistor in 1934. In the mid-1940s, John Bardeen and Walter Brattain (US) built a semiconducting triode for use in military radar equipment. After the end of World War II, Schockley, Bardeen and Walter Brattain (US) worked on using semiconductors to replace vacuum tubes in electrical systems. In December 1947, they created a germanium point-contact transistor – the first solid-state electronic transistor. In June 1948, Shockley designed a grown-junction transistor; a prototype was built in 1949. German physicists Herbert F. Mataré and Heinrich Welker invented a transistor they called the transistron in August 1948. In 1950, Shockley developed a bipolar junction transistor. Morgan Sparks (US) at Bell Labs made the new transistor into a useful device. General Electric and RCA produced an alloy-junction transistor – a type of bipolar junction transistor – in 1951. By 1953, transistors were being used in products such as hearing aids and telephone exchanges. Dick Grimsdale (UK) built the first transistor computer in 1953. Also in 1953, Philco (US) invented the first surface-barrier transistor. In the early 1950s, Bell Labs also produced the first tetrode and pentode transistors. Around the same time, the spacistor was created, but it was soon obsolete. In 1954, teams led by Morris Tanenbaum (US) at Bell Labs and Gordon Teal (US) at Texas Instruments, working independently, invented the first silicon transistor.  Also in 1954, Bell Labs produced the first diffusion transistor, while in 1955, Bell made the first diffused silicon mesa transistor, which was developed commercially by Fairchild Semiconductor (US) in 1958. Also in 1955, Tanenbaum and Calvin Fuller (US) invented a much improved silicon transistor. The first gallium-arsenide Schottky-gate field-effect transistor was invented by Carver Mead (US) in 1966.

From left: John Bardeen (1908-1991), William Shockley (1910-1989) and Walter Brattain (1902-1987) at Bell Labs in 1948.

9
HUMAN ANATOMY (1543)
The modern study of human anatomy began with physician Andreas Vesalius (Belgium), whose seven-volume 1543 treatise, De fabrica corporis humani, provided a detailed, well-researched and systematic study of the human body that corrected many errors of the past. Anatomical study has a long history before and after Vesalius. Ancient Egyptian treatises on anatomy date to 1600 BCE. Ancient Greek anatomists include Alcmaeon, Acron (480 BCE), Pausanias (480 BCE), Empedocles (480 BCE), Praxagoras (300 BCE?), Herophilus (280 BCE?); and Erasistratus (260 BCE?). Aristotle conducted empirical studies in the 4^th Century BCE and began the study of comparative anatomy. Galen, a Greek living in the Roman Empire in the 2^nd Century CE, was the first major anatomist.  He was highly influential into the modern era, but performed few human dissections and propagated some serious errors. Italian physician Mondino de Luzzi performed the first human dissections since Ancient Greece between 1275 and 1326. In the late 15^th Century, Leonardo da Vinci dissected approximately 30 human bodies and made detailed drawings, until the Pope ordered him to stop.  In 1541, Giambattista Canano (Italy) published illustrations of each muscle and its relation with the bones.

An engraved portrait of Andreas Vesalius (1514-1564) taken from his 1543 treatise.

TELESCOPIC OBSERVATIONS (1609-1610)
In 1609 and 1610, Galileo Galilei built a series of progressively more powerful telescopes and began making detailed scientific observations of the heavens. In addition to providing support for the Copernican/Keplerian heliocentric model, Galileo discovered four of the moons of Jupiter; the phases of Venus; sunspots; lunar mountains and craters; and masses of stars in the Milky Way, which was thought to be made of clouds. Galileo published his discoveries in Sidereus Nuncius (Starry Messenger) in 1610, which became a bestseller.

Two of Galileo Galilei’s original telescopes from the early 1600s.

CELLS (1665)
Seventeenth Century scientist Robert Hooke (England) used the newly-invented microscope to make detailed observations of biological organisms and other materials.  He published his results in 1665 in a book titled Micrographia.  In the book, Hooke coined the term ‘cell’ to describe the small compartments he observed in plant tissues, including cork.  One theory is that the term came from the resemblance of the plant cells to the cells of a honeycomb; others say they reminded Hooke of monk’s living quarters, known as cells.  The importance of cells in biological systems would not be fully recognized until Theodor Schwann and Matthias Schleiden (Germany) proposed their cell theory in 1838-1839.

A microscope built by Christopher Cook for Robert Hooke (1635-1703) in the 17th Century.

MICROORGANISMS (1674-1675)
Antonie van Leeuwenhoek was a 17th Century Dutch amateur scientist and inventor who was fascinated by the microscope, and built at least 25 microscopes in his lifetime.  In 1674 and 1675, van Leeuwenhoek turned his lens on pond water and was surprised to find an entire universe of tiny living creatures that humans could not see with the naked eye.  Van Leeuwenhoek called his discoveries ‘animalcules’ but we now refer to them as microorganisms.  Most of the microorganisms van Leeuwenhoek described belonged to the Protista, a group one-celled creatures, although some were probably multi-celled larvae of larger animals, such as insects and crustaceans.  In 1676, van Leeuwenhoek was the first to see and describe bacteria.  Science historians recognize van Leeuwenhoek as the first microbiologist.

A portrait of Antonie van Leeuwenhoek (1632-1723) by Jan Verkolje from between 1670 and 1693. It is located in the Museum Boerhaave in Leiden.

THE ELECTRIC BATTERY (1800)
Although there is some evidence of primitive batteries from the first centuries of the Common Era in Mesopotamia and India, the modern precursor to the electric battery was the Leyden Jar, which was invented in 1745-1746. Benjamin Franklin coined the term ‘battery’ to describe a set of linked Leyden jars because of its resemblance to a battery of artillery pieces.  Then, in 1791, Italian scientist Alessandro Volta published the results of experiments showing that two metals joined by a moist intermediary could create electric energy. In 1800, Volta used this principle to create the voltaic pile, the first true battery. Over the next century, many scientists developed Volta’s invention further: William Cruickshank (UK) invented the trough battery in 1800; William Sturgeon (UK) improved upon the design in 1835; John Daniell (UK) invented the Daneill cell in 1836; Golding Bird (UK) invented the Bird cell in 1837; John Dancer (UK) invented the porous pot Daniell cell in 1838; William Grove (UK) invented the Grove cell in 1844; Gaston Planté (France) invented the lead-acid battery in 1859; Callaud (France) created the gravity cell in the 1860s; Johann Poggendorff (Germany) created the Poggendorff cell; Georges Leclanché (France) invented the Lelanché cell in 1866; and the first dry cells were invented independently by Carl Gassner (Germany), Frederick Hellesen (Denmark) and Yai Sakizo (Japan) in 1886-1887.

One of Allessandro Volta’s (1745-1827) early voltaic piles on display at his museum in Como, Italy.

MODERN ATOMIC THEORY (1808)
In 1808, John Dalton (UK) theorized that all matter was made of very small indivisible particles called atoms, that each element is made of different atoms, that each element’s atoms are identical, that atoms combine to make chemical compounds and are combined, separated or rearranged in chemical reactions.  The individual elements of this atomic theory were confirmed experimentally over the next two centuries.

An 1834 portrait of John Dalton (1766-1844) by Charles Turner.

THE ELECTRON (1897)
In the mid-19^th Century, Richard Laming suggested that atoms consist of a core surrounded by small charged particles. George Johnstone Stoney (Ireland) proposed in 1874 that electricity consisted of charged ions that had a measurable charge. Hermann von Helmholz (Germany) suggested in 1881 that the positive and negative charges were divided into basic parts and both were atoms of electricity. In 1891, Stoney coined the name ‘electron’ for the fundamental unit of electricity. Experiments leading up to the discovery of the electron began with German physicist Johann Wilhelm Hittorf’s conductivity work in 1869; the discovery of cathode rays by Eugen Goldstein (Germany) in 1876; and the development of a high vacuum cathode ray tube by Sir William Crookes (UK) in the 1870s. Arthur Schuster (Germany/UK) performed cathode ray experiments that allowed him to estimate the charge-to-mass ratio of the electron.  In 1896-1897, J.J. Thomson, assisted by John S. Townsend and H.A. Wilson (UK), performed a series of experiments that conclusively identified the cathode ray emissions as particles with a definite mass and a negative charge.  They also showed that these particles were identical even when produced in different contexts (heating, illumination, radioactivity). George Fitzgerald (Ireland) proposed the name ‘electron’ for Thomson’s particle. In 1900, Henri Becquerel (France) showed that beta rays emitted by radioactive elements were electrons. The charge of the electron was measured more carefully by Robert Millikan and Harvey Fletcher (US) in a 1909 experiment, the results of which were published in 1911.

J.J. Thomson (1856-1940).

GENES ARE MADE OF DNA (1944)
In a groundbreaking 1928 experiment, Frederick Griffith (UK) found a ‘transforming’ principle that could change one type of bacteria to another. Over the next 15 years, scientists at the Rockefeller Institute for Medical Research in New York sought to isolate the transformative substance by working with bacteria and bacteriophage viruses.  In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty (US) published their surprising results: the substance that contained the genetic information was a nucleic acid, DNA (deoxyribonucleic acid), not a protein as supposed.  The scientific community was reluctant to accept the results of the Avery-MacLeod-McCarty experiment.  In 1952, Alfred Hershey and Martha Chase (US) followed up with conclusive proof that DNA is the substance of genes, which led to general acceptance by scientists.

From left: Oswald Avery (1877-1955), Colin MacLeod (1909-1972) and Maclyn McCarty (1911-2005).

8
THE SPEED OF LIGHT (1676)
Ole Rømer (Denmark) determined in 1676 that light travels at a finite speed, contrary to the common belief that light traveled infinitely fast. Christiaan Huygens (The Netherlands) used Rømer’s results to calculate the speed of light to be 220,000 kilometers/second. In 1704, Isaac Newton (England) calculated the time for light to travel from the sun to the Earth as “seven or eight minutes” (the actual time is 8 minutes, 19 seconds). James Bradley (England) discovered the phenomenon known as ‘aberration of light’ in 1729 and adjusted the calculation of the sun-earth time to 8 minutes, 12 seconds. In the 19th Century, James Clerk Maxwell (UK) proposed that light was a type of electromagnetic wave, and that all such waves traveled at the same speed. Hippolyte Fizeau (France) made a calculation of 313,300 km/sec in 1849 without using astronomical measurements. Albert Michelson and Edward Morley (US) conducted an experiment in 1887 that measured light at 185,000 miles per second. A 1928 experiment by Michelson refined the speed of light to 186,284 miles per second.  The current estimate for the speed of light is 186,282 miles per second (officially 299,792,458 meters per second).

Ole Rømer (1644-1710).

THE STEAM ENGINE (1698)
Humans have been using the steam from boiling water to do mechanical work since ancient times, but practical designs only arrived in the 17th Century.  Jerónimo de Ayanz y Beaumont (Spain) patented a steam engine in 1606 for removing water from mines.  In 1679, Denis Papin (France/England) developed a steam digester, a precursor to the steam engine.  British engineer Thomas Savery’s pistonless steam pump of 1698 was the first practical design based on Papin’s ideas.  Thomas Newcomen’s (UK) 1712 piston-driven “atmospheric-engine” proved to be the first commercially viable steam engine.  In 1725, Savery and Newcomen built a steam engine for pumping water from collieries.  Between 1765 and 1774, James Watt (UK) improved on the Newcomen engine by making it condensing and double acting, which hugely increased its efficiency.  A high pressure steam engine was developed by Oliver Evans (US) in 1804.  Further improvements followed throughout the 19th Century.

Thomas Savery (c. 1650-1715).

THE TELEGRAPH (1837)
One of the first steps towards an electrical telegraph was taken in 1750 by Benjamin Franklin (US), who created a device that sent an electrical signal across a conductive wire that was registered at a remote location.  An electrochemical telegraph was created by Francisco Salva Campillo (Spain) in 1804; Samuel von Sömmering (Germany) made an improved version in 1809.  The messages could be transmitted a few kilometers and would release a stream of bubbles in a tube of acid, which had to be read to determine the letter or number. In 1823, Francis Ronalds (UK) created the first working electrostatic telegraph using eight miles of wire in insulated glass tubing attached to clocks marked with letters of the alphabet.  Baron Pavel Schilling von Canstatt (Estonia) created an electromagnetic telegraph in 1832, but it was Carl Friedrich Gauss and Wilhelm Weber (Germany) who built the first electromagnetic telegraph used for regular communication, in 1833.  David Alter (US) invented the first American electric telegraph in 1836 but never used it to make a practical system. The first commercial electrical telegraph was created by William Cooke and Charles Wheatstone (UK); it was patented in May 1837 and successfully demonstrated in July 1837; they installed the system between two railway stations 13 miles apart in 1838.  Edward Davy (UK) also demonstrated a telegraph system in 1837 and patented it in 1838 although he did not pursue it. Samuel Morse (US) independently invented his own electrical telegraph in 1837, while his assistant Alfred Vail developed Morse code. Morse sent the first telegram using his system on January 11, 1838, but it was not until 1844 that he sent his famous message, “What hath God wrought” from Washington, D.C. to Baltimore, Maryland.  Telegraph lines connected the east and west coasts of the US by 1861 and by 1866, a trans-Atlantic telegraph cable linked Europe and the US.

A telegraph key designed by Samuel Morse (1791-1872) and Alfred Vail (1807-1859), from 1844-1845.

THE LAW OF CONSERVATION OF ENERGY (1842)
According to the law of conservation of energy, energy can change form, but it cannot be created or destroyed; because the total energy of a system does not change over time, the energy is said to be conserved.  German chemist Karl Friedrich Mohr gave one of the first statements of the law in 1837. The key concept that heat and mechanical work are equivalent was first stated by Julius Robert von Mayer (Germany) in 1842. James Prescott Joule (UK) reached the same conclusion independently in 1843, as did Ludwig A. Colding (Denmark). In 1844, William Robert Grove (UK) suggested that mechanics, heat, light, electricity and magnetism were all manifestations of a single force, a notion he published in 1846. Drawing on the work of Joule and others, Hermann von Helmholtz (Germany) reached conclusions similar to Grove’s in an 1847 book, which brought about wide acceptance of the idea. In 1850, William Rankine (UK) first coined the phrase ‘law of conservation of energy’ to describe the principle.

Hermann von Helmholtz (1821-1894).

ANESTHESIA (1842)
Ancient physicians used various herbs, including Solanum, opium and coca to induce unconsciousness and/or relieve pain in their patients, as well as alcohol.  There is some evidence that Medieval Arabs used an inhaled anesthetic.  In the late 12th Century, in Salerno, Italy, physicians used a ‘sleep sponge’ soaked in a solution of opium and various herbs, which was held under the patient’s nose.  The sleep sponge was used by Ugo Borgognoni and his son Theodoric (Italy) in the 13th Century. In 1275, Spanish physician Raymond Lullus invented what would later be called ether. He and Swiss physician Paracelsus experimented with animals but not humans.  In 1772, Joseph Priestley (England) discovered nitrous oxide, or laughing gas, and in 1799, British chemist Humphry Davy discovered the gas’s anesthetic properties by experimenting on himself and his friends. Morphine was discovered in 1804 by Friedrich Sertürner (Germany) but it was only widely used as an anesthetic after the invention of the hypodermic syringe.  In 1842, American physician Crawford Long became the first to use ether as an anesthetic for human surgery when he removed two small tumors from James Venable, one of his students, in a painless procedure, but the operation was not publicized until 1849.  In a widely publicized 1846 event, Boston dentist William Morton administered inhaled ether to a patient in Massachusetts General Hospital, after which a surgeon painlessly removed a tumor. Ether was eventually replaced by other chemicals due to its flammability.  Cocaine, which was first identified in 1859, became the first effective local anesthetic in 1884 when Austrian physician Karl Koller used it during eye surgery.

An artist’s rendering of William Morton’s 1846 use of general anesthesia, an event that became much more well known than Crawford Long’s 1842 breakthrough.

THE LOCATION OF GENES (1910)
Through his experiments with fruit flies (Drosophila melanogaster), biologist Thomas Hunt Morgan (US) proved that genes are carried on chromosomes and are the mechanical basis for heredity. In so doing, Morgan established the modern science of genetics.

Thomas Hunt Morgan (1866-1945) in the fly room at Columbia University.

7
MOVABLE TYPE PRINTING (1040)
THE PRINTING PRESS (1450)
Block printing was first invented in Japan in about 700 CE. Bi Sheng (China) invented movable type printing in 1040.  He made the characters from wood at first but found that ceramics worked better. Choe Yun-ui (Korea) was the first to use metal for the type, in 1234. The technology did not spread to Europe. Johannes Gutenberg (Germany) invented movable type printing independently between 1440 and 1450.  Gutenberg’s major innovation was to adapt the already-existing screw press to print his pages.  He also created a special metal alloy for the type; invented a device for moving type quickly; and developed a new, superior ink.  The result was the production of higher quality printing at a much faster pace. Offset printing was invented by Aloys Senefelder (Germany) in 1796. The cast iron printing press, which reduced the force needed and doubled the size of the printed area, was invented by Lord Stanhope (UK) in 1800. Between 1802 and 1818, Friedrich Koenig (Germany) created a steam-powered press with rotary cylinders instead of a flatbed.  In 1843, Richard M. Hoe (US) invented a steam-powered rotary printing press. Linotype printing was invented by Ottmar Mergenthaler (US) in 1884.

A replica of Johannes Gutenberg’s (1398-1468) printing press (left) and workshop located in St. George’s, Bermuda.

BOYLE’S LAW (1662)
Boyle’s Law states that as the volume of a gas increases, the pressure of the gas decreases according to an inverse mathematical proportion. The relationship between pressure and volume of gases was first recognized by Richard Towneley and Henry Power (UK) in 1661, but Irish scientist Robert Boyle confirmed the relationship and published his results in 1662 with a mathematical formula, the first to accompany a natural law. Boyle’s assistant Robert Hooke (UK) built the experimental apparatus. Edme Mariotte (France) independently reached the same result in 1676.

Johann Kerseboom’s 1689 portrait of Robert Boyle (1627-1691).

THE CALCULUS (1666, 1674)
Sir Isaac Newton (England) and Gottfried Leibniz (Germany) independently invented the infinitesimal calculus in the mid-17^th Century. Newton appears to have priority over Leibniz, although the question of who was the inventor was the subject of much controversy at the time. An unpublished manuscript of Newton’s supports his claim to have been working on ‘fluxions and fluents’ as early as 1666. Leibniz began his work in 1674 and first introduced the concept of differentials in 1675, as he explained to Newton in a 1677 letter. Leibniz’s first publication on calculus using differentials was in 1684. Newton explained his geometrical form of calculus in his Principia of 1687, but did not publish his fluxional notation until 1693 and not fully until 1704.  Precursors to Newton and Leibniz included Pierre de Fermat (France) in 1636, René Descartes (France) in 1637, Blaise Pascal (France) in 1654, John Wallis (England) in 1656, and Newton’s teacher Isaac Barrow (England) in 1669. Bonaventura Cavalieri (Italy) developed his method of indivisibles in the 1630s and 1640s, and computed  Cavalieri’s  quadrature formula.  Evangelista Torricelli (Italy) extended this work to other curves such as the cycloid in the 1640s, and the formula was generalized to fractional and negative powers by Wallis in 1656. In a 1659 treatise, Fermat is credited with an ingenious trick for evaluating the integral of any power function directly. Fermat also obtained a technique for finding the centers of gravity of various plane and solid figures, which influenced further work in quadrature. In a 1668 book, James Gregory (England) published the first statement and proof of the fundamental theorem of the calculus, stated geometrically, and only for a subset of curves. Mathematical developments after Newton and Leibniz include those of Augustin Louis Cauchy (France) in 1821; Karl Weierstrauss (Germany) in the 1850s; and Bernhard Riemann (Germany) in the 1850s.

A portrait of Gottfried Leibniz (1646-1716) by Christoph Bernhard Francke.

THE LEYDEN JAR (1745)
The Leyden jar is the prototype electrical condenser and the first capacitor; it was capable of storing static electric charge. The Leyden jar was invented independently in 1745 by German cleric Ewald Georg von Kleist and in 1746 by Dutch scientists Pieter van Musschenbroek and Andreas Cunaeus at the University of Leyden in The Netherlands. Leyden jars were used in many early experiments on electricity. Daniel Gralath (Poland) was the first to join multiple Leyden jars to each other in parallel to increase the stored charge, a formation for which Benjamin Franklin (US) coined the term “battery.”

An artist’s imagining of the discovery of the Leyden jar by Andreas Cunaeus (1743-1797) in the laboratory of Pieter van Musschenbroek (1692-1761).

URANUS (1781)
Uranus, the seventh planet from the sun, had been recognized possibly as early as 188 BCE, and again by John Flamsteed (England) in 1690 and Pierre Lemonnier (France) between 1750 and 1769, but it was not identified as a planet due to its dimness and slow orbit. William Herschel (England) first observed Uranus in March 1781, although he originally identified it as a comet. When Anders Johan Lexell (Finland/Sweden) computed the object’s orbit in 1781, he concluded it was a planet, not a comet, the same conclusion reached by Johann Elert Bode (Germany) about the same time. Herschel acknowledged that he had discovered a new planet in 1783.  Herschel suggested the name Georgium Sidus, after King George III, but it was Bode’s suggestion of Uranus, the father of Saturn, that eventually won out.

A replica of the telescope that William Herschel (1738-1822) used to discover Uranus.

ELECTROMAGNETIC INDUCTION (1831)
In electromagnetic induction, an electromotive force is produced across a conductor when it is exposed to an electromagnetic field. Michael Faraday (UK) and Joseph Henry (US) independently discovered this phenomenon in 1831. Because Faraday published his results first, he is usually credited with the discovery. James Clerk Maxwell (UK) later devised the mathematical principle underlying electromagnetic induction, naming it Faraday’s Law. Electromagnetic induction is the principle underlying electrical generators, transformers and many other electrical machines.

A sketch of Michael Faraday’s (1791-1867) electromagnetic induction experiment. The battery (right) sends a current to the coil (A), which creates a magnetic field but no current. When the small coil is moved in and out of the large coil (B) the magnetic field changes and a current flows.

CELL THEORY (1838)
Cell theory holds that (1) All living organisms are composed of one or more cells or the products of cells; (2) The cell is the most basic unit of life; and (3) All cells arise from pre-existing, living cells.  German botanist Matthias Schleiden proposed the first two premises of cell theory in 1838 to describe the plant kingdom.  In 1839, Theodor Schwann (Germany) extended Schleiden’s theory to animals.  Barthelemy Dumortier (Belgium) had proposed a similar theory in 1832, and Schleiden’s theory adopted Dumortier’s erroneous belief that cells were created by a crystallization process either from other cells or from outside. This portion of the theory was refuted by Robert Remak (Poland/Germany), Rudolf Virchow (Germany), and Albert Kolliker (Switzerland) in the 1850s. In 1855, Virchow proposed the third premise of cell theory, that all cells arise only from pre-existing cells.

Matthias Schleiden (1804-1881).

THE FIRST LAW OF THERMODYNAMICS (1850)
The first law of thermodynamics is derived from the law of conservation of energy, as applied to thermodynamic systems. The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. The first law of thermodynamics states that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings. The first law of thermodynamics was stated by Rudolf Clausius (Germany) in 1850. The principles were developed by William Rankine (UK) in the 1850s. The law was conceptually revised by George H. Bryan (UK) in 1907 to state, “When energy flows from one system or part of a system to another otherwise than by the performance of mechanical work, the energy so transferred is called heat.” Max Born (Germany/UK) revised this reformulation in 1921 and 1949.

Rudolf Clausius (1822-1888).

THE AUTOMOBILE (1885)
The first automobiles powered by internal combustion engines used gases instead of gasoline.  Samuel Brown (UK) used hydrogen to fuel his vehicle in 1826.  John Joseph Etienne Lenoir (Belgium) also used hydrogen, then coal gas, to power his Hippomobile in 1860. In 1870, Siegfried Marcus (Austria) used liquid fuel to propel a handcart, known as “the Marcus car.” He developed a more sophisticated four-seat vehicle in 1888-1889. Edouard Delamare-Debouttevile (France) built a gas-powered automobile in 1884. German inventor Karl Benz made his first automobile in 1885 – the first with a practical high-speed internal combustion engine – and started production in 1888. Gottlieb Daimler and Wilhelm Maybach (Germany) designed and built the first true automobile (not a carriage with a motor) from scratch in 1885. John William Lambert (US) built a three-wheeler in 1891, the same year that Henry Nadiq (US) built a four-wheeler. René Panhard and Emile Lavassor (France) built the first automobile with a spray carburetor in 1891. Charles Duryea tested the first US gasoline-powered automobile in Massachusetts in 1893. Frederick William Lanchester (UK) built an early automobile in 1895. Canadian George Foss built a single-cylinder gasoline car in 1896. In 1908, Ford Motor Company (US) introduced the mass-produced Model T, which was considered the first affordable automobile for the middle class.  More than 15 million Model T Fords were produced in factories in the US and Europe between 1908 and 1927.

An 1886 photograph of a gasoline-powered automobile designed by Gottfried Daimler (1834-1900).

INSULIN (1921)
In a series of experiments beginning in 1869 in Germany, scientists identified the islets of Langerhans in the pancreas and determined that these islets secreted a substance that controlled blood sugar levels.  Absence of this secretion caused diabetes mellitus.  Early attempts to treat diabetes with general pancreatic fluids had mixed results. Frederick Banting (Canada), working with medical student Charles Best (Canada), finally isolated and extracted the substance, now known as insulin, in 1921. James Collip (Canada) was instrumental in developing a purified extract.  The first successful treatment of a human diabetic occurred in 1922. Later the same year, Eli Lilly and Co. developed a method for producing large quantities of insulin. In 1923, Banting and John MacLeod (UK) were able to purify insulin for use in humans. Frederick Sanger (UK) identified the molecular structure of insulin in the 1950s. In the early 1960s, Panayotis Katsoyannis (US) and Helmut Zahn (Germany) independently invented the first synthetic insulin, but it was not specifically designed for humans. Scientists in China synthesized insulin in 1966. In 1977, a team of scientists (Arthur Riggs, US; Keiichi Itakura, Japan/US; and Herbert Boyer, US) created the first genetically engineered synthetic ‘human’ insulin.  It went on the market in 1982 as Humulin.

Frederick Banting (1891-1941) (right) and Charles Best (1899-1978) with one of the diabetic dogs they used to test insulin.

GALAXIES (1924)
In 1914, Arthur Stanley Eddington (UK) hypothesized that what were called spiral nebula were actually distant galaxies. In 1924, Edwin Hubble (US) conclusively proved that the Milky Way is just one of many millions of galaxies in the universe. Precursors to Hubble included Thomas Wright (UK), who speculated in 1750 that the Milky Way was a flattened disk of stars (a galaxy) and that some nebulae might be separate galaxies.  Lord Rosse (Ireland/UK) in 1845 detected individual stars in some nebulae. Vesto Slipher (US) studied nebulae and detected red shifts in 1912.  Heber Curtis (US) found evidence to support independent galaxies in 1917. In 1922, Ernst Öpik (Estonia) proved that the Andromeda Galaxy is separate from the Milky Way.

At just over 2.5 million light years from Earth, the Andromeda Galaxy is one of the closest galaxies to the Milky Way.

ROCKETRY (1926)
Modern rocketry was born in 1926 when Robert H. Goddard (US) launched the first liquid fuel rocket in Auburn, Massachusetts.  His invention led to the V-2 and the ICBM missiles as well as the rockets that sent satellites into orbit, men to the moon, and probes into deep space. The first rockets, fueled by gunpowder, were made by the Chinese in the 13^th Century for war and fireworks. They spread to the Mongols, who then brought them to Europe and the Muslim world, including the Ottoman Empire, in the 13^th, 14^th and 15^th centuries. The Kingdom of Mysore in southern India in the 1780s and 1790s developed an artillery rocket that used iron cylinders to contain the combustible element, which significantly improved range. William Congreve (UK) adapted the Mysore rocket to create the Congreve rocket. In 1844, William Hale (UK) altered the design of the Congreve rocket to improve its accuracy significantly.

Robert Goddard (1882-1945) with his first liquid fueled rocket in 1926.

THE NEUTRON (1932)
Ernest Rutherford proposed the existence of the neutron in 1920 to explain the disparity between the atomic number of an atom’s nucleus (i.e., the number of positively-charged protons) and the atomic mass. Some scientists believed that all the atomic mass came from protons, but that the negative charge of electons present in the nucleus canceled out some of the protons’ positive charge. But Viktor Ambartsumian and Dmitri Ivanenko (USSR) proved in 1930 that electrons could not exist in the nucleus and there must be neutral particles present. Walther Bothe and Herbert Becker (Germany) discovered unusual radiation in 1931, a result that was pursued in 1932 by Irène Joliot-Curie and Frédéric Joliot (France). Following up on the strange radiation found by the German and French scientists, James Chadwick (UK) in 1932 definitively identified the neutron, an uncharged particle approximately the same mass as the proton. The discovery of the neutron was a key in the development of nuclear reactors and atomic weapons.

James Chadwick (1891-1974).

THE COMPUTER (1945)
The Ancient Greeks made analog computing machines to perform astronomical calculations, including the Antikythera mechanism and astrolabe (c. 150-100 BCE) and Hero of Alexandria’s automata and programmable cart (c. 10-70 CE).  Abu Rayhan al-Biruni (Persia) invented the planisphere in 1000 CE; Abu Ishaq Ibrahim al-Zarqali (Moorish Spain) invented an equatorium and latitude-independent astrolabe about 1015 CE.  In China, Su Song created an astronomical clock in 1090 CE.  John Napier (Scotland) invented Napier’s Bones, an abacus-like device, in 1617. William Oughtred (UK) and others invented the slide rule in 1622. In 1623, Wilhelm Schickard (Germany) invented a calculating clock that was destroyed in a fire in 1624. Blaise Pascal (France) created a mechanical calculator (the Pascaline) in 1642 and built 20 copies, nine of which survive. Gottfried Wilhelm von Leibniz (Germany) invented the Stepped Reckoner in 1672; he also described the binary number system. In 1801, Josephe-Marie Jacquard (France) used punch cards to control a loom weaving a pattern. Charles Xavier Thomas de Colmar (France) made the first successful mass-produced mechanical calculator – the Thomas Arithmometer – in 1820. Between 1833 and 1837, Charles Babbage (UK) used a punch card system to design an analytical engine that, if ever completed, would have been the first programmable computer.  (In 1843, Per Georg and Edward Schulz of Sweden built a working model of an older, less sophisticated Babbage design – the 1822 difference engine.)  Beginning in the 1880s, a number of other mechanical calculators arrived that were based on Colmar’s Arithmometer, such as: the comptometer (Dorr Felt, US, 1887); the Addiator (Louis Troncet, France, 1889); the Yazu Arithmometer (Ryoichi Yazu, Japan, 1903); the Monroe (Jay R. Monroe, US, 1912); the Addo-X (AB Addo, Sweden, 1918); and the Curta (Curt Herzstark, Austria, 1948). Late in the 1880s, Herman Hollerith (US) used punch cards on a machine that could store and read the data contained on them by using a tabulator and a key punch machine. The machine was used to tabulate the 1890 U.S. Census.  Hollerith’s company eventually became IBM.  In the first half of the 20th Century, a number of analog computers were developed, usually for specific purposes. These include the Dumaresq (John Dumaresq, UK, 1902); Arthur Pollen’s fire-control system (UK, 1912); the differential analyzer (H.L. Hazen and Vannevar Bush/MIT, US, 1927); the FERMIAC (Enrico Fermi, Italy/US, 1947); MONIAC (US, 1949); Project Cyclone (Reeves, US, 1950); Project Typhoon (RCA, US, 1952); and the AKAT-1 (Jacek Karpiński, Poland, 1959).  In 1909, Percy Ludgate, of Ireland, apparently unaware of Babbage’s work, independently designed a programmable mechanical computer. In 1936, Alan Turing (UK) published a paper that described the Turing Machine – the theoretical basis for all modern computers.  John von Neumann (Hungary/US) invented a computer architecture based on Turing’s theory.  In a 1937 MIT master’s thesis, Claude Shannon (US) showed how electronic relays and switches can realize the expressions of Boolean algebra. In 1937, George Stibitz (US), of Bell Labs, invented and built the first relay-based calculator to use binary form – the Model K.  Starting in 1936, Konrad Zuse (Germany) built a series of progressively more complex programmable binary computers with memory: the Z1 (1938) never worked reliably, but the Z3 (May 1941) is considered by some the first working programmable fully automatic modern computer that meets the criteria for Alan Turing’s “universal machine.”  In 1939, John V. Atanasoff and Clifford E. Berry (US) at Iowa State created the Atanasoff-Berry Computer, which was electronic and digital but not programmable. In 1940, George Stibitz and his team produced and demonstrated their Complex Number Calculator. In 1943, Max Newman, Tommy Flowers and others (UK) built the Mk I Colossus, a computer designed to break the German encryption system, building on 1941 work by Britons Turing and Gordon Welchman (who in turn built on 1938 work by Marian Rejewski, of Poland). Some consider Colossus to be the world’s first electronic programmable computing device. The improved Mk II Colossus followed in 1944. Also in 1944, the Harvard Mark I began operation, after being built at IBM’s Endicott labs by a team headed by Howard Aiken, starting in 1939. Beginning in 1943, the U.S. Government sponsored the development of ENIAC under the lead of John Mauchly and J. Presper Eckert (US) at the University of Pennsylvania. When it began operating at the end of 1945, ENIAC met all of Alan Turing’s criteria for a true computer. Also in 1945, Konrad Zuse developed the Z4, which also met Turing’s criteria. Improvements to ENIAC in 1948 made it possible to execute stored programs set in function table memory.  Frederic C. Williams, Tom Kilburn and Geoff Tootill (UK) at Victoria University of Manchester built the Manchester Small-Scale Experimental Machine, or “Baby” in 1948, the first stored-program computer. Baby led to the Manchester Mark 1, which became operational in 1949. The Mark 1, in turn, led to the first commercial computer, the Ferranti Mark 1, in 1951.  Maurice Wilkes (UK) at Cambridge developed the EDSAC in 1949.  Not to be outdone, Australians Trevor Pearcey and Maston Beard built CSIRAC in 1949. Another commercial computer was the LEO I, made by J. Lyons & Co. (UK) in 1951.  Also in 1951, the U.S. Census Bureau purchased a UNIVAC I (essentially a variation of ENIAC using a new metal magnetic tape) from Remington Rand. After years of delays, EDVAC, Eckert and Mauchly’s follow-up to ENIAC, began operations in 1951 at the Ballistics Research Lab.  In 1952, IBM began marketing the 701, its first mainframe computer. In 1954, IBM released the IBM 650, a smaller, more affordable computer. Maurice Wilkes (UK) invented microprogramming in 1955.  In 1956, IBM introduced the first hard disk drive – it could store five megabytes of data.  Beginning about 1953, transistors began replacing vacuum tubes in computers. The invention of the integrated circuit, or microchip, led to the invention of the microprocessor in the late 1960s.

J. Presper Eckert (1919-1995) (left) and John Mauchly (1907-1980) with the ENIAC computer in the 1940s.

6
WHEELED VEHICLES (c. 4000-3500 BCE)
Civilizations in Mesopotamia, the Indus Valley, the Northern Caucasus and Central Europe all invented vehicles with wheels of solid wood between 4000 and 3500 BCE. The earliest clear depiction of a wheeled vehicle was found in Poland and dates to 3500-3350 BCE. The oldest surviving wheel was found in the Ljubljana Marshes in Slovenia and dates to approximately 3250 BCE. Wheeled vehicles are found in the Indus Valley by 3000-2000 BCE. The spoke-wheeled chariot was invented in Russia and Kazakhstan some time between 2200 and 1550 BCE, and reached China and Scandanavia by 1200 BCE.  Wire wheels and pneumatic tires were invented in the mid-19th Century.

These wooden fragments from 3000 BCE are thought to be the oldest existing remains of a wheel and axle.

EUCLIDIAN GEOMETRY (300 BCE)
Greek mathematician Euclid, who lived in Alexandria, Egypt, published his Elements in about 300 BCE, setting out the fundamentals of what is now called Euclidean geometry. Many of the axioms, postulates and proofs in the Elements were originally discovered by others, but Euclid fit them all into a single comprehensive system. After Euclid, Archimedes (3rd Century BCE) developed equations for volumes and areas of various figures and Apollonius of Perga (late 3rd Century-early 2nd Century BCE) investigated conic sections. In the 17^th Century, René Descartes and Pierre de Fermat (France) developed analytic geometry, an alternative method that focused on turning geometry into algebra. Also in the 17^th Century, Girard Desargues (France) invented projective geometry.

This fragment of a copy of Euclid’s Elements dating to c. 100 CE was found at Oxyrhynchus in Egypt.

PAPER (200-100 BCE)
Although the invention of paper is traditionally attributed to Ts’ai Lun (China) in 105 CE, strong evidence indicates that the pulp process was developed in China some time earlier in the 2nd Century BCE, during the Han Dynasty. The first recipe may have included tree bark, cloth rags, hemp and fishing nets. The earliest use of paper was to wrap and pad delicate objects such as mirrors. The use of paper for writing is first seen in the 3rd Century CE. Paper was used as toilet tissue from at least the 6th Century CE. In the Tang Dynasty (618-907 CE), paper was used to make tea bags, paper cups and paper napkins. In the Song Dynasty (960-1279 CE), paper was used to make bank notes, or currency. Paper was introduced into Japan between 280 and 610 CE. In America, the Mayans developed a type of paper called amatl, made from tree bark, beginning in 5th Century CE. The Islamic world obtained the secret of papermaking by the 6th Century CE, when it was being made in Pakistan. The knowledge had spread to Baghdad by 793 CE, to Egypt by 900 CE and to Morocco by 1100 CE. In Baghdad, an inventor discovered a way to make thicker sheets of paper, a crucial development. The first water-powered pulp mills were built in 8th Century Samarkand (modern-day Uzbekistan). In 1035, a traveler noted that Cairo market sellers were wrapping customers’ purchases in paper. The first European papermaking occurred in Toledo, Spain in 1085 CE. The first French paper mill was established by 1190 CE. Arab merchants introduced paper into India in the 13th Century. The first definitive reference to a water-powered paper mill in Europe is from 1282 in Spain. Paper was expensive to make until after 1844, when Charles Fenerty (Canada) and F.G. Keller (Germany) independently developed processes for using wood pulp to make paper, instead of recycled fibers.

These scraps of hemp paper, made in China about 100 BCE, were used for wrapping.

THE MECHANICAL CLOCK (976 CE)
Prior to the invention of the mechanical clock, humans kept time using sundials (a type of shadow clock), hourglasses, water clocks and candle clocks. Chinese inventors improved on the water clock by adding escapements.  Liang Lingzan and Yi Xing designed and built a mechanized water clock with the first known escapement mechanism in 725 CE. Islamic scientists had also made improvements on the water clock, including a clock given as a gift to Charlemagne in 797 CE by Harun al-Rashid of Baghdad.  In 976 CE, Zhang Sixun (China) was the first to replace the water in his clock tower with mercury.  In 1000 CE, Pope Sylvester brought water clocks to Europe.  In 1088, Su Song (China) further improved on Zhang’s design in his astronomical clock tower, nicknamed ‘Cosmic Engine.’ The first geared water clock was invented by Arab engineer Ibn Khalaf al-Muradi in Spain in the 11th Century. There is some evidence of mechanical clocks that used falling weights instead of water in France in 1176 and England in 1198. Al-Jazari (Mesopotamia) built numerous clocks in the early 13th Century; there is evidence of an Arabic mechanical clock in a 1277 Spanish book. There is also evidence of mechanical clocks in England in 1283 and 1292, as well as Italy and France. The oldest surviving mechanical clock is at Salisbury Cathedral (UK) and dates to 1386.  Spring-driven clocks first appeared in the 15th Century. Clocks indicating minutes and seconds also begin to appear in the 15th Century. Jost Bürgi (Switzerland) invented the cross-beat escapement in 1584. Around the same time, the first alarm clocks were invented. The first pendulum clock was invented by Christiaan Huygens (The Netherlands) in 1656. A pendulum clock uses a weight that swings back and forth in a precise time interval, thus making this type of clock much more precise than previous designs. Galileo Galilei (Italy) had been exploring the properties of pendulums since 1602 and he designed a pendulum clock in 1637, but died without completing it.  With the assistance of clockmaker Salomon Coster (The Netherlands), Huygens designed and built a pendulum clock that realized Galileo’s dream.

An illustration from a book by Su Song (1021-1101) showing his 1088 Cosmic Engine clock tower.

EARTH’S MAGNETIC FIELD (1600)
The Chinese were aware by 1 CE that a magnet will align with north and south directions. About 200 CE, Chinese scientists discovered that magnetic north and true north were different. In the 16^th Century, Georg Hartmann (Germany) and Robert Norman (England) independently discovered magnetic inclination, the angle between the magnetic field and the horizontal. In 1600, William Gilbert (England) published the results of his experiments using a small model of Earth, which led to his discovery that the Earth is a giant magnet, thus explaining why compasses point north. He also predicted accurately that the Earth has an iron core. Carl Friedrich Gauss (Germany) was the first to measure the Earth’s magnetic field in 1835. The true cause of the magnetic field was only discovered in the 20^th Century, after the dominant theory – that the Earth is made of magnetic rocks – was disproved. In 1919, Sir Joseph Larmor (UK) proposed that a self-exciting dynamo could be the mechanism. W.M. Elsasser and Edward Bullard (UK) showed in the 1940s that the motion of a liquid core could produce a self-sustaining magnetic field.

The title page from the 1600 book De Magnete, by William Gilbert (1544-1603), in which it was first proposed that the Earth had a magnetic field.

THE CALCULATOR (1642)
The invention of logarithms by John Napier (Scotland) in 1614 made multiplying easier and thus made calculators practical. In 1632, William Oughtred (England) invented the slide rule. The first mechanical calculator, the Pascaline, was invented by Blaise Pascal (France) in 1642. Gottfried Leibniz (Germany) made a multiplication machine in 1671, but it did not improve on Pascal’s. Several machines were made in the 18th Century, including that of Poleni (Italy). The first commercial mechanical calculator was the Arithmometer of Thomas de Colmar (France), which was invented in 1820 but not marketed until 1851. Charles Babbage (UK) invented the difference machine in 1822 and a calculating machine in 1834-1835, which were programmable and precursors to the computer but were never built.  Americans Frank S. Baldwin, Jay R. Monroe, and W. T. Ohdner all produced calculators in the second half of the 19^th Century. Other machines included the 1886 calculating machine of William Seward Burroughs (US), the comptometer of Dorr E. Felt (US) from 1887, and Swiss inventor Otto Steiger’s “Millionaire” in 1894.  James Dalton (US) introduced the Dalton Adding Machine in 1902, the first with push buttons.  The Curta calculator, invented by Curt Herzstark (Austria) in 1948, was the last popular mechanical calculator. Casio (Japan) introduced the first all-electric calculator, the Model 14-A, in 1957, which was built into a desk. British Bell announced its all-electronic desktop calculators – the ANITA Mk VII and Mk VIII – in 1961. The ANITAs were among the last to use vacuum tubes.  The 1963 Friden EC-130 (US) used transistors. In 1964, Sharp (US) produced the CS-10A and Industria Macchine Elettroniche (Italy) announced the IME 84. Similar models followed from these and other companies, including Canon, Olivetti, SCM, Sony, Toshiba and Wang.  The next development was the hand-held pocket calculator. In 1967, Jack Kilby, Jerry Merryman and James Van Tassel (US) at Texas Instruments made a prototype of the Cal Tech, although it was still too large to fit in a pocket.  In the 1970s, manufacturers reduced size by switching from transistors to integrated circuits. The first microchip pocket calculators were the Sanyo Mini Calculator, the Canon Pocketronic (based on Kilby’s Cal Tech) and the Sharp micro Compet, all in 1970. Sharp brought out the EL-8 in 1971.  Mostek (US) made the MK6010 the same year.  Also in 1971, Pico Electronics and General Instrument collaborated on the Monroe Digital III, a single chip calculator.  Busicom (Japan) made the first truly pocket-sized calculator, the 1971 LE-120A “Handy”, at 4.9 X 2.8 X 0.9 inches.  The first US pocket-sized device was the Bowmar Brain from late 1971.

The Pascaline, from 1652, a calculator invented by Blaise Pascal (1623-1662).

BIOLOGICAL CLASSIFICATION SYSTEM (1735)
Aristotle (Ancient Greece) was the first to systematically classify living things into categories in the 4^th Century BCE; he introduced the concepts of genus and species.  In 1552, Konrad Gesner (Switzerland) developed a system that distinguished genus from species and order from class. Other early classification systems were developed by Andrea Caesalpino (Italy) in 1583, John Ray (UK) in 1686, Augustus Quirinus Rivinus (Germany) in 1690, and Joseph Pitton de Tournefort (France) in 1694. Beginning in 1735, Carolus Linnaeus (Sweden) developed the modern system of taxonomy for living organisms by establishing three kingdoms divided into classes, orders, families, genera and species.  Classification was based on physical characteristics, often the sexual organs. He also adopted the binomial system of naming by using the genus and species name.  Since the 1960s, biologists have adapted Linnaean taxonomy to include evolutionary relationships, looking at the DNA of the organism rather than relying on physical characteristics only.

A 1775 portrait of Carl Linnaeus (1707-1778) by Alexander Roslin. It is now in Gripsholm Castle in Sweden.

THE SPINNING JENNY (1764)
The spinning jenny is a multi-spindle spinning frame that was invented by James Hargreaves (England) in 1764. Dependent on the recently-invented flying shuttle, the spinning jenny held more than one ball of yarn so it could make more yarn in a shorter time, thus reducing cost and increasing productivity. The technology was replaced in about 1810 by the spinning mule.

A spinning jenny at Belper North Mill in Derbyshire, UK.

EVOLUTION (1809)
Although the notion that biological organisms change over time had ancient roots, Jean-Baptiste Lamarck (France) proposed the first fully-developed theory of evolution, or transmutation of species, in his Zoological Philosophy in 1809. Early formulations of the idea of evolution come from Epicurus (Ancient Greece) in the 3^rd Century BCE; Lucretius (Ancient Rome) in the 1^st Century BCE; Augustine of Hippo (Ancient Rome/Algeria) in the 4^th Century CE; and Ibn Khaldun (Tunisia) in 1377. More sophisticated concepts of evolution, with or without divine intervention, came from Gottfried Leibniz (Germany) in the early 18^th Century; Benoît de Maillet (France) in 1748; and Pierre Louis Maupertuis (France) in 1751. Charles Bonnet (Switzerland) first used the term evolution to refer to species development in 1762. Between 1749 and 1788, G.L.L. Buffon (France) suggested that each species is just a well-marked variety that was modified from an original form by environmental factors. In 1753, Denis Diderot (France) wrote that species were always changing through a constant process of experiment where new forms arose and survived or not based on trial and error. James Burnett, Lord Monboddo (England), suggested between 1767 and 1792 that man had descended from apes and that organisms had transformed their characteristics over long periods of time in response to their environments. In 1796, Charles Darwin’s grandfather, Erasmus Darwin, published Zoönomia, which proposed that “all warm-blooded animals have arisen from one living filament”, a theme he developed in his 1802 poem Temple of Nature. The mechanism for evolution was a source of much controversy. Lamarck proposed that organisms acquired new characteristics during their lifespans (such as longer necks from stretching to reach food on trees), which they then passed down to their offspring. He also believed in spontaneous generation of species. Many scientists rejected these ideas.  Prominent evolutionists in the years after Lamarck included Étienne Geoffroy Saint-Hilaire (France), Robert Grant (UK) (whose pupils included a young Charles Darwin), Robert Jameson (UK), and Robert Chambers (UK), whose anonymous Vestiges of the Natural History of Creation proposed that evolution was progressively leading to better and better organisms. It was not until 1858 that Charles Darwin and Alfred Russell Wallace provided a convincing mechanism for evolution: natural selection. In the years after Darwin, developments in genetics, molecular biology and paleontology have brought about many changes to the field now known as evolutionary biology.

Jean-Baptiste Lamarck (1744-1829).

THE SECOND LAW OF THERMODYNAMICS (1824)
The second law of thermodynamics states that the entropy of an isolated system never decreases, because isolated systems always evolve toward thermodynamic equilibrium, which is a state with maximum entropy. The earliest statement of the law was by Sadi Carnot (France) in 1824, who, while studying steam engines, postulated that no reversible processes exist in nature. Beginning in 1850, Rudolph Clausius (Germany) set out the first and second laws of thermodynamics, although it is his 1854 formulation that was most highly regarded: “Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.” William Thomson, Lord Kelvin (UK) reformulated the second law in 1851 as: “It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects.”

A graphic description of the Second Law of Thermodynamics.

THE ELECTRIC GENERATOR & THE DYNAMO (1831)
Michael Faraday (UK) created the first disk generator in 1831. Hippolyte Pixii (France) made the first alternating current generator in 1832 and the first oscillating direct current generator in 1833. Charles Wheatstone (UK) created a magneto-electric generator in 1840. Anyos Jedlik (Hungary) created electromagnetic rotating devices between 1852-1854. Werner von Siemens (Germany) made a generator with double-T armature and slots windings in 1856. Wheatstone, von Siemens and Samuel Alfred Varley (UK) independently invented the dynamo-electric machine (dynamo) in 1866-1867. Zénobe Gramme (Belgium) made the first anchor ring motor in 1871. J.E.H. Gordon (UK) invented an alternating current generator in 1882. William Stanley, Jr. (US) of Westinghouse Electric demonstrated an alternating current generator in 1886. In 1891, Sebastian Ziani de Ferranti and Lord Kelvin (UK) invented the Ferranti-Thompson alternator. Also in 1891, Nikola Tesla (Serbia/US) patented a high-frequency alternator.

A drawing of Michael Faraday’s original disk generator. The horseshoe-shaped magnet (A) created a magnetic field through the disk (D). The turning of the disk induced an electric current, which traveled radially from the center toward the rim. The current then flowed through the sliding spring contact (m), through the external circuit, and back into the center of the disk through the axle.

THE ELECTRIC LIGHT (1878-1880)
Humphry Davy (UK) invented an arc lamp in 1801 but it was not very bright and did not last very long.  James Bowman Lindsay (Scotland) invented an incandescent electric light in 1835 but failed to pursue it.  Others who produced light bulbs were: Warren de la Rue (UK) in 1840; Frederick de Moleyns (UK) in 1841; John W. Starr (US) in 1845; Jean Eugène Robert-Houdin (France) in 1851; Joseph Swan (UK) in 1860; Alexander Lodygin (Russia) in 1872; and Henry Woodward and Mathew Evans (Canada) in 1874. A.E. Becquerel (France) invented a fluorescent lamp in 1867. In 1878, Joseph Swan and Charles Stearn (UK) developed an effective light bulb using a carbon rod from an arc lamp, but it was not commercially viable due to the high current required and short lifetime. Swan switched to a carbon filament by 1880 and began installing light bulbs in British homes. Thomas Edison (US) began experimenting with light bulbs in 1878 and tested a long-lasting carbon filament bulb in 1879. He began installing light bulbs in 1880. Lewis Latimer, an Edison employee, made further improvements on the Edison bulb between 1880 and 1882. Meanwhile Hiram Maxim and William Sawyer (US) set up a competitor to Edison. In 1897, Walther Nernst (Germany) made an incandescent bulb that did not require a vacuum. Carl Auer von Welsbach (Austria) made the first commercial metal filament lamp in 1898. Frank Poor (US) also made improvements in 1901. In 1903, Willis Whitney (US) made a metal-coated carbon filament that did not blacken the bulb.  In 1915 Irving Langmuir invented a tungsten filament.  Peter Cooper Hewitt (US) made the first mercury vapor lamp in 1903 and Georges Claude (France) invented the neon light bulb in 1911.

Light bulbs from 1878-1880 from Joseph Swan (1828-1914) (left) and Thomas Edison (1847-1931).

RADIO (1895)
James Clerk Maxwell (Scotland) established the mathematical basis for propagating electromagnetic waves through space in a paper published in 1873.  David E. Hughes (Wales/US) was probably the first to intentionally send a radio signal through space in 1879 using his spark-gap transmitter, although the achievement was misunderstood at the time. In 1880, Alexander Graham Bell and Charles Sumner (US) invented the photophone, a wireless telephone that transmitted sound on a beam of light.  In 1885, Thomas Edison (US) invented a method of electric wireless communication between ships at sea.  In 1886, Heinrich Hertz (Germany) conclusively demonstrated the transmission of electromagnetic waves through space to a receiver.  Édouard Branly (France) improved the receiver device in 1890.  In 1892, Nikola Tesla (Serbia/US) invented the Tesla coil, which generated alternating current electricity; in 1893 Tesla developed a wireless lighting device and in 1898 he demonstrated a remote controlled boat.  In 1894, Sir Oliver Lodge (UK) improved Branly’s receiver, calling it a coherer, and demonstrated a radio transmission in 1894.  In the same year, Lodge showed the reception of Morse code signals by a wireless receiver.  Also in 1894, Jagadish Chandra Bose (India) demonstrated transmission of radio waves over distance; Bose developed an improved transmitter and receiver in 1899.  Guglielmo Marconi (Italy/UK) read Lodge’s and Tesla’s papers in 1894 and built his first radio devices in early 1895.  By the end of 1895, he had developed a device that could transmit radio waves 1.5 miles. In 1896, Marconi moved to England, where he presented his device to Sir William Preece at the British Telegraph Service. By 1897, Marconi had patented his device and started his own wireless business, which established radio stations at various locations.  In 1899, Marconi sent radio waves across the English Channel; he sent the first transatlantic message, possibly as early as 1901.  Alexander Popov (Russia) built and demonstrated improved versions of both the transmitter and receiver, first in May 1895 for a scientific group and then a public display in March 1896.  There is some evidence that Popov set up a radio transmitter with two-way communication between a naval base and a battleship in 1900. Beginning in 1899, Ferdinand Braun (Germany) made significant improvements to the design of wireless devices, including inventing the closed circuit system and increasing the distance the signals would carry. Roberto Landell de Moura, a Brazilian priest and scientist, invented a radio in 1900 that could transmit a distance of eight kilometers. In 1904, Sir John Fleming (UK) invented the vacuum electron tube, which became the basis for radio telephony. Lee de Forest (US) invented the triode amplifying tube in 1906.  In 1912, Edwin H. Armstrong (US) invented the regenerative circuit, which allowed long-distance sound reception.  Armstrong also discovered frequency modulation, or FM radio, in 1933.

A photograph of Guglielmo Marconi (1874-1937) with his radio shortly after his 1896 arrival in England.

THE EARTH’S ATMOSPHERE (1902)
The Earth’s atmosphere is made up of the following layers: (1) troposphere (0 to 7 miles) (the top of the troposphere is called the tropopause); (2) stratosphere (7-31 miles); (3) mesophere (31-50 miles); (4) thermosphere (50-440 miles) and (5) exosphere (440 miles and up). The ozone layer is located in the stratosphere, usually between 9.3-21.7 miles.  The ionosphere includes the mesosphere, the thermosphere and part of the exosphere (31-621 miles). In 1902, Léon Philippe Teisserenc de Bort (France) and German scientist Richard Assmann both discovered independently the atmosphere is divided into troposphere and stratosphere. Oliver Heaviside (UK) proposed the existence of the conducting layer known as the ionosphere, in 1902. Also in 1902, Arthur Edwin Kennelly (Ireland/US) discovered some of the radio and electrical properties of the ionosphere. Robert Watson-Watt (UK) coined the term ‘ionosphere’ in 1926.  Edward V. Appleton (UK) experimentally confirmed the existence of the ionosphere in 1927. Lloyd Berkner (US) measured the ionosphere’s height and density in the 1950s. Charles Fabry and Henri Buisson (France) discovered the ozone layer in 1913.  G.M.B. Dobson (UK) studied the ozone layer and set up a worldwide network of ozone monitoring stations between 1928 and 1958.
A diagram of the Earth’s atmosphere.

THE AIRPLANE (1903)
Humans have tried to fly since ancient times.  Abbas Ibn Firnas (Berber/Andalusia) built a glider in the 9th Century; Eilmer of Malmesbury (UK) tried it in the 11th Century; and Leonardo da Vinci (Italy) designed a man-powered aircraft in 1502.  Sir George Cayley (UK) designed fixed-wing airplanes from 1799 and built models from 1803.  He built a successful glider in 1853.  In 1856, Jean-Marie Le Bris (France) took the first powered flight when a horse pulled his glider, the Albatross, across a beach.  John J. Montgomery (US) made a controlled flight in a glider in 1883, as did Otto Lilienthal (Germany), Percy Pilcher (UK) and Octave Chanute (France/US) about the same time. Between 1867 and 1896, Lilienthal made numerous heavier-than-air glider flights.  Clément Ader (France) built a steam-powered airplane in 1890 and may have flown 50 meters in it.  Hiram Maxim (US/UK) built an airplane powered by steam engines in 1894 that had enough lift to fly, but was uncontrollable and never actually flew. Lawrence Hargrave (Australia) experimented with box kites and rotary aircraft engines in the 1890s. In 1896, American Samuel Pierpont Langley’s Aerodrome No. 5 made the first successful sustained flight of an unmanned, engine-driven heavier-than-air craft, but his attempts at manned flight in 1903 did not succeed. There is some evidence that Gustave Whitehead (Germany/US) flew his Number 21 powered monoplane at Fairfield, Connecticut (US) in 1901, two and a half years before the Wright Brothers, but the matter is subject to debate.  Most believe that Orville and Wilbur Wright (US) accomplished “the first sustained and controlled heavier-than-air powered flight” (FAI) on December 17, 1903 at Kill Devil Hills, North Carolina.  By 1905, the third version of the Wright Brothers’ airplane was capable of fully controllable, stable flight for substantial periods.  Traian Vuia (France) flew in a self-designed, fully self-propelled, fixed wing aircraft with a wheeled undercarriage in 1906.  Jacob Ellehammer (Denmark) also flew a monoplane in 1906. In 1906, Alberto Santos Dumont (Brazil) flew 220 meters in less than 22 seconds, without the assistance of a catapult. In 1908-1910, Dumont designed a number of Demoiselle airplanes that were well received. In 1908 and 1909, Louis Blériot (France) designed airplanes that were improvements over earlier models.  The first jet aircraft was the German Heinkel He 178, first tested in 1939, followed by the Messerschmitt Me 262 in 1943. The first aircraft to break the sound barrier was the Bell X-1, in 1947. The first jet airliner was the de Havilland Comet, introduced in 1952. The first widely successful commercial jet was the Boeing 707, which arrived in 1958. The Boeing 747 was the largest passenger jet from 1970 until 2005, when it was surpassed by the Airbus A380.

Orville Wright (1871-1948) observes as his brother Wilbur Wright (1867-1912) pilots an airplane in the first powered flight on December 17, 1903 at Kill Devil Hills in North Carolina.  Photograph by John T. Daniels.

SUPERCONDUCTIVITY (1911)
Superconductivity is a phenomenon in which certain materials experience zero electrical resistance and expulsion of magnetic fields when cooled below a critical temperature. Heike Kamerlingh Onnes (The Netherlands) discovered lack of electrical resistance in liquid helium in 1911. In 1933, Fritz Walther Meissner and Robert Ochsenfeld (Germany) discovered that substances undergoing superconductivity expelled their magnetic fields, which became known as the Meissner Effect. In 1935, Fritz and Heinz London (Germany) developed a mathematical explanation for superconductivity. Lev Landau and Vitaly Ginzburg (USSR) proposed a phenomenological theory of superconductivity in 1950. John Bardeen, Leon Cooper and John Scheiffer (US) developed a complete microscopic theory of superconductivity (the BCS theory) in 1957. The Landau-Ginzburg and BCS models were reconciled through the work of N.N. Bogolyubov (USSR) in 1958 and Lev Gor’kov (USSR) in 1959.

Heike Kamerlingh Onnes (1853-1926).

VITAMINS (1912)
That certain diseases were caused by the lack of particular nutrients was known by the Ancient Egyptians. In 1747, Scottish physician James Lind discovered that citrus fruits prevented scurvy. Deprivation experiments allowed late 18^th and early 19^th Century scientists to identify a lipid from fish oil, then called ‘antirachitic A’ (later identified as vitamin D), that cured rickets. In a series of experiments with mice in 1881, Nikolai Lunin (Russia) found that an unidentified natural component of milk prevented scurvy. Takaki Kanehiro (Japan) performed an experiment on Japanese naval crews showing that a diet of only white rice lacked a nutrient that prevented beriberi. In 1897, Christiaan Eijkman (The Netherlands) showed that a diet of unpolished white rice led to beriberi in chickens, while polished rice prevented it. In 1907, Norwegian physicians Axel Holst and Theodor Frølich conducted a series of experiments with guinea pigs that set the stage for the discovery of ascorbic acid, or vitamin C. In 1910, Umetaro Suzuki (Japan) became the first scientist to isolate a vitamin complex, which he called aberic acid (later Orizanin and ultimately identified as vitamin B1, or thiamin) but the discovery received little attention. In 1912, Frederick Hopkins (UK) conducted a series of experiments that led him to the conclusion that some foods contained what he called ‘accessory factors’ that were necessary for functioning. Casimir Funk (Poland) independently repeated Suzuki’s results in 1912, calling the micronutrients a “vitamine” for vital amines, although the name was shortened to vitamin when it became clear that not all vitamins were amines. Elmer V. McCollum and M. Davis (US), discovered vitamin A in 1912–1914. McCollum also discovered vitamin B in 1915-1916. Sir Edward Mellanby (US) discovered vitamin D in 1920; McCollum also isolated vitamin D in 1922. Also in 1922 Sir Herbert McLean Evans (US) discovered vitamin E. D.T. Smith and E.G. Hendrick (US) discovered vitamin B2 (riboflavin) in 1926. Henrik Dam (Denmark) and Edward Adelbert Doisy (US) discovered vitamin K in 1929. Paul Karrer (Switzerland) determined the structure for beta-carotene, the precursor of vitamin A, in 1930. Between 1928 and 1932, a Hungarian team led by Albert Szent-Györgyi and Joseph L. Svirbely, and an American team led by Charles Glen King, first identified and isolated vitamin C (ascorbic acid). The discovery was confirmed by Karrer and Norman Haworth (UK). Vitamin C was the first vitamin to be synthesized in the laboratory, by Haworth and Edmund Hirst in 1933-1934, and independently by Tadeus Reichstein (Poland) in 1933.

Frederick Gowland Hopkins (1861-1947).

CHEMICAL BONDING (1916)
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electrostatic force of attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction. In 1704, Sir Isaac Newton (England) proposed that “particles attract one another by some force, which in immediate contact is exceedingly strong, at small distances performs the chemical operations, and reaches not far from the particles with any sensible effect.”  In 1801, Jöns Jakob Berzelius (Sweden) developed a theory of chemical bonding that emphasized the electronegative and electropositive character of the combining atoms. By the mid-19th century, Edward Frankland (UK), F.A. Kekulé (Germany), A.S. Couper (UK), Alexander Butlerov (Russia), and Hermann Kolbe (Germany), developed the theory of valency (originally called ‘combining power’), which held that compounds joined due to an attraction of positive and negative poles. In 1916, American chemist Gilbert N. Lewis developed the modern concept of the electron-pair bond, in which two atoms may share one to six electrons, thus forming the single electron bond, a single bond, a double bond, or a triple bond. According to Lewis, “An electron may form a part of the shell of two different atoms and cannot be said to belong to either one exclusively.” Also in 1916, Walther Kossel (Germany) put forward a theory that assumed complete transfers of electrons between atoms, and was thus a model of ionic bonds. Both Lewis and Kossel structured their bonding models on that of Abegg’s rule of 1904. In 1927, Danish physicist Oyvind Burrau was the first to describe a simple chemical bond in mathematically complete quantum terms.  Walter Heitler (Germany) and Fritz London (Germany/US) invented a more practical approach in 1927, which is now called valence bond theory. In 1929, Sir John Lennard-Jones (UK) introduced the linear combination of atomic orbitals molecular orbital method (LCAO) approximation.

Notes of cubic chemical bonds made by Gilbert Lewis (1875-1946) in 1902.

HUMAN EVOLUTION (1925)
Even before publication of Charles Darwin’s The Origin of Species, skulls of Neanderthals, a close relative of modern man, had been discovered in Belgium (1829), Gibraltar (1848) and the Neander Valley in Germany (1856). Eugène Dubois (The Netherlands) discovered a fossil skeleton of “Java Man”, now called Homo erectus, in Java in 1891. It was Australian scientist Raymond Dart’s 1924 discovery (published in 1925) of a fossilized skull of a new species of hominid, Australopithecus africanus, in Taung, South Africa, that convinced many in the scientific community that humans had evolved from other species in Africa. (Dart’s find became known as the Taung Child.)  Subsequent discoveries have included British scientist Louis Leakey’s 1964 discovery of Homo habilis in Tanzania; American Donald Johanson’s discovery of an almost complete skeleton of Australopithecus afarensis, known as “Lucy”, in Ethiopia in 1974; British scientist Mary Leakey’s 1978 discovery of 3.5 million year old fossilized human footprints in Tanzania; and the discovery of a 1.6 million year old Homo erectus skeleton in Kenya in 1984 by Richard Leakey (UK) and Alan Walker (UK). In 1994, Meave Leakey (UK) discovered Australopithecus anamensis, which lived in Kenya and Ethiopia about 4 million years ago. Tim White (US) discovered the 4.2 million year old Ardipithecus ramidus in Ethiopia in 1995. In 2000, Martin Pickford (UK) and Brigitte Senut (France) found a bipedal hominid in Kenya from six million years ago that they named Orrorin tugenensis.  In 2001, Michel Brunet (France) found a skull of a 7.2 million year old bipedal hominid in Chad, which he named Sahelanthropus tchadensis. In addition to the fossil record, since the 1960s, much of the study of human evolution has been conducted through analysis of the DNA of living humans and apes.

Raymond Dart (1893-1988) with the Taung Child skull.

CHEMICAL ORIGINS OF LIFE (1953)
While no one has yet definitively determined how life began, a number of theories have been proposed and at least one famous experiment conducted.  In a famous 1871 letter, Charles Darwin speculated:

It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present.— But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts,—light, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter w^d be instantly devoured, or absorbed, which would not have been the case before living creatures were formed.

In 1922 and 1924, Alexander Oparin (USSR) suggested that life could have arisen from basic organic chemicals in the Earth’s primordial ocean given a strongly reducing atmosphere (methane, ammonia, hydrogen and water vapor) and the forces of natural selection. J.B.S. Haldane (UK) made similar proposals in 1926 and 1929, in which he suggested that an ‘oily film’ would have enclosed self-reproducing molecules, creating the first cells. Both Oparin and Haldane suggested that complex organic molecules might begin to self-reproduce while still inanimate. The first experiment to test the Oparin-Haldane theory was conducted by Stanley Miller and Harold Urey (US) in 1953. They simulated an early Earth atmosphere and ocean by placing liquid water, methane, ammonia and hydrogen in a sealed container with a pair of electrodes.  They heated the water to induce evaporation, and fired sparks between the electrodes to simulate lightning, then cooled the environment to allow the products in the atmosphere to condense.  The result was the production of many organic compounds, including all the amino acids needed to make proteins, and sugars. No nucleic acids were created.  Many others have followed up the experiment. In 1961, Joan Oró (Spain) was able to create a nucleotide base from hydrogen cyanide and ammonia in water.  As scientists have learned more about the early Earth’s atmosphere and other conditions, revised experiments have been conducted.

A diagram of the Miller-Urey experiment.

QUARKS (1964)
Murray Gell-Mann and George Zweig (US) independently proposed the quark model, also known as quantum chromodynamics, in 1964. They suggested that there were three types of quarks (up, down and strange) and that all hadrons (including protons and neutrons) were composed of combinations of quarks and antiquarks. In 1965, Sheldon Lee Glashow and James Bjorken (US) proposed charm, the fourth quark. Experiments by Jerome Friedman, Henry Kendall, and Richard Taylor (US) in 1968 using the Stanford Linear Accelerator eventually revealed the existence of the up, down and strange quarks. In 1973, Makoto Kobayashi and Toshihide Maskawa (Japan) proposed two more quarks: top and bottom. The charm quark was observed in 1974 by Burton Richter and Samuel Ting (US). In 1977, the bottom quark was observed by Leon Lederman (US). A team at Fermilab found the top quark in 1995.

Murray Gell-Mann (1929- ).

A 2011 photograph of George Zweig (1937- ).

GENETIC ENGINEERING & GENETICALLY MODIFIED ORGANISMS (1972)
The modification of living organisms has been part of human culture since the domestication of plants and animals through selective breeding and hybridization, which dates back many thousands of years.  After the discoveries of genetics and the chemistry of DNA, scientists began to learn how to modify and engineer biological organisms through manipulation of their genes.  In 1927, H.J. Muller (US) first used x-rays to create genetic mutations in plants.  Barbara McClintock and Harriet Creighton (US) showed direct physical recombination in DNA in 1931. In 1967, scientists discovered DNA ligases, which could join pieces of DNA together. In the late 1960s, Stewart Linn (US) and Werner Arber (Switzerland) discovered restriction enzymes. In 1970, Hamilton Smith (US) used restriction enzymes to target DNA at a specific location and separate the pieces.  Also in 1970, Morton Mandel and Akiko Higo (US) inserted a bacteriophage virus into the DNA of the E. coli bacteria.  In 1972, Paul Berg (US) created the first recombinant DNA molecules.  Also in 1972, Herbert Boyer and Stanley Cohen (US) inserted recombinant DNA into bacterial cells using a technique called DNA cloning. They then created the first genetically modified organism by inserting a gene for resistance to an antibiotic into bacteria that had no such gene, making the bacteria resistant. Later, they placed a frog gene into a bacterial cell.  In 1973, Rudolf Jaenisch (Germany/US) inserted foreign DNA into a mouse.  In 1974, Cohen, Annie Chang and Herbert Boyer (US) created a genetically modified DNA organism. Beginning in 1976, recombinant DNA research has been subject to regulation in the US. Frederick Sanger (UK) developed a way to sequence DNA in 1977.  In 1979, scientists were able to modify bacteria to produce human insulin.  In 1981, Frank Ruddle (US), Frank Constantini and Elizabeth Lacy (UK) were able to pass new genes into subsequent generations by inserting foreign DNA into a mouse embryo.  In 1983, Michael Bevan, Richard Flavell (UK) and Mary-Dell Chilton (US) inserted new genetic material into a tobacco plant – the first genetically modified plant.  In 1983, Kary Mullis (US) identified the polymerase chain reaction, which amplified small sections of DNA. In 1984, mice were genetically modified to predispose them to cancer. In the late 1980s, electroporation – the use of electricity to make a cell membrane more porous – increased scientists’ ability to insert foreign DNA into cells. In 1989, Mario Capecchi (US), Martin Evans (UK) and Oliver Smithies (UK/US) were the first to manipulate a mouse’s DNA to turn off a gene. After the discovery of microRNA in 1993, Craig Mello and Andrew Fire (US) were able to silence genes in mammalian cells in 2002 and in an entire mouse in 2005. The first of many commercial enterprises featuring genetic engineering was Genentech, founded by Boyer and Robert Swanson (US) in 1976. The release of GMOs into the environment has been a source of controversy and has generated protests around the world.

A diagram of one form of genetic engineering.

THE HUBBLE SPACE TELESCOPE (1990)
The US (with assistance from Europe) launched the Hubble Space Telescope and its 2.4 meter (7.9 ft.) mirror into low Earth orbit in April 1990. After an adjustment to its mirror in 1993, the telescope has been able to observe distant space objects in the ultraviolet, visible and infrared spectra. Its images have helped scientists: (1) determine the rate of expansion of the universe (the Hubble constant); (2) accurately measure the age of the universe; (3) identify that the expansion of the universe in accelerating; (4) locate black holes in the center of galaxies; (5) create deep field images of distant galaxies; (6) understand the nature of the early universe; (7) identify and measure the effects of dark energy; and (8) measure the atmospheres of extrasolar planets. Over 9,000 papers based on Hubble data have been published in peer-reviewed journals.  As of September 2014, the Hubble Space Telescope was still operating.

A photo of the Hubble Space Telescope taken by the crew of the Space Shuttle Columbia in 2002.

5
ATOMIC THEORY (ATOMISM) (c. 500 BCE)
The idea that all matter is composed of tiny particles called atoms, known as the atomic theory, or atomism, was proposed in India by Jain philosophers of the Ajivika and Carvaka schools in the 6^th Century BCE. Ancient Greek philosophers Leucippus and Democritus advocated atomism in c. 500 BCE. Epicurus adopted a form of atomism in the 3^rd Century BCE, and his ideas were promoted by Roman philosopher-poet Lucretius in the 1^st Century BCE. In the 2^nd Century BCE, Kanada (India) founder of the Vaisheshika philosophy, held that that the world was composed of atoms, but that there were different kinds of atoms for each element, while the Jains and the Greeks believed that all atoms were alike. It is not clear if the Indian and Greek atomistic philosophies developed independently or whether one influenced the other.

A bust of Democritus (460-370 BCE).

GUNPOWDER (800-900 CE)
Historians believe that Chinese alchemists invented gunpowder in the 9th Century CE while they were looking for a chemical that would make them immortal. They soon found out the explosive potential for their discovery and it was used in creating many weapons, including fireworks (10th Century CE), flamethrowers (1000) and bombs (1220).  The Chinese had perfected the recipe by the mid-14th Century.  The Mongols learned about gunpowder when they conquered China in the mid-13th Century and spread it throughout the world during their subsequent invasions.  The Arab world obtained gunpowder in the mid-13th Century. The Mamluks used cannons against the Mongols in 1260. In 1270, Syrian chemist Hasan al-Rammah described a method for purifying saltpeter in making gunpowder. Europeans first saw gunpowder used by the Mongols at the Battle of Mohi in 1241.  Roger Bacon (UK) referred to gunpowder in a 1267 book.  The first known use of gunpowder used by Europeans in battle was during the 1262 siege of the Spanish city of Niebla by Castilian King Alfonso X.  By 1350, cannons were a common sight in European wars.  India had gunpowder technology from at least 1366 CE, if not earlier. In the late 14th Century, European powdermakers began adding liquid and ‘corning’ the powder, which improved performance significantly.

A 14th Century illustration of a phalanx-charging fire-gourd, a type of Chinese fire lance that was powered by gunpowder.

THE SCIENTIFIC METHOD (1267)
The scientific method is the set of techniques and principles used in investigating phenomena, obtaining new knowledge and correcting or assimilating prior knowledge.  The scientific method is based on empirical and measurable evidence and rests on certain rational principles. According to the Oxford English Dictionary, the scientific method involves “systematic observation, measurement and experiment, and the formulation, testing and modification of hypotheses.” The scientific method contrasts with the very influential method proposed by Aristotle of reasoning from first principles. Muslim scientists such as Jabir ibn Hayyan (721-815 CE) and Alkindus (801-873 CE) were among the first to use experiment and quantification to test theories. Use of the scientific method is clear in Arab Iraqi scientist Ibn al-Haytham’s Book of Optics (1021) and Persian scholar Kamal al-Din al-Farisi’s early 14th Century revision of the Optics. Abu Rayhan al-Biruni (Persia) used a quantitative scientific method in studying mineralogy, sociology and mechanics in the 1020s and 1030s. Persian scientist and physician Ibn Sina (Avicenna) set out a method using hypotheses in The Book of Healing, from 1027. In the 1220s, Robert Grosseteste (England) published a commentary of Aristotle’s Posterior Analytics in which he set out some aspects of the scientific method, including (1) take particular observations to create a universal law, and then use the universal law to predict particular observations; and (2) verify scientific principles through experimentation. Roger Bacon (England) followed up on Grosseteste’s work in his 1267 Opus Majus, which systematically set out the principles of the scientific method. The scientific method’s next champion did not arise for almost 400 years.  Francis Bacon (England) sought to overturn the Aristotelian methods used in science education and practice by focusing on inductive reasoning and experimentation, especially in his Novum Organum of 1620, which reintroduced the scientific method to the modern world. In the first half of the 17^th Century, Galileo Galilei (Italy) promoted the scientific method in the face of Aristotelianism, by using observation, experiment, and inductive reasoning, and by changing his views based on the empirical findings. René Descartes (France) provided philosophical premises for the scientific method in 1637. In 1687, Sir Isaac Newton (England) set out four rules of reasoning in science that embodied principles of the new scientific method. After philosopher David Hume (Scotland) attacked inductive reasoning beginning in 1738, scientists and philosophers sought to rehabilitate scientific knowledge. These included Hans Christian Ørsted (Denmark) in 1811, John Herschel (UK) in 1831, William Whewell (UK) in 1837 and 1840, and John Stuart Mill (UK) in 1843, and William Stanley Jevons (UK) in 1873 and 1877. Claude Bernard (France) applied the scientific method to medicine in 1865. Charles Sanders Peirce (US) articulated the modern scheme for testing hypotheses and the importance of statistical knowledge in science in 1878. Karl Popper proposed a revision of the scientific method in 1934 by stating that a scientific hypothesis must be falsifiable. Not all scientists agreed with Popper, including Thomas Kuhn, in 1962, who noted that different scientists worked differently and falsifiability was not a methodology that scientists actually follow.

A statue of Roger Bacon (1219-1294) at the Oxford University Museum of Natural History, UK.

THE TELESCOPE (1608)
The first telescopes were known as refractors because they used lenses to collect and magnify light. The earliest versions were made in 1608 by Hans Lippershey, Zacharias Jansen and Jacob Metius (The Netherlands). Galileo Galilei (Italy) built a series of improved refractor telescopes beginning in 1609. In 1655, Christiaan Huygens (The Netherlands) developed a compound eyepiece refractor based on a theory by Johannes Kepler (Germany). In 1668, Isaac Newton (England) invented the first reflector telescope, which used a mirror instead of a lens to collect light. Laurent Cassegrain (France) improved on the reflector in 1672. Further improvements were made throughout the 18th Century.

Sir Isaac Newton’s first reflecting telescope. Photograph by Peter Macdiarmid/Getty Images.

ATMOSPHERIC PRESSURE & THE BAROMETER (1643)
Atmospheric pressure, also known as air pressure, is the force exerted on a surface by the weight of the air above that surface in the atmosphere of the Earth. A barometer measures atmospheric pressure, which can forecast short-term changes in the weather. Evangelista Torricelli (Italy), a student of Galileo’s, discovered atmospheric pressure and invented the mercury barometer in 1643. Torricelli built on previous discoveries. In 1630, Giovanni Battista Baliani (Italy) conducted an experiment in which a siphon failed to work. Galileo Galilei (Italy) explained the result by noting that power of a vacuum held up the water, but that at a certain point the weight of the water was too much for the vacuum. René Descartes (France) designed an experiment to determine atmospheric pressure in 1631. Having read of Galileo’s ideas, Raffaele Magiotti and Gasparo Berti (Italy) devised an experiment between 1639 and 1641 in which Berti filled a long tube with water, plugged both ends, and stood the tube in a basin of water. Berti then unplugged the bottom of the tube. The result was that only some of the water flowed out, and the water in the tube leveled off at 10.3 meters, the same height Baliani observed in the siphon. Above the water in the tube was a space that appeared to be a vacuum. Torricelli analyzed the results from a different angle: instead of explaining the phenomenon with a vacuum, he chose to challenge common understanding and claim that the air itself had weight, and exerted pressure on the water. From this, he concluded that he could create a device that would measure the pressure of the atmosphere. By using mercury, which is 14 times heavier than water, he could use a tube only 80 centimeters long instead of 10.5 meters. He also discovered that the barometer measured different pressures on rainy days and sunny days. Blaise Pascal and Pierre Petit (France) repeated and perfected Torricelli’s experiment in 1646, showing that the liquid used did not change the results.  Pascal had his brother-in-law, Florin Perier (France) perform another experiment which showed that the barometer (and therefore the air pressure) became lower as one increased in altitude, thus proving that the weight of the air was the cause of the barometer’s movements. In 1654, Otto von Guericke (Germany) demonstrated that a vacuum could exist, and he invented a pump that could create a vacuum. In 1661, Robert Boyle (Ireland) took advantage of the vacuum pump to discover Boyle’s Law.

A diagram of the mercury barometer invented by Evangelista Torricelli (1608-1647).

PROBABILITY THEORY (1654)
Probability theory is a branch of mathematics that analyzes random phenomena. In the 16^th Century, Gerolamo Cardano (Italy) took the first steps toward probability theory in his attempts to analyze games of chance. The next developments came from Pierre de Fermat and Blaise Pascal (France), who originated probability theory in 1654. Christiaan Huygens (The Netherlands) published a book on probability in 1657. Books by Jacob Bernoulli (Switzerland) in 1713 and Abraham de Moivre (France) in 1718 developed the mathematical basis for probability theory. The fundamentals of probability and statistics were set down by Pierre-Simon Laplace (France) in a 1812 treatise. Richard von Mises (Austria-Hungary) made advances in the 20^th Century, and modern probability theory was established by Andrey Nikolaevich Kolmogorov (USSR) and later Bruno de Finetti (Italy).

Daniel Bernoulli (1700-1782).

FOSSILS (1669)
While some early scientists, such as Leonardo da Vinci (Italy) in c. 1500, hypothesized that fossils were the remains of living things, this notion did not gain wide acceptance for many centuries.  In 1665, Athanasius Kircher (Germany) suggested that giant fossil bones belonged to a race of extinct giant humans. When Robert Hooke (England) looked at petrified wood through a microscope in 1665, he suggested that it and fossil seashells were formed when living trees and shells were filled with water containing “stony and earthy particles.” In 1668, however, Hooke proposed that fossils told us about the history of life on Earth, a radical idea at the time. Danish cleric Nicholas Steno is credited with first identifying the true nature of fossils. In 1667, he dissected a shark’s head and noticed that common fossils called tongue stones were actually shark’s teeth. Steno then began studying rock strata and published in 1669 a work that systematically disproved many of the prior theories about fossils (such as the theory that they grew inside of rocks like crystals). He proposed that fossils were the remains of living organisms that had become buried in layers of sediment, which had then hardened and formed horizontal layers of rock.  One of the obstacles to acceptance of Steno’s theory was the existence of fossils of organisms that did not resemble any living creatures. More than a century later, in 1796, Georges Cuvier (France) definitively proved that some creatures that had lived on Earth in the past were now extinct. The next advances came from William Smith (UK), who studied fossils in the different layers of rock and, between 1799 and 1819, proposed the law of superposition (younger rocks lay atop older rocks) and the principle of faunal succession, which would allow scientists to compare fossils from different areas.

In a 1667 book, Nicolas Steno (1638-1686) compared the head of a contemporary shark with fossil shark’s teeth.

THE STEAMBOAT (1803)
Denis Papin (France) made a ship powered by his steam engine, mechanically linked to paddles in 1704, although it did not create sufficient pressure to be practical.  Jonathan Hulls (England) received a patent for a Newcomen steamboat in 1736, but there is little evidence of any real success. William Henry (US) built several steamboats in 1763 and after but had little success with them. Marquis Claude de Jouffroy (France) made a steam-powered ship in 1783, the paddle steamer Pyroscaphe, which worked for 15 minutes and then stopped. John Fitch (US) and William Symington (Scotland) made similar boats in 1785. Symington and Patrick Miller (Scotland) made a boat with manually-cranked paddle wheels between double hulls in 1785, with a successful try-out in 1788. Using Symington’s design, Alexander Hart (UK) built and launched a successful steamboat in 1801. The same year, Symington designed a second steamboat with a horizontal steam engine linked directly to a crank, the Charlotte Dundas, which was built by John Allan (UK) and the Carron Company. Its maiden voyage was in 1803. The same year, Robert Fulton (US) observed the Charlotte Dundas and, with engineer Henry Bell (UK), designed his own steamboat, which he sailed on the Seine in 1803. Fulton then brought the boat to the US where as the North River Steamboat (later the Clermont), it carried passengers between New York City and Albany, New York in 1807. Other names in the steamboat saga include: J.C. Perier (France), 1775; James Rumsey (US), 1787; and Oliver Evans (US), 1804.

An artist’s depiction of the Charlotte Dundas under way.  The steamboat, which was designed by William Symington (1764-1831), provided the inspiration for Robert Fulton (1765-1815).

MOLECULES (1811)
The atomists of Ancient Greece theorized that different atoms connected to one another in different ways depending on the substance involved. Iron atoms, they supposed, had hooks to connect to other iron atoms, while water atoms were slippery. When the atom theory saw a resurgence in the 17^th Century, Pierre Gassendi (France) adopted some of the Ancient Greek ideas. Sir Isaac Newton (England), on the other hand, suggested in 1704 that particles attract one another by a force that is strong at short distances. Irish chemist Robert Boyle first discussed the concept of the molecule in his 1661 treatise, The Sceptical Chymist, in which he suggested that matter is made of clusters of particles or corpuscles of various shapes and sizes and that chemical reactions rearrange those clusters. In 1680, Nicolas Lemery (France) hypothesized that acidic substances had points, while alkalis had pores, and the points locked into the pores to create Boyle’s clusters. In 1738, Daniel Bernoulli (Switzerland) proposed his kinetic theory of gases, which presumed that gases consist of great numbers of clusters of atoms. William Higgins (Ireland) proposed a theory describing the behavior of clusters of ultimate particles in 1789. John Dalton (UK) published in 1803 the atomic weight of the smallest amount of certain compounds. Italian chemist Amedeo Avogadro published a paper in 1811 that coined the word ‘molecule’, although he uses it to refer to both molecules and atoms. Later, in setting out Avogadro’s Law, Avogadro distinguished between atoms and molecules for the first time. Jean-Baptiste Dumas (France) built on Avogadro’s findings in 1826 and Marc Antoine Auguste Gaudin (France) clearly stated the implications of the molecular hypothesis in 1833, where he suggests molecular geometries and molecular formulas that are consistent with atomic weights. In 1857-1858, German chemist Friedrich August Kekulé proposed that every atom in an organic molecule was bonded to every other atom, and he showed how carbon skeletons could form in organic molecules. At about the same time, Archibald Couper (UK) developed a theory of molecular structure complete with a new form of notation very similar to that used today. In 1861, Joseph Loschmidt (Austria) self published a booklet with a number of new molecular structures. August Wilhelm von Hofmann (Germany) made the first stick and ball models of molecules in 1865. Summing up the knowledge gained so far, James Clerk Maxwell (UK) published an article in 1873 entitled, ‘Molecules’ in which he defined a molecule as “the smallest possible portion of a particular substance.”

A model of two water molecules.

AVOGADRO’S LAW (1811)
In 1811, Amedeo Avogadro (Italy) first stated the law that equal volumes of all gases at the same temperature and pressure contain the same number of molecules. This law had the effect of reconciling French chemist Joseph Louis Gay-Lussac’s 1808 law on volumes and combining gases with British physicist John Dalton’s atomic theory.

A graphic illustration of Avogadro’s Law.

UNIFORMITARIANISM (1830)
Before the 18^th Century, most scientists believed that the Earth was very young and that its features were the result of sudden, catastrophic processes. Beginning in the mid-1700s, some geologists challenged the prevailing theory. In 1759, Mikhail Vasilevich Leomonsov (Russia) suggested that the Earth’s topography is result of very slow natural activity, including uplift and erosion.  Beginning in 1785, James Hutton (Scotland) proposed that the Earth formed through the gradual solidification of molten rock at a slow rate, by the same processes, particularly erosion and vulcanism, that occur today.  Hutton called this process Plutonism to contrast with Neptunists, who believed that the Biblical Flood was the cause of much geology.  The implication of this theory was the then-shocking notion that the Earth was millions of years old. The study of fossils in rock layers by William Smith (UK) in the 1790s and independently by French scientists Georges Cuvier and Alexandre Brogniart in 1811 introduced the notion of stratigraphy for determining the relative age of different rocks. In his Principles of Geology, the first volume of which was published in 1830, Scottish geologist Charles Lyell set out the voluminous evidence for uniformitarianism (a form of Hutton’s plutonism) over catastrophism. Charles Darwin read the volumes of Lyell’s Principles while serving as naturalist on the Beagle in the 1830s and uniformitarianism ultimately provided the geological basis for his theory of evolution by natural selection.
A diagram of some of the processes of uniformitarianism.

THE ELECTRIC MOTOR (1833-1834)
Choosing a single inventor for the electric motor would ignore the complexity of the machine’s development, but there is a reasonable argument that the first true electric motor or motors were invented in 1828, 1833 or 1834 (see below).  Andrew Gordon (Scotland) created a simple electrostatic motor as early as the 1740s.  André-Marie Ampère (France) invented the solenoid in 1820. Peter Barlow (UK) invented Barlow’s wheel, an early homopolar motor, in 1822. Ányos István Jedlik (Hungary) made the first commutated rotary electromagnetical engine in 1828. William Sturgeon (UK) made a commutated rotating electric machine in 1833, the same year that Joseph Saxton (US) made a magneto-electric machine. Thomas Davenport (US) created a battery-powered direct current (DC) motor in 1834 and obtained a patent for a motor in 1837 but high battery power cost made the invention impractical.  Moritz von Jacobi (Germany/Russia) made a 15-watt rotating motor in 1834 and the first useful rotary electrical motor in 1838. Sibrandus Stratingh and Christopher Becker (The Netherlands) built an electrical motor in 1835 that powered a small model car.  Between 1837 and 1842, British railway pioneer Robert Davidson made electric motors for a lathe and a locomotive.  Solomon Stimpson (US) made a 12-pole electric motor with segmental commutator in 1838. Truman Cook (US) made the first electric motor with PM armature in 1840. Paul-Gustave Froment (France) made the first motor that translated a linear electromagnetic piston’s energy to a wheel’s rotary motion in 1845. Zénobe Gramme (Belgium) made the first anchor ring motor in 1871. Galileo Ferraris (Italy) made the first alternating current (AC) commutatorless induction motor with two-phase AC windings in space quadrature in 1885. Nikola Tesla (Serbia/US) made three different two-phase four-stator-pole motors, including a synchronous motor with separately excited DC supply to rotor winding in 1886-1889.  Frank Sprague (US) built a constant-speed DC motor in 1886.  The three-phase cage induction motor, the most frequently produced machine for 1 kW and above, was first built by Michael Dolivo-Dobrowolsky (Russia) in 1889.

An artist’s rendering of the electric motor invented by Thomas Davenport (1802-1851) in 1834.

THE ICE AGES (1837)
As with many scientific discoveries, the concept of ice ages developed slowly over time. The first inklings of a theory were provided by scientists and others seeking to explain the presence of large erratic boulders and moraines, who suggested that glaciers had placed them in their current locations in the past.  These included: Pierre Martel (Switzerland) in 1744; James Hutton (Scotland) in 1795; Jean-Pierre Perraudin (France) in 1815; Göran Wahlenberg (Sweden) in 1818; Johann Wolfgang von Goethe (Germany) in 1820; Ignaz Venetz (Switzerland) in 1829; and Ernst von Bibra (Germany) in 1849-1850. In 1824, Jens Esmark (Denmark/Norway) proposed that changes in climate caused a sequence of worldwide ice ages. Robert Jameson (Scotland) accepted and promoted Esmark’s ideas, as did Albrecht Reinhard Bernhardi (Germany), who speculated in 1832 that former polar ice caps may have reached the temperate zones. Momentum began to build when Venetz convinced Jean de Charpentier (Switzerland/Germany) of his glaciation theory and de Charpentier presented a paper on the subject in 1834. German botanist Karl Friedrich Schimper gave lectures in Munich in 1835-1836 in which he proposed that erratic boulders were the result of global times of obliteration, when the climate was cold and water was frozen. Schimper spent the summer of 1836 in the Swiss Alps with de Charpentier and Louis Agassiz (Switzerland), during which time Agassiz became convinced of the glaciation theory. Agassiz and Schimper developed a theory of a sequence of glaciations in 1836-1837.  Schimper coined the term ‘ice age’ in 1837. The reception from the scientific community was cool, so Agassiz set out to collect more data to support the theory, which he published in 1840. Widespread acceptance of the theory did not come until 1875, when James Croll (UK) became the first to propose a convincing mechanism to explain the ice ages.  In his book Climate and Time, in their Geological Relations, Croll hypothesized that cyclical changes in the Earth’s orbit could have triggered the growth of the glaciers.  The orbital changes were later proven experimentally.
An illustration of the extent of glaciation at the height of the last ice age, about 20,000 years ago.

ABSOLUTE ZERO (1848)
Absolute zero is the lower limit of the thermodynamic temperature scale. It is the state at which the enthalpy and entropy of a cooled ideal gas reaches its minimum value of zero. Absolute zero equals −273.15° Celsius, −459.67° Fahrenheit and 0° Kelvin. Robert Boyle (Ireland) was one of the first to propose the idea of an absolute zero, or primum frigidum, in 1665. Eighteenth Century scientists accepted the idea of absolute zero and tried to calculate it. Some calculations were more accurate than others.  While the calcuations of Guillaume Amontons (France)  (–240° C) in 1702 and Johann Heinrich Lambert (Switzerland) (-270° C) in 1779 were relatively close to the actual figure, Pierre-Simon Laplace and Antoine Lavoisier (France) put the number at  –600° C or colder, while in 1808, John Dalton (UK) suggested a value of –3000° C.  In 1848, William Thomson, Lord Kelvin (UK), arrived at -273.15° C, the temperature for absolute zero that is still recognized today.  Kelvin’s scale is based on Carnot’s theory of the motive power of heat and is independent of the properties of any particular substance.

William Thomson, Lord Kelvin (1824-1907).

MASS STEEL PRODUCTION (1851, 1855)
Humans had been making steel since at least 2000 BCE, but before the 19th Century, steel manufacturing was a slow, expensive process that required the use of carbon-free wrought iron; as a result it was impossible to produce steel in mass quantities.  In 1740, Benjamin Hunstman (UK) developed the crucible technique, which increased the cost and duration of the process but increased quality. Beginning in 1847, William Kelly (US) began to experiment with reducing carbon content by blowing air through the molten iron and by 1851, he had developed a process that greatly improved the purity of the finished product and allowed for production of mass quantities of steel. In the UK, Henry Bessemer independently invented a similar process, which he patented in 1855 and which bears his name. Shortly afterwards, Robert Mushet (UK) improved the Bessemer process, creating a more malleable final product.  In 1878, Sidney Thomas (UK) designed a way to reduce phosphorus residue in the Bessemer process, increasing the quality of the steel.  In the late 20th Century, the Bessemer process was replaced by the basic oxygen process, which allowed better control of the chemistry.

A Bessemer converter that operated until 1978.  It is now located at Kelham Island Museum, UK.

FERMENTATION (1857)
While fermentation has been used by humans to make fermented beverages and foods since at least 7000 BCE, the scientific explanation for the process only became understood in the 19^th Century. In 1837 and 1838, Theodor Schwann (Germany), Charles Cagniard de la Tour (France) and Friedrich Traugott Kützing (Germany), working independently, concluded that fermentation was caused by yeast, a living organism. In 1857, Louis Pasteur (France) demonstrated that lactic acid fermentation is carried out by living bacteria. In 1897, Eduard Buechner (German) isolated the enzyme in yeast that caused fermentation.

A diagram of the chemical reactions leading to lactic acid fermentation.

THE INTERNAL COMBUSTION ENGINE (1860)
The history of the internal combustion engine (ICE) is long and complex.  Components of the system were invented as long ago as the 3rd Century CE.  In the 17th Century, Christiaan Huygens (The Netherlands) created a rudimentary ICE piston engine when he used gunpowder to drive water pumps for the Versailles palace gardens.  In the 1780s, Alessandro Volta (Italy) built a toy pistol, in which an electric spark exploded a mix of air and hydrogen, firing a cork. In 1791, John Barber (UK) received a patent for a turbine.  In 1794, Robert Street (UK) built the first compressionless engine. In 1807, Nicéphore Niépce (France) powered a boat, the Pyréolophore, with an ICE, fueling it with moss, coal dust and resin. In 1807, Swiss engineer François Isaac de Rivaz built an ICE powered by a mix of hydrogen and oxygen, and ignited by an electric spark. In 1823, Samuel Brown patented the first industrial ICE, a compressionless model.  Nicolas Léonard Sadi Carnot (France) established the theoretical basis for idealized heat engines in 1824.  In 1826, Samuel Morey (US) received a patent for a compressionless ICE. In 1833, Lemuel Wellman Wright (UK) invented a table-type gas engine with a double acting gas engine and, for the first time, a water-jacketed cylinder. In 1838, William Barnett (UK) received a patent for the first machine with in-cylinder compression. Between 1853 and 1857, Eugenio Barsanti and Felice Matteucci (Italy) invented and patented an engine using the free-piston principle that was possibly the first 4-cycle engine. In 1856, Pietro Benini (Italy) built an engine that supplied five horsepower.  Later, he developed more powerful engines with one or two pistons.  In 1860, Jean Joseph Etienne Lenoir (Belgium) produced and sold the first two-stroke gas-fired ICE with cylinders, pistons, connecting rods, and flywheel – Lenoir is generally recognized as the inventor of the ICE. In 1861, Alphonse Beau de Rochas (France) received the first patent for a four-cycle engine. In 1862, German inventor Nikolaus Otto built and sold a four-cycle free-piston engine that was indirect-acting and compressionless. Alphonse Beau de Roche, France set out the ideal operating cycle for a four-stroke ICE in 1862. In 1865, Pierre Hugon (France) created the Hugon engine, similar to the Lenoir engine, but with better economy, and more reliable flame ignition. In 1867, Nikolaus Otto and Eugen Langen (Germany) introduced a free piston engine with less than half the gas consumption of the Lenoir or Hugon engines.  In 1870, Siegfried Marcus (Austria) put the first mobile gasoline engine on a handcart. In 1872, American George Brayton invented Brayton’s Ready Motor, which used constant pressure combustion, and was the first commercial liquid fueled ICE. In 1876, Nikolaus Otto, working with Gottlieb Daimler and Wilhelm Maybach (Germany), began developing and patenting the four-cycle engine.  In 1878, Dugald Clerk (UK) designed the first two-stroke engine with in-cylinder compression. In 1879, Karl Benz (Germany), working independently, received a patent for a two-stroke gas ICE using De Rochas’s four-stroke design. In 1885, Benz designed and built a four-stroke engine to use in an automobile. In 1882 James Atkinson (UK) invented the Atkinson cycle engine, which had one power phase per revolution together with different intake and expansion volumes. In 1884, British engineer Edward Butler constructed the first gasoline ICE. Butler also invented the spark plug, ignition magneto, coil ignition and spray jet carburetor. Rudolf Diesel (Germany) invented the diesel engine in 1892 and Felix Wankel (Germany) invented the rotary engine in 1956.

The internal combustion engine invented by Jean Joseph Etienne Lenoir (1822-1900) in 1860.

THE TELEPHONE (1876)
The telephone evolved from the telegraph.  Numerous inventors sought to develop acoustic telegraphy, to send sound waves over the electrical wires.  Antonio Meucci (US), an Italian immigrant, created a voice communication device about 1854 that he described to the US Patent Office in an 1871 patent caveat.  Johann Philipp Reis (Germany) created a device in 1860 that could transmit music and speech, although usually indistinctly. There is some evidence that Innocenzo Manzetti (Italy) may have created a telephone in 1864. In 1870, Cromwell Varley (UK) created a machine that could transmit sounds, but not distinct speech. Poul la Cour (Denmark) made a similar machine in 1874.  In 1875, Elisha Gray (US) invented a tone telegraph that could transmit musical notes. Gray filed a patent caveat for a true telephone with a water transmitter on the same day in 1876 that Alexander Graham Bell (Scotland/Canada/US) filed a patent application for his telephone. In future models, however, Bell did not use the water transmitter. The invention of the carbon microphone in 1877 by Thomas Edison and Emile Berliner (US) and independently by David Hughes in the UK, further improved the telephone.

A replica of the transmitter component of the original 1876 telephone made by Alexander Graham Bell (1847-1922).

A replica of Bell’s original receiver.

MITOSIS (1879)
Hugo von Mohl (Germany) described the splitting of one cell into two (mitosis) in the cells of living organisms in 1839, including the appearance of cell plate between daughter cells during cell division. Carl Nageli (Germany) observed cell division and chromosomes in 1842, but he thought what he was seeing was an anomaly. Walther Flemming (Germany) used aniline dyes to study salamander embryos beginning in 1879.  Flemming made the first accurate counts of chromosomes and observed longitudinal splitting of chromosomes.  His 1882 book on cell division was seminal. Additional work was done by Edouard Van Beneden (Belgium) and Eduard Strasburger (Poland/Germany), who identified chromosome distribution during mitosis. In 1888, Heinrich Wilhelm Gottfried von Waldeyer-Hartz (German) coined the term ‘chromosome’ to name what Flemming had described.

A whitefish blastula cell undergoing mitosis.

RADIO WAVES (1887)
In 1867, James Clerk Maxwell (Scotland) predicted the existence of radio waves, electromagnetic waves that are radiated by charged particles as they accelerate. Heinrich Hertz (Germany) proved the existence of radio waves by generating them experimentally in his laboratory in 1887. He also showed that the radio waves traveled at the speed of light.

A replica of the 1887 radio wave experiment by Heinrich Hertz (1857-1894).

BLOOD TYPES (1901)
During experiments with blood transfusion, Karl Landsteiner (Austria) identified types A, B and O blood (the ABO blood group) in 1901. Alfred von Decastello and Adriano Sturli (Austria) identified the AB blood type in 1902. Czech physician Jan Jansky discovered the four basic blood groups independently and published the finding in a little-noticed 1907 paper. William Lorenzo Moss (US) made similar discoveries, which were published in 1910.  In 1910-1911, Ludwik Hirszfeld (Poland) and Emil von Durgern (Germany) discovered that ABO blood groups are inherited. Felix Bernstein (Germany) determined the chromosomal basis for blood groups in 1924. In 1937, Landsteiner, together with Alexander Wiener (US), identified the Rhesus group. In 1945, Robin Coombs, Arthur Mourant and Rob Race (UK) developed the Coombs blood test. At present, 33 human blood group systems have been identified, and more than 600 blood group antigens.

Karl Landsteiner (1868-1943).

THE THIRD LAW OF THERMODYNAMICS (1906)
The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero Kelvin is exactly equal to zero. Walter Nernst (Germany) first formulated the law in 1906; in 1912, Nernst stated the law as follows: “It is impossible for any procedure to lead to the isotherm T = 0 in a finite number of steps.” Gilbert N. Lewis and Merle Randall (US) proposed an alternative version of the law in 1923: “If the entropy of each element in some (perfect) crystalline state be taken as zero at the absolute zero of temperature, every substance has a finite positive entropy; but at the absolute zero of temperature the entropy may become zero, and does so become in the case of perfect crystalline substances.” A later formulation of the third law, known as the Nernst-Simon statement, is: “The entropy change associated with any condensed system undergoing a reversible isothermal process approaches zero as temperature approaches 0 K, where condensed system refers to liquids and solids.”

Walther Nernst (1864-1941).

PLASTIC (1907)
Prior to the invention of Bakelite, the first completely synthetic plastic, chemists made artificial plastics from naturally-occurring nitrocellulose mixed with other materials. Alexander Parkes (UK) invented Parkesine in 1856, a thermoplastic celluloid based on nitrocellulose treated with solvents. John W. Hyatt modified Parkesine to create Celluloid in 1869. In an effort to find a substitute for shellac, Leo Baekeland, a Belgian-born chemist working in the US, invented Bakelite, which contains no naturally-occurring ingredients, in 1907. The chemical name of Bakelite is polyoxybenzylmethylenglycolanhydride.  In 1922, Hermann Staudinger (Germany) set out the theoretical background of macromolecules and polymerization on which the modern plastics industry rests.  In 1958, Robert Banks and Paul Hogan (US) invented polypropylene and devised a low-pressure method for producing high-density polyethylene.

This Bakelite radio was sold by General Electric in Australia in 1932.

THE UNCERTAINTY PRINCIPLE (1927)
The uncertainty principle holds that there is a mathematically-determined fundamental limit to knowing precisely and simultaneously certain pairs of physical properties (known as complementary variables) of a particle, such as position and momentum. Werner Heisenberg (Germany) first articulated the uncertainty principle in 1927 by stating that the more precisely a particle’s position is determined, the less precisely its momentum can be known, and vice versa. The uncertainty principle is sometimes confused with the observer effect, which states that measurements of certain systems cannot be made without affecting the system.

A diagram explaining the uncertainty principle.

THE WEAK NUCLEAR FORCE & THE WEAK INTERACTION (1933)
The weak interaction is the mechanism responsible for the weak nuclear force, one of the four basic forces of nature, along with electromagnetism, gravity and the strong nuclear force. The weak interaction is responsible for radioactive decay and nuclear fusion of subatomic particles; caused by emission or absorption of W and Z bosons. Fermions (which include quarks, leptons and certain particles made from them) also interact through weak interaction. Ernest Rutherford (NZ/UK) proposed the weak nuclear force in 1899 to explain beta decay of radioactive elements. Enrico Fermi (Italy) first suggested the existence of the weak interaction in 1933 in explaining beta decay. Fermi thought it was a force with no range, dependent on contact. (It is now believed that the weak force is a non-contact force with a finite range.) In 1956, Clyde Cowan and Frederick Reines (US) showed that electrons and antineutrinos were released in beta decay. The same year, Tsung-Dao Lee and Chen Ning Yang (China/US) predicted that the weak force did not follow parity, the symmetry of the other forces. In 1968, Sheldon Glashow (US), Abdus Salam (Pakistan) and Steven Weinberg (US) proved that the weak interaction and electromagnetism were two aspects of the same force, now known as the electroweak force. W and Z bosons were first experimentally detected by Carlo Rubbia (Italy) and Simon van der Meer (The Netherlands) in 1983.

Carlo Rubbia (1934- ) (right) and Simon van der Meer (1925-2011), who jointly won the Nobel Prize in Physics in 1984 for discovering the W and Z bosons.

ATOMIC WEAPONS (1945)
The road to the atom bomb began in 1934, when Hungarian scientist Leó Szilárd proposed bombarding radioactive atoms with neutrons to form a nuclear chain reaction, an idea he patented and then transferred to the British Admiralty so it would be kept secret.  In 1938, Otto Hahn and Fritz Strassmann (Germany) split the uranium atom, a fact explained and confirmed by Lise Meitner and Otto Robert Frisch (Germany) in January 1939. Meitner and Frisch named the process ‘fission’ by analogy to biological processes. Scientists at Columbia University repeated the experiment in January 1939. In August 1939, fearing that the Germany would produce a fission-based weapon, Szilárd wrote and Albert Einstein (Germany) signed a letter of warning to US President Franklin Roosevelt, who responded by setting up a committee to study the matter, which only received significant funding after the US entered World War II in December 1941. In 1940 and 1941, the British took the lead in conducting research into uranium and potential weapons. The US research did not begin in earnest until September 1942, with the start of the Manhattan Project, led by General Leslie Groves, which took over the British research.  Robert Oppenheimer (US) led the Manhattan Project’s team of physicists.  In addition to the Los Alamos laboratory, an Oak Ridge, Tennessee facility produced the rare uranium-235 isotope needed for a chain reaction. The project also used plutonium-239, a byproduct of a uranium-238 reaction, as a basis for a fission weapon. The Manhattan Project ultimately produced two types of fission bombs: a uranium-235 gun-type weapon (“Little Boy”) and a plutonium-239 implosion-type bomb (“Fat Man”). The first atomic weapon – a plutonium implosion bomb – was detonated at Los Alamos, New Mexico on July 16, 1945, releasing the equivalent of 19 kilotons of TNT. On August 6, 1945, the US dropped a uranium gun-type bomb on Hiroshima, Japan. On August 9, 1945, the US dropped a plutonium implosion-type bomb on Nagasaki, Japan. The two bombings resulted in the deaths of approximately 220,000 people, mostly civilians. The USSR tested its first fission bomb on August 29, 1949. In 1950, the US began developing the much more powerful thermonuclear or hydrogen bomb, which uses fission to create a fusion reaction; the first bomb was tested in 1952, releasing energy equal to 10.4 megatons of TNT. The USSR followed with its first thermonuclear bomb test on August 12, 1953.

The first atomic bomb explodes on July 16, 1945 at Almogordo, New Mexico.

INFORMATION THEORY (1948)
Information theory is a branch of applied mathematics, electrical engineering, and computer science that involves the quantification of information. Information theory was developed by Claude E. Shannon (US) in 1948 to find fundamental limits on signal processing operations such as compressing data and on reliably storing and communicating data.  Since its inception, information theory has expanded and has been applied in numerous contexts.

Claude E. Shannon (1916-2001).

THE LASER (1958-1960)
A laser (an acronym for ‘light amplification by stimulated emission of radiation’) emits light through optical amplification based on the stimulated emission of electromagnetic radiation. Albert Einstein (Germany) established the theoretical basis for lasers and masers in a 1917 paper. Other aspects of the science were developed by Rudolf Ladenburg (Germany) in 1928; Valentin Fabrikant (USSR) in 1939; Willis E. Lamb and R.C. Retherford (US) in 1947 (stimulated emission) and Alfred Kastler (France) in 1950 (optical pumping). Charles Hard Townes, with students James Gordon and Herbert Zeiger (US) created the first microwave amplifier, or maser, in 1953, although it was incapable of continuous output.  In the USSR, Nikolay Basov and Aleksandr Prokhorov had solved the continuous output problem using a quantum oscillator in 1952, but results were not published until 1954-1955.  In 1957, Townes and Arthur Leonard Schawlow (US), at Bell Labs, began working on an infrared laser, but soon changed to visible light, for which they sought a patent in 1958.  Also in 1957, Columbia University grad student Gordon Gould, after meeting with Townes, began working on the idea for a ‘laser’ using an open resonator. Prokhorov independently proposed the open resonator in 1958.  In 1959, Gould published the first paper using the term ‘laser’ and filed for a patent the same year.  In 1960, the US Patent Office granted Townes’ and Schawlow’s patent and denied Gould’s.  The first working laser was created by Theodore Maiman (US) in 1960, but it was only capable of pulsed operation.  Also in 1960, Ali Javan (Iran/US), William Bennett and Donald Herriott (US) made the first gas laser.  In 1962, Robert N. Hall (US) invented the first laser diode device.  The same year, Nick Holonyak, Jr. (US) made the first semiconductor laser with a visible emission, although it could only be used in pulsed-beam operation.  In 1970, Zhores Alferov (USSR), Izuo Hayashi (Japan) and Morton Panish (US) independently developed room-temperature, continual-operation diode lasers. In 1987, following years of patent litigation, a Federal judge ordered the US Patent Office to issue patents to Gordon Gould for the optical pump and gas discharge lasers.

The first working laser, which was made by Theodore Maiman (1927-2007) in 1960.

PULSARS (1967)
A pulsar (short for ‘pulsating star’) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. On November 28, 1967, Antony Hewish and Jocelyn Bell Burnell (UK) became the first scientists to observe a pulsar, which had a pulse period of 1.33 seconds. Walter Baade (Germany/US) and Fritz Zwicky (Switzerland/US) had predicted neutron stars in 1934, and in early 1967, Franco Pacini (Italy) suggested that a rotating neutron star with a magnetic field would emit radiation. In 1968, David Staelin, Edward C. Reifenstein III and Richard Lovelace (US) discovered a pulsar in the Crab Nebula with a 33 millisecond pulse period and a rotation speed of 1,980 revolutions per minute. Joseph Hooton Taylor, Jr. and Russell Hulse (US) discovered the first pulsar in a binary system in 1974.  A team led by Don Backer (US) discovered the first millisecond pulsar, with a rotation period of 1.6 milliseconds, in 1982.

A Chandra X-ray image of the Vela Pulsar, which is located inside the Milky Way galaxy about 950 light years from Earth and has a pulse period of 89 milliseconds.

THE INTERNET (1969)
The development of the Internet was a complex, many-faceted process. It is impossible to identify one person who invented the Internet, and it is very difficult to choose a point in time when the Internet was invented, but there is a rational basis for choosing 1969, as shown below. Leonard Kleinrock’s (US) July 1961 paper on packet switching theory at MIT was an early precursor to the Internet, as was a series of “Galactic Network” memos by J.C.R. Licklider (US), also at MIT, in August 1962.  In October 1962, Licklider became the first computer research program head at DARPA (Defense Advanced Research Projects Agency), where he convinced Ivan Sutherland, Robert Taylor and MIT’s Lawrence G. Roberts (US) of the importance of networking.  Kleinrock convinced Roberts to use packets rather than circuits.  The first wide-area computer network was built in 1965 when Roberts and Thomas Merrill (US) connected the TX-2 computer in Massachusetts to the Q-32 in California.  In 1966, Roberts went to DARPA, where he developed the computer network concept and put together a plan for the ARPANET, which he published in 1967.  Parallel research on networks had been going on at RAND (1962-1965) (esp. Paul Baran (US)) and National Physical Laboratory (NPL) (1964-1967) (esp. Donald Davies and Roger Scantlebury (UK)). After Roberts and DARPA refined the ARPANET’s specifications, in 1968, they chose Frank Heart’s (US) team at Bolt Beranek and Newman (BBN) to built the packet switches, called Interface Message Processors (IMPs).  Robert Kahn (US) at BBN; Howard Frank (US) at Network Analysis Corp.; and Kleinrock, played significant roles. In September 1969, BBN installed the first IMP at UCLA, which became the first node. Doug Engelbart’s Stanford Research Institute (SRI) provided the second node. The first message was sent between UCLA and SRI in October 1969. Four computers were linked by the end of 1969 and many more joined in the next few years. In December 1970, S. Crocker (US) and his Network Working Group finished the ARPANET’s initial host-to-host protocol, the Network Control Protocol (NCP). Also in 1970, NPL started the Mark I network. In 1971, the Merit Network and Tymnet networks became operational. Kahn successfully demonstrated the ARPANET at a conference in October 1972. Also in 1972, Louis Pouzin in France began an Internet-like project called Cyclades, that was based on the notion that the host computer, not the network, should be responsible for data transmission. Cyclades was eventually shut down, but the Internet eventually adopted its basic principle. The first trans-Atlantic transmission occurred in 1973, to University College of London.  In 1974, a proposal was made to link ARPA-like networks into a larger inter-network that would have no central control.  Also in 1974, the International Telecommunication Union developed X.25 packet switching network standards. The PC modem was invented by Dennis Hayes and Dale Heatherington in 1977. The first bulletin board system was invented in 1978. Usenet was invented in 1979 by Tom Truscott and Jim Ellis (US) and CompuServe was launched the same year. In 1981, the National Science Foundation created CSNET, the Computer Science Network, which linked to ARPANET.  In 1982, the TCP/IP protocol suite, invented by Vinton Cerf (US), was formalized.  ARPANET computers were required to switch from the NCP protocol to the TCP/IP protocols by January 1, 1983.  In 1984, the system of domain names was adopted – the first .COM domain name was registered in 1985.  In 1986, NSF created NSFNET, which was linked with ARPANET. In 1988, Internet Relay Chat was first introduced.  America Online (AOL) was launched in 1989.  In 1990, ARPANET was decommissioned in favor of NSFNET. NSFNET was decommissioned in 1995 when it was replaced by networks operated by several commercial Internet Service Providers.

A 1969 diagram of the Arpanet network.

A recent visualization of routing paths through a portion of the Internet.

STRING THEORY (1970)
According to string theory, all elementary particles are actually made of vibrating one-dimensional objects called strings. String theory purports to unite all four basic forces in one explanatory framework. String theory requires multiple spatial dimensions; one version of the theory requires 11 dimensions. While some physicists have embraced string theory, others have criticized it because it is difficult (some say impossible) to test its predictions. A precursor to string theory was S-matrix theory, proposed by Werner Heisenberg (Germany) in 1943. Some physicists expanded on the theory in the 1950s, particularly Tullio Regge (Italy), Geoffrey Chew and Steven Frautschi (US). The theory eventually developed into the dual resonance model of Gabriele Veneziano in 1968. The scattering amplitude that Veneziano predicted was essentially a closed vibrating string. Then in 1970, Yochiro Nambu (Japan/US), Holger Bech Nielsen (Denmark) and Leonard Susskind (US) proposed a theory that represented nuclear forces as one-dimensional vibrating strings.  John H. Schwarz (US) and Joel Scherk (France) proposed bosonic string theory in 1974. Michael Green (UK) and John Schwarz proposed the existence of supersymmetric strings, or superstrings, in the early 1980s. Between 1984 and 1986, a number of scientific discoveries occurred that have been termed the first superstring revolution. These discoveries resulted in a number of rival versions of the theory. In 1994, Edward Witten (US) suggested that the five different versions of string theory were all different limits on an 11-dimensional theory he called M-theory, an announcement that led to the second superstring revolution between 1994 and 1997. Chris Hull and Paul Townsend (UK) played important roles in this phase. Some scientists believe that the Large Hadron Collider at CERN may be able to produce enough energy to provide experimental evidence for string theory.

Edward Witten (1951- ).

THE HUMAN GENOME (2003)
Discussions began in the US in 1984 to sequence the entire human genome. The Human Genome Project began in 1986 through the US National Institutes of Health and the Department of Energy, but the actual project did not begin until 1990. Researchers from all over the world identified the genetic sequencing of human DNA using samples from approximately 270 individuals. A first draft of the genome was announced in 2000, and the project was declared finished in 2003. Celera Genomics undertook a parallel human genome project in the private sector in 1990s, which was much faster and less expensive than the government’s Human Genome Project. (Some pointed out that Celera was able to finish so quickly in part because it was able to freely obtain all the Human Genome Project’s results daily as they were placed online for the public, while Celera refused to share its results on proprietary grounds.) In 2001, Craig Venter of Celera Genomics and Francis Collins of the Human Genome Project, jointly published their decoding of the human genome.

A graphic depiction of the human genome.

4
THE LEVER (2600 BCE)
A lever is a simple machine consisting of a beam or rigid rod that pivots at a fixed hinge, or fulcrum, thereby amplifying an input force to provide a greater output force. Greek scientist philosopher Archimedes first correctly stated the mathematical principle behind the lever in the 3rd Century BCE. Pappus of Alexandria quotes Archimedes as saying of the lever, “Give me a place to stand, and I shall move the Earth with it.” Although there is no written evidence of levers prior to Archimedes, historians believe that the Ancient Egyptians must have had levers in order to construct the pyramids and other massive monuments weighing more than 100 tons in the 3^rd Millenium BCE.

Three types of levers.

THE HINDU-ARABIC NUMERAL SYSTEM (1-600 CE)
Also known as the Indo-Arabic Counting System, the Hindu-Arabic Numeral System was the first counting system to include a zero and is the basis for most of subsequent mathematics. This positional decimal numeral system was invented in India, but there is much debate about the date.  Some scholars believe there is evidence for a 1st Century CE date, while others say the earliest evidence is from the 3rd or 4th Century CE.  All agree that the system was in use by 600 CE.  The system began to spread elsewhere: Severus Sebokht (Syria) mentions it in 662 CE and Muslim scholar al-Qifti (Egypt) cites an encounter between a Caliph and an Indian mathematics book in 776 CE.  Persian mathematician Al-Khwarizmi gave a treatise on the system in an 825 CE book and Arab mathematician Al-Kindi did the same in 830 CE.  Arabic numerals first appear in Europe in a 976 CE Spanish text.  Italian mathematician Fibonacci sought to promote the system in a book published in 1202, but the system did not become standard in Europe until after the printing press was invented after 1540.

This chart shows the changes in numerals from Hindu India, to the Islamic world, and then to Europe.

MEDICINE AS A SCIENCE (1025)
The Canon of Medicine, a five-book encyclopedia by Persian scientist, physician and philosopher Ibn Sina (often referred to by the Latinate form of his name, Avicenna) set out in a systematic way the medical knowledge and procedures known in the 11^th Century. While the Canon relies primarily on Galenic medical theories, Ibn Sina also adopts Aristotle’s explanations in some cases and drew from many other sources, including Chinese texts from 310 CE and 610 CE. In his introduction, Ibn Sina sets out his belief in medicine as a science, that the physician must determine the causes of both health and disease before the body can be restored to health. The book contains specific instructions on diagnosis and treatments, including surgical procedures, and analyzes the efficacy of over 600 different drugs or herbal remedies. Originally written in Arabic, the Canon was translated into Latin by Gerard of Cremona (Italy) in the 13^th Century, allowing it to become the premier textbook for European medical education in the medieval period.

A page from a 1597 Arabic copy of the Canon of Medicine, by Ibn Sina (c. 980-1037 CE).

SUPERNOVAE (1572)
A supernova occurs when a star suffers a catastrophic explosion, causing it to increase greatly in brightness. The explosions of supernovae radiate enormous amounts of energy and normally expel most or all of the star’s contents at a velocity of 30,000 km/s, which sends a shock wave and an expanding shell of gas and dust (called a supernova remnant) into interstellar space. Supernovae generate much more energy than novae. There are two types of supernova: the first occurs when nuclear fusion suddenly reignites in a degenerate star due to accumulation of material from a companion star; the second occurs when a massive star undergoes sudden gravitational collapse.  The first supernovae to be observed were those occurring in the Milky Way galaxy that were visible to the naked eye.  Chinese astronomers observed a supernova in 185 CE. Chinese and Islamic astronomers described a supernova in 1006. A widely-seen supernova in 1054 created the Crab Nebula. Tycho Brahe (Denmark) described a supernova in Cassiopeia 1572 and Johannes Kepler  (Germany) described one in 1604.  The first supernova in another galaxy was seen in the Andromeda galaxy in 1885. Prior to 1931, supernovae were not distinguished from ordinary novae. Based on observations at Mt. Wilson Observatory, Walter Baade (Germany/US) and Fritz Zwicky (Switzerland/US) created a new category for supernovae, a term they began using in a series of 1931 lectures and announced publicly in 1933. In 1941, Zwicky and Rudolph Minkowski (Germany/US) developed the modern supernova classification scheme. In the 1960s, astronomers began to use supernova explosions as ‘standard candles’ to measure astronomical distances. More recently, scientists have been able to determine the dates and locations of supernovae that occurred in the past based on their aftereffects.

A multiwavelength X-ray, infrared, and optical compilation image of the supernova remnant of the supernova observed by Johannes Kepler in 1604 (SN 1604).

THE MICROSCOPE (c. 1590)
Roger Bacon first proposed the idea of a microscope in 1267, but it was not until about 1590 that two Dutch eyeglass makers, Hans Lippershey & Zacharias Jansen, made the first compound optical microscope.  (‘Optical’ because it used visible light and lenses to magnify objects and ‘compound’ because it used multiple lenses, allowing for much greater magnification than the single lens, or simple optical microscope.) Galileo Galilei (Italy) developed a compound microscope in 1609, and Cornelius Drebbel (The Netherlands) created one in 1619.  Early microscope researchers were Robert Hooke (UK), who published a book of drawings from the microscope entitled Micrographia in 1665, containing numerous scientific discoveries, including the first description of a biological cell, and Antonie van Leeuwenhoek (The Netherlands), who made many discoveries in the 1670s.

A replica of the microscope used by Antonie van Leeuwenhoek (1632-1723), which magnified objects 270 times.

THE MOONS OF JUPITER (1609-1610)
Chinese astronomer Gan De reportedly observed a moon orbiting Jupiter about 364 BCE. It is Galileo Galilei (Italy), however who is credited with discovering the four largest moons of Jupiter – Ganymede, Callisto, Io and Europa – by making observations using progressively stronger telescopes in 1609 and 1610. E.E. Barnard (US) discovered a fifth moon, Amalthea, in 1892.  Using photographic telescopes, additional moons were discovered in 1904, 1905, 1908, 1914, 1938, 1951, and 1974.  A 14^th moon was discovered in 1975. The Voyager space probes found three more moons in 1979. Between 1999 and 2003, a team led by Scott S. Sheppard and David C. Jewitt (US) found 34 additional moons, most of them very small (averaging 1.9 miles in diameter) with eccentric orbits. Between 2003 and 2014, scientists have discovered 17 additional moons, bringing the total to 67.

A view of Jupiter and the four moons discovered by Galileo, as seen through a 10″ Meade LX200 telescope.

THE LIGHT SPECTRUM (1666)
In the 13^th Century, Roger Bacon (England) suggested that rainbows were produced the same way that light produced colors when passed through a glass or crystal. In 1666, Isaac Newton (England) discovered that visible white light is composed of a spectrum of colors. He made this discovery by studying the passage of light through a dispersive prism, which refracted the light into the colors of the rainbow: red, orange, yellow, green, blue and violet. He also found that the multicolored spectrum could be recomposed into white light by a lens and a second prism. He published his results in 1671.

Light dispersion of a mercury-vapor lamp with a prism made of flint glass. Photo by D-Kuru (2009).

LIGHT THEORY (1675 (particle); 1678 (wave); 1862 (electromagnetic); 1900 (quanta))
Light is electromagnetic radiation that is visible to the human eye.  Scientists now accept that light has wavelike and particle-like qualities. Explanations for the nature of light began with the Ancient Greeks, including Empedocles in the 5th Century BCE, who believed that sight results from a beam of light emitted by the eye. Euclid questioned the ‘beam from the eye’ theory in 300 BCE with a thought experiment, although he supposed the theory could be true if the speed of light was infinite. Lucretius (Ancient Rome) in 55 BCE supposed that light consisted of atoms moving from the sun to the Earth. Ptolemy in the 2^nd Century CE discussed refraction of light. Indian Hindu philosophers in the early centuries of the common era proposed a particle theory of light, but Indian Buddhists in the 5^th and 7^th centuries CE suggested that light was composed of flashes of energy. In 1604, Johannes Kepler found that the intensity of a light source varies inversely with the square of one’s distance from that source. René Descartes (France) theorized in 1637 that light was a mechanical property of the luminous body and the medium transmitting the light. The modern particle theory of light was proposed by Pierre Gassendi (France) and published after his death in the 1660s. Isaac Newton (England) adopted the particle theory in 1675 (with a final version published in 1704), stating that corpuscles of light were emitted from a source in all directions. He also explained diffraction, polarization and (incorrectly) refraction. Further work on polarization of light was done by Étienne-Louis Malus (France) in 1810 and Jean-Baptiste Biot (France) in 1812.  Although Newton’s particle theory was dominant for at least a century, others found that light had wavelike properties.  Robert Hooke (England) invoked a wave theory of light to explain the origin of colors in 1665 and expanded on the theory in 1672. Christiaan Huygens (The Netherlands) developed a mathematical wave theory of light in 1678. The theory predicted interference patterns, which were confirmed by Thomas Young (UK) in 1801. In 1746, Leonhard Euler (Switzerland) argued that wave theory provided a better explanation for diffraction than particle theory. Augustin-Jean Fresnel (France) developed a separate wave theory in 1817, which received support from Siméon Denis Poisson (France). Measurements of the speed of light in 1850 supported wave theory. Wave theory suffered a non-fatal blow in 1887.  Huygens had proposed that waves were propagated by a luminiferous aether, but the Michelson-Morley experiment in 1887 proved that the aether did not exist. Meanwhile, as the result of experiments performed in 1845-1847, Michael Faraday (UK) suggested that light was a form of electromagnetic wave, which could be propagated in a vacuum. In 1862 and then in 1873, James Clerk Maxwell (UK) took the results of Faraday’s experiments and provided a mathematical basis for the conclusion that light, electricity and magnetism were all forms of the same wave force.  Heinrich Hertz (Germany) provided experimental confirmation of Maxwell’s theory by propagating electromagnetic, or radio waves in his laboratory in 1886-1887. In 1900, Max Planck (Germany) proposed that light and other electromagnetic radiation consisted of waves that could gain and lose energy only in finite amounts or quanta. German physicist Albert Einstein’s 1905 paper on the photoelectric effect suggested that quanta were real, and Arthur Holly Compton (US) in 1923 showed that certain behavior of X-rays could be explained by particles, but not waves. In 1926, Gilbert N. Lewis (US) named the electromagnetic quanta ‘photons.’

A chart that explores the wave-particle duality of light and other electromagnetic radiation.

BACTERIA (1676)
One of the earliest and most common forms of life on Earth, bacteria are usually a few micrometers long. Antonie van Leeuwenhoek (The Netherlands) first observed bacteria in 1676, using a single-lens microscope. After 1773, Otto Frederik Müller distinguished bacillum and spirillum bacteria. Christian Gottfried Ehrenberg (Germany) coined the term ‘bacterium’ in 1828 to describe certain rod-shaped bacteria. Robert Koch (Germany) identified the bacteria that cause anthrax (1875), tuberculosis (1882) and cholera (1883). In 1977, Carl Woese (US) recognized that, based on their ribosomal RNA, some organisms formerly considered bacteria belonged to another domain or kingdom, the Archaea.

A cross-section diagram of an average bacterium.

COMETS (1705)
While comets have been observed since ancient times, the first modern scientific theory of comets was developed by Tycho Brahe (Denmark), who measured the parallax of the Great Comet of 1577 and determined that it must exist outside the Earth’s atmosphere.  Isaac Newton (England) demonstrated the orbit of the comet of 1680 in his Principia of 1687. In 1705, Edmund Halley (England) analyzed 23 appearances of comets between 1337 and 1698 and concluded that three of the appearances were the same comet, which he predicted would return in 1758-1759. (Three French mathematicians further refined the date.) When the comet returned as scheduled, it was named Halley’s Comet. Scientists of the 17^th, 18^th and 19^th centuries proposed various theories for the composition of comets, but it was Fred Lawrence Whipple (US) who suggested in 1950 that comets were made of ice mixed with dust and rock – the ‘dirty snowball’ theory. A number of observations appeared to confirm this view, but in 2001, high resolution images of Comet Borrelly showed no ice, only a hot, dry, dark surface. Another probe, which crashed into Comet Temple 1 in 2005, found that most of the ice is beneath the surface.

Comet McNaught in 2007.

REFRIGERATION & THE REFRIGERATOR (1748)
William Cullen (Scotland) invented artificial refrigeration at the University of Glasgow in 1748. Oliver Evans (US) created the vapor-compression refrigeration process in 1805.  Jacob Perkins (US) took Evans’s process and built the first actual refrigerator in 1834.  John Gorrie (US) invented the first mechanical refrigeration unit in 1841. Further improvements were made by Alexander Twining (US) in 1853; James Harrison (Scotland/Australia) in 1856; Ferdinand Carré (France) in 1859; Andrew Muhl (France/US) in 1867 and Carl von Linde (Germany) in 1895. Electrolux produced the first electric refrigerator in 1923.

This photo is said to show a refrigerator made by Jacob Perkins (1766-1849) built in the early 19th Century.

THE LIGHTNING ROD (1749)
A lightning rod is a metal rod or other object mounted on top of a building or other elevated structure that is electrically bonded using a wire or electrical conductor to connect with a ground through an electrode, in order to protect the structure if lightning hits it. For thousands of years, builders in Sri Lanka have protected their buildings from lightning by installing metal tips made of silver or copper on the highest point. The Leaning Tower of Nevyansk in Russia, which was built between 1721 and 1745, is crowned with a metal rod that is grounded and pierces the entire building, but it is not known whether it was intended as a lightning rod. Benjamin Franklin (US) invented the lightning rod in 1749. Prokop Diviš (Bohemia) independently invented the grounded lightning rod in 1754.

This lightning rod at the Franklin Institute in Philadelphia, Pennsylvania, is believed to have been made by Benjamin Franklin (1706-1790).

LIGHTNING (1752)
Lightning was in the air in the late 1740s and early 1750s. Benjamin Franklin (US) listed a dozen analogies between lightning and electricity in his notebooks in 1749. Similar speculation by Jean Antoine Nollet (France) led to a French essay contest on the topic, which was won in 1750 by Denis Barbaret (France), who said lightning was caused by the triboelectric effect. Jacques de Romas (France) proposed a similar theory in a 1750 memoir; he also claimed to have suggested a test of the theory using a kite. In 1752, Franklin proposed to test the theory by using rods to attract lightning to a Leyden jar. The experiment was carried out by Thomas-François Dalibard in May 1752 and by Franklin himself in June 1752, but using a kite instead of a rod. He attached a key to the kite string, which was connected to a Leyden jar. Although the kite was not struck by lightning, static electricity was conducted to the key, and Franklin felt a shock when he moved his hand near the key. Georg Wilhelm Richmann (Germany/Russia) was killed by electrocution while attempting to recreate the experiment in St. Petersburg in 1753.

An 1876 rendering of Benjamin Franklin’s kite-flying experiment by Currier & Ives. © Museum of the City of New York/Corbis.

COMBUSTION (1772)
Combustion is a sequence of exothermic chemical reactions between a fuel and an oxidant that is accompanied by the production of heat and the conversion of chemical species. The release of heat can produce light in the form of glowing or flames. Modern scientific attempts to determine the nature of combustion began in 1620, when Francis Bacon (England) observed that a candle flame has a structure. At about the same time, Robert Fludd (England) described an experiment in a closed container in which he determined that a burning flame used up some of the air.  Otto von Guericke (Germany) demonstrated in 1650 that a candle would not burn in a vacuum. Robert Hooke (England) suggested in 1665 that air had an active component that, combined with combustible substances when heated, caused flame. Antoine-Laurent Lavoisier (France) was the first to give an accurate account of combustion when in 1772 he found that the products of burned sulfur or phosphorus outweighed the initial substances, and he proposed that the additional weight was due to the combining of the substances with air. Later Lavoisier concluded that the part of the air that had combined with the sulfur was the same as the gas released when English chemist Joseph Priestley heated the metallic ash of mercury, which was the same as the gas described by Carl Wilhelm Scheele (Sweden) as the active fraction of air that sustained combustion. Lavoisier gave the name ‘oxygen’ to the gas found by Priestley and Scheele.

A drawing of one of the experiments Antoine Lavoisier (1743-1794) conducted to discover the nature of combustion.

THE HOT-AIR BALLOON (1783)
The ability to raise small unmanned balloons into the air using hot air was known in China from the 3rd Century CE.  French brothers Jacques and Joseph Montgolfier built the first hot-air balloons capable of carrying human passengers in the late 18th Century.  They tested their design first with no passengers on June 4, 1783, then on September 19, 1783 with a sheep, a duck and a rooster, who survived an eight-minute flight. Then, on November 21, 1783, French scientist Pilâtre de Rozier and the Marquis d’Arlandes, an Army officer, climbed aboard a Montgolfier balloon to make the first untethered manned flight. They traveled for 25 minutes, covered a distance of five miles and attained an altitude of 3,000 feet before safely landing.  Among those in the audience were King Louis XVI and Benjamin Franklin.

A 1786 illustration of the first manned balloon flight, just three years earlier.

THE COTTON GIN (1793)
A cotton gin separates the cotton seeds from the fibers, a task previously done by hand. Primitive labor-intensive gins had been invented in India (5th Century CE) and elsewhere, but American Eli Whitney’s 1793 hand-powered cotton gin was the first mechanical cotton gin that efficiently separated fibers and seeds from large amounts of cotton.  Whitney’s invention revolutionized the U.S. cotton industry and led to the growth of slave labor in the South. Modern cotton gins are automated and much more productive than Whitney’s original.

Eli Whitney (1765-1825) created this working model of his cotton gin in 1800 to use in court while defending his patent against multiple infringers.

THE LOCOMOTIVE (1804)
In 1804, Richard Trevithick’s first steam locomotive pulled a train containing 10 tons of iron and 70 passengers in five cars approximately nine miles near Merthyr Tydfil in Wales.  The first commercially successful steam locomotives were built by Matthew Murray (UK) in 1812 (Salamanca); and Christopher Blackett & William Hedley (UK) in 1813 (Puffing Billy).  George Stephenson (UK) improved on Trevithick’s and Hedley’s designs by adding a multiple fire tube boiler in 1814 with the Blücher and again in 1825 with the Locomotion and in 1929 with The Rocket.  The largest steam-powered locomotive was the Union Pacific’s Big Boy, (US) of 1941. Steam locomotives were gradually phased out in the first half of the 20th Century, to be replaced by diesel and electric locomotives.

An 1862 photo of the 1813 steam locomotive Puffing Billy.

PHOTOGRAPHY (1826)
Joseph Nicéphore Niépce (France) created what was probably the first photograph on bitumen-covered pewter in 1826. His photographic method required an exposure of eight hours or more, and the final image was only viewable when held at an angle. In 1835 William Talbot (UK) created a method using a paper negative that allowed multiple positive prints from the same exposure. In 1837, Louis-Jacques-Mandé Daguerre (France) created a process with a much shorter exposure time and much clearer images, called daguerrotypes. Unfortunately, there was no way to make multiple copies of daguerrotypes, which were direct positive images on silver plate. Alexandre Becquerel and Claude Niepce de Saint-Victor (France) produced the first color images between 1848–1860. John Carbutt (US) produced the first commercially successful celluloid film in 1888. Also in 1888, George Eastman (US) introduced the hand-held Kodak camera with roll film. Kodak also introduced Kodachrome, the first commercial color film with three emulsion layers, in 1935.

The first photograph, from 1826, “View from the Window at Le Gras.”

OHM’S LAW (1827)
Ohm’s Law states that the ratio of the potential difference between the ends of a conductor and the current flowing through it is constant, and that ratio equals the resistance of the conductor. (Alternately, the law states that the current through a conductor between two points is directly proportional to the potential difference across the two points.)  Ohm’s Law establishes the relationship between strength of electric current, electromotive force, and circuit resistance. Henry Cavendish (England) arrived at a formulation of Ohm’s Law in 1781 but he did not communicate his results at the time.  Georg Ohm conducted experiments on resistance in 1825 and 1826 and published his results, including a more complicated version of Ohm’s Law, in 1827.

A diagram of Ohm’s Law.

THE SYNTHESIS OF UREA (1828)
Dutch scientist Herman Boerhaave discovered urea in urine in 1727. In 1828, Friedrich Wöhler (Germany) synthesized urea by treating silver cyanate with ammonium chloride. This was the first artificial synthesis of an organic compound from inorganic materials. It had important consequences for organic chemistry and also provided evidence against vitalism, the notion that living organisms are fundamentally different from inanimate matter.

An 1856 lithograph of Friedrich Wöhler (1800-1882).

DINOSAURS (1831)
In ancient times, dinosaur fossils were explained as the bones of a giant race of humans that have vanished from the Earth.  More scientific approaches came in the 19^th Century. In 1808, Georges Cuvier (France) identified a German fossil as a giant marine reptile that would later be named Mosasaurus. He also identified another German fossil as a flying reptile, which he named Pterodactylus. Cuvier speculated, based on the strata in which these fossils were found, that large reptiles had lived prior to what he called “the age of mammals.” Cuvier’s speculation was supported by a series of finds in Great Britain in the next two decades. Mary Anning (UK) collected the fossils of marine reptiles, including the first recognized ichthyosaur skeleton, in 1811, and the first two plesiosaur skeletons ever found, in 1821 and 1823. Many of Anning’s discoveries were described scientifically by the British geologists William Conybeare, Henry De la Beche, and William Buckland. Anning first observed that stony objects known as “bezoar stones”, which were often found in the abdominal region of ichthyosaur skeletons, often contained fossilized fish bones and scales when broken open, as well as sometimes bones from small ichthyosaurs. This led her to suggest to Buckland that they were fossilized feces, which he named coprolites. In 1824, Buckland found and described a lower jaw that belonged to a carnivorous land-dwelling reptile he called Megalosaurus. That same year Gideon Mantell (UK) realized that some large teeth he had found in 1822 belonged to a giant herbivorous land- dwelling reptile that he named Iguanodon because the teeth resembled those of an iguana. In 1831, Mantell published an influential paper entitled “The Age of Reptiles” in which he summarized the evidence for an extended time during which giant reptiles roamed the Earth.  Based on the appearance of the different giant reptile fossils in the rock strata, Mantell divided the era into three intervals, which anticipated the modern division of the Mesosoic era into the Triassic, Jurassic, and Cretaceous periods. In 1832, Mantell found a partial skeleton of an armored reptile he called Hylaeosaurus. In 1841 the English anatomist Richard Owen created a new order of reptiles, which he called Dinosauria, to contain Megalosaurus, Iguanodon, and Hylaeosaurus.

A portrait of Mary Anning (1799-1847) and her dog, painted before 1833.

THE STELLAR PARALLAX (1838)
A stellar parallax is the apparent shift of position of a nearby star against the background of distant objects that is made possible by the movement of the Earth in its orbit. Once a stellar parallax is measured, the distance to the star can be determined using trigonometry. The distance of most stars from the Earth makes stellar parallax so difficult to detect that some scientists argued that it did not exist. For example, James Bradley tried but could not measure stellar parallaxes in 1729.  Then, in 1838, Friedrich Bessel (Germany) measured the stellar parallax for the star 61 Cygni using a Fraunhofer heliometer.  This discovery was closely followed by Thomas Henderson (Scotland) for the star Alpha Centuri in 1839, and Friedich von Struve (Germany) for the star Vega in 1840.

An 1839 portrait of Friedrich Bessel (1784-1846).

NEPTUNE (1846)
Neptune is the eighth and farthest planet from the sun. It is the fourth largest planet by diameter and the third largest by mass. In 1821, French astronomer Alexis Bouvard published tables of the orbit of Uranus that contained significant discrepancies, which led to the prediction of another planet. In 1835, Benjamin Valz (France), Friedrich Bernhard Gottfried Nicolai (German) and Niccolo Cacciatore (Italy) each independently conjectured that a trans-Uranian planet caused the otherwise inexplicable discrepancies in the historical record of the orbits of both Halley’s comet and Uranus. Using Bouvard’s tables, both Urbain Jean Joseph Le Verrier (France) and John Couch Adams (UK), working independently, calculated the location where the new planet should be found in 1846. On September 23, 1846, German astronomer Johann Gottfried Galle, with the assistance of Heinrich Louis d’Arrest, observed the new planet within one degree of the predicted location.

A 1989 photograph of Neptune taken by the Voyager 2 spacecraft.

BOOLEAN LOGIC (1847)
George Boole (UK/Ireland) developed Boolean algebra and Boolean logic in books published in 1847 and 1854. Boolean algebra has been fundamental in the development of digital electronics and is used in set theory and statistics. Many are familiar with it as the basis for computer database search engines.

George Boole (1815-1864).

ASPIRIN (1853)
Aspirin is acetylsalicylic acid.  In ancient times, plants containing salicylate, such as willow, were used to prepare medicines.  There are references to it in Egyptian manuscripts from between 2000 and 1000 BCE and Hippocrates mentions salicylic tea to reduce fever in 400 BCE. Willow bark extract was a common remedy in the 18th and early 19th centuries, after which pharmacists began to experiment with and prescribe chemicals related to salicylic acid.  French chemist Charles Frédéric Gerhardt first produced acetylsalicylic acid in the lab in 1853. A pure form of the chemical was synthesized by Felix Hoffmann (Germany), a chemist with the Bayer Company, in 1897, and it was soon marketed all over the world. Sales rose after the flu epidemic of 1918, but dropped after the introduction of acetaminophen in 1956 and ibuprofen in 1962. Aspirin sales once again increased in the last decades of the 20th Century, when scientists discovered aspirin’s anti-clotting benefits.

Felix Hoffmann (1868-1946).

SPONTANEOUS GENERATION DISPROVED (1859)
It took a long time for the idea of spontaneous generation – that living things could arise from non-living matter – to die. Francesco Redi (Italy) proved in 1668 that maggots did not spontaneously generate from rotten meat but were hatched from tiny eggs laid by flies. Lazzaro Spallanzani (Italy) conducted an experiment in 1768 that supported Redi’s conclusion and contradicted the 1745 experiment of John Needham that seemed to support spontaneous generation. Louis Pasteur (France) put the final nails in spontaneous generation’s coffin in 1859 with an experiment in which no life grew in a sterile flask for a year until the neck of the flask was removed and microorganisms had access to the liquid inside. John Tyndall conducted further investigations in 1875-1876 to support Pasteur’s work and dispel any lingering objections to his conclusion, although his experiments were plagued by airborne bacterial spores.

A diagram of Louis Pasteur’s experiment disproving spontaneous generation.

OIL WELLS & OIL DRILLING (1859)
Digging or drilling for underground oil dates back to the 4th Century CE in China, where drill bits were attached to bamboo poles to dig wells of up to 800 feet deep. People in Arabian countries and Persia dug for oil as far back as the 9th Century.  Also from the 9th to the 16th centuries, those living near Baku, in modern-day Azerbaijan, hand dug holes of up to 115 feet.  Also in Baku, the first offshore drilling began in 1846.  The first recorded land-based commercial oil well was begun in Oil Springs, Ontario in 1858. But American Edwin Drake’s drilling operation in Titusville, Pennsylvania in 1859 was the first oil well using modern principles.  One of Drake’s key innovations was the drive pipe – he drove a cast iron pipe into the ground and then lowered the drill through the pipe, thus preventing the hole from collapsing.

A replica of the engine house and derrick at Drake’s Well in Titusville, Pennsylvania.

ANTISEPTIC (1867)
An antiseptic is a substance applied to living tissue or skin to kill microbes and reduce the possibility of infection or sepsis. Sumerian clay tablets from 2150 BCE and writings of Hippocrates (c. 400 BCE) and Galen (c. 130-200 CE) all advocate the use of antiseptic agents. In the early 13^th Century, Italian surgeons Hugh of Lucca and Theodoric of Lucca disregarded Galen’s view that pus was good and cleaned pus from wounds, then used wine to clean the wound and prevent infection. In an 1843 paper that was reissued in 1855, Oliver Wendell Holmes (US) advocated cleaning of medical instruments to prevent the spread of puerperal fever. In 1861, Ignaz Semmelweis (Austria) recommended that physicians wash their hands in chlorine solution before assisting in childbirth. While serving in the Confederate Army in the American Civil War in the early 1860s, George H. Tichenor (US) used alcohol on wounds. The adoption of antiseptic practices only became mainstream after British surgeon Joseph Lister’s 1867 paper, Antiseptic Principle of the Practice of Surgery, in which he advised the use of carbolic acid to create a sterile surgical environment.

A 1902 photograph of Joseph Lister (1827-1912).

HORMONES (1902)
Hormones are signaling molecules produced by the glands of living organisms that are transported to distant target organs by the circulatory system in order to regulate physiology and behavior. In 1894, George Oliver and Eduard Albert Sharpey-Schaeffer (UK) demonstrated the effect of an extract of the adrenal gland (the hormone adrenaline), which contracted blood vessels and muscles and raised blood pressure. In 1902, Ernest Starling and William Bayliss (UK) discovered secretin, which upon stimulation was released from the duodenum and carried to the pancreas, where it stimulated the pancreas to release digestive juices into the intestine. In 1905, Starling and Bayliss coined the term ‘hormone’ to describe secretin and similar substances. Edward C. Kendall isolated the thyroid hormone thyroxin in 1915. The same year, Walter Bradford Cannon (US) demonstrated the close connection between endocrine glands and emotions.

A diagram showing the sources of some human hormones.

EQUIVALENCE OF MASS AND ENERGY (1905)
Albert Einstein’s famous equation ‘E = mc^2’ states the physical law that matter and energy are two forms of the same substance, that one can be converted to the other, and that the amount of energy produced by converting (i.e., destroying) even a small amount of mass is enormous, as it is proportional to the square of the speed of light. A number of precursors led up to Albert Einstein’s revolutionary equation. In 1717, Isaac Newton wondered whether particles of mass and particles of light might be converted into one another. Emanuel Swedenborg (Sweden) speculated in 1734 that matter was made of points of potential motion. Numerous physicists at the end of the 19^th and beginning of the 20^th Century sought to understand how electromagnetic fields affect the mass of charged particles. Albert Einstein (Germany) first introduced a mass-energy equivalence equation in his 1905 paper on special relativity; it was later reduced to the famous form of E = mc². The equivalence of mass and energy has been experimentally proven in both directions.  In 1932, John Cockcroft and E.T.S. Walton (UK) broke apart an atom, releasing energy, and found that the total mass of the fragments had decreased slightly, proving the conversion of mass into energy. In 1933, Irène and Frédéric Joliot-Curie (France) detected the conversion of energy into mass when they photographed a photon (a quantum of electromagnetic energy) converting into two subatomic particles.

A sculpture entitled “The Theory of Relativity”, designed by Scholz & Friends, was displayed in Berlin, Germany in 2006 as part of the “Walk of Ideas.”

THE PHOTOELECTRIC EFFECT (1905)
Many metals emit electrons when light shines on them, a phenomenon known as the photoelectric effect. Heinrich Hertz (Germany) discovered the photoelectric effect in 1887. In 1905, Albert Einstein (Germany) discovered that the results of experiments measuring the photoelectric effect could be explained if light energy was carried in discrete quantized packets, or quanta. Einstein’s explanation lent support to quantum theory.

A diagram of the photoelectric effect.

BROWNIAN MOTION (1905)
Brownian motion refers to the random movements of particles suspended in a liquid or gas fluid that result from collisions with smaller atoms or molecules in the fluid. Dutch scientist Jan Ingenhousz described the irregular motion of coal dust particles on the surface of alcohol in 1785, an early example of Brownian motion. The official discovery of Brownian motion took place in 1827, when Scottish botanist Robert Brown noted the unusual random movements of pollen grains suspended in water. Thorvald N. Theile (Denmark) provided the mathematical underpinnings of Brownian motion in 1880, and Louis Bachelier (France) used the model of Brownian motion to explain the stochastic processes of economic markets in a 1900 thesis. In 1905, Albert Einstein (Germany) explained Brownian motion as the result of the larger particle (e.g., pollen grain) being moved by individual molecules of the fluid in which it is suspended (e.g., water). Einstein’s explanation proved definitively that atoms and molecules exist. The predictions of Einstein’s paper were verified experimentally in 1908 by Jean Perrin (France).

An animated example of Brownian motion.

BLACK HOLES  (1916)
A black hole is a region of spacetime, the gravitational pull of which is so strong that nothing, not even electromagnetic radiation, can escape it. The boundary of the region from which there is no escape is called the event horizon. According to the general theory of relativity, a mass that is sufficiently compact will deform spacetime enough to form a black hole. Some scientists believe supermassive black holes lie at the center of many galaxies, including the Milky Way. John Michell (UK) in 1783 and Pierre-Simon Laplace (France) in 1796 both suggested that some objects might have such strong gravitational fields that light could not escape. In 1916, soon after Albert Einstein (Germany) published his general theory of relativity, Karl Schwarzschild (Germany) was the first to show mathematically that Einstein’s theory predicted black holes under certain conditions. Johannes Droste (The Netherlands?) followed up Schwarzschild’s findings in 1916-1917, finding that Schwarzschild’s solution to general relativity created a singularity (where some terms became infinite) at a point known as the Schwarzschild radius, which defines the event horizon. Arthur Eddington (UK) showed in 1924 that the singularity disappeared after a change of coordinates. Subrahmanyan Chandrasekhar (India) showed in 1931 that stars and other objects above a certain mass (1.4 solar masses) were inherently unstable and would eventually collapse. In 1939, Robert Oppenheimer (US) and others predicted that neutron stars larger than three suns would collapse into black holes. In 1958, David Finkelstein (US) was the first to describe a black hole as a region of space from which nothing could escape. Important theoretical discoveries about the nature of black holes were made by Roy Kerr (NZ) in 1963, Ezra Newman (US) in 1965, Werner Israel (Germany/South Africa/Canada), Brandon Carter (Australia) and David Robinson. The term ‘black hole’ was first used by journalist Ann Ewing in 1964; John Wheeler used the term in a 1967 lecture. Roger Penrose and Stephen Hawking (UK) showed in the late 1960s that singularities appear in generic solutions of general relativity. In the early 1970s, Hawking, Carter, James Bardeen (US) and Jacob Bekenstein (Mexico/Israel) formulated black hole thermodynamics and Hawking showed in 1974 that black holes should give off black body radiation. Black holes cannot be detected directly, but indirect evidence exists. The first indirect evidence of a black hole in an X-ray binary system, Cygnus X-1, was discovered by Charles Thomas Bolton, Louise Webster and Paul Murdin in 1972. Numerous other candidates have since been found.

An artist’s depiction of Cygnus X-1, which scientists believe is a black hole. It formed when a large star collapsed on itself. This black hole pulls matter from the supergiant blue star beside it. Image Credit: NASA/CXC/M.Weiss

THE PAULI EXCLUSION PRINCIPLE (1925)
According to the Pauli exclusion principle, no two electrons in an atom can be in the same quantum state; in other words, two electrons must have opposite spin, thus cancelling each other, and there can be no more than two in the same orbital. A number of discoveries led up to the articulation of the principle by Austrian physicist Wolfgang Pauli in 1925. In 1916, for example, Gilbert N. Lewis (US) stated that the atom tends to hold an even number of electrons in the shell and especially to hold eight electrons that are normally arranged symmetrically at the eight corners of a cube. In 1919, Irving Langmuir (US) suggested that the periodic table could be explained if the electrons in an atom were connected or clustered in some manner.  In 1922, Niels Bohr updated his model of the atom by assuming that certain numbers of electrons (for example 2, 8 and 18) corresponded to stable “closed shells.” Pauli tried to explain these empirical findings as well as the results of experiments on the Zeeman effect in atomic spectroscopy and in ferromagnetism. A 1924 paper by Edward Stoner (UK) pointed out that for a given value of the principal quantum number (n), the number of energy levels of a single electron in the alkali metal spectra in an external magnetic field, where all degenerate energy levels are separated, is equal to the number of electrons in the closed shell of the noble gases for the same value of n. This led Pauli to realize that the complicated numbers of electrons in closed shells can be reduced to the simple rule of one electron per state, if the electron states are defined using four quantum numbers. For this purpose he introduced a new two-valued quantum number, identified by Samuel Goudsmit and George Uhlenbeck (The Netherlands/US) as electron spin.

A 1945 photograph of Wolfgang Pauli (1900-1958).

TELEVISION (1925-1927)
As with so many technological advancements, choosing a specific date and a particular inventor fails to appreciate the contributions of so many people over a long period of time. Nevertheless, the chronology below supports the conclusion that the invention of television reached critical mass between 1925 and 1927, particularly with the work of Baird, Zworykin, Jenkins, Farnsworth and Bell Labs. An early milestone in the history of television was the first known transmission of a still image over an electronic wire by Abbe Giovanna Caselli (Italy) in 1862. In 1877, George Carey (US) designed a machine that would use selenium to allow people to see electrically-transmitted images; by 1880, he had built a primitive system with light-sensitive cells. In 1884, Paul Nipkow (Germany) sent images over wires with 18 lines of resolution using a rotating metal disk – this was the mechanical approach. In 1906, Lee De Forest (US) invented the Audion vacuum tube, which could amplify electronic signals. In the same year, Boris Rosing (Russia) combined a cathode ray tube with Nipkow’s disk to make a working television. In 1907, Rosing begin developing an electronic scanning method of reproducing images using a cathode ray tube – this was the electronic approach. In 1908, Alan Campbell-Swinton (UK) described how a cathode ray tube could be used as a transmitting and receiving device in a television system.  In 1909, Georges Rignoux and A. Fournier (France) demonstrated instantaneous transmission of images in a mechanical system. In 1911, Rosing and Vladimir Zworykin (Russia) developed a mechanical/electronic system that transmitted crude images. In 1923, Zworykin (now in the US) patented a TV camera tube, the iconoscope, and later the kinescope, or receiver, although a 1925 demonstration was unimpressive. On March 25, 1925, John Logie Baird (Scotland) demonstrated transmission of silhouette images. In May, 1925, Bell Labs transmitted still images. On June 13, 1925, Charles Francis Jenkins (US) transmitted the silhouette image of a moving toy over a distance of five miles.  Also in 1925, Zworykin patented a color TV system. On December 25, 1925, Kenjiro Takayanagi (Japan) demonstrated a mechanical/electronic system with 40 lines of resolution. In the USSR, Leon Theremin developed a series of increasingly higher resolution television systems, from 16 lines in 1925 to 100 lines in 1927. On January 26, 1926, Baird demonstrated a system with 30 lines of resolution, running at five frames per second, showing a recognizable human face.  Also in 1926, Kálmán Tihanyi (Hungary) solved the problem of low sensitivity to light in television cameras through charge-storage. In 1927, Philo Farnsworth patented the Image Dissector, the first complete electronic television system. On April 7, 1927, Herbert Ives and Frank Gray of Bell Labs (US) demonstrated a mechanical television system that produced much higher-quality images than any prior system. Charles Jenkins (US) received the first television station license in 1928. In 1929, Zworykin demonstrated both transmission and reception of images in an electronic system. After a series of improvements to his design, Farnsworth transmitted live human images in 1929. In 1931, Jenkins invented the Radiovisor and began selling it as a do-it-yourself kit. Manfred von Ardenne (Germany) demonstrated a new type of system in 1931. Farnsworth gave a public demonstration of an all-electronic TV system, with a live camera, on August 25, 1934. The BBC began the first public television service on November 2, 1936 with 405 lines of resolution.  In 1937, the BBC adopted new equipment that was far superior to prior systems.  In 1940, Peter Goldmark invented a mechanical color TV system with 343 lines of resolution.  In 1941, the US adopted a 525-line standard. In 1943, Zworykin developed an improved camera tube that allowed recording of night events. In 1948, USSR began broadcasting at 625-lines of resolution, which was eventually adopted throughout Europe. Cable television was introduced in 1948 to bring television to rural areas. Videotape broadcasting was introduced in 1956 by Ampex. In 1962, the launching of the Telstar satellite permitted international broadcasting. Color televisions began to outnumber black & white TVs in the 1970s.  Satellite television began in 1983. High definition TV appeared in 1998. Analog broadcast TV ended on June 12, 2009, leaving digital television remaining.

John Logie Baird (1888-1946), with one of his earliest television systems. In the end, it was the electronic path of Zworkin and Farnsworth that carried the day.

ANTIBIOTICS (1928)
People in ancient Egypt, China and Mesoamerica used molds to treat infected wounds. The Germ Theory of Disease, propagated by Louis Pasteur (France) in the mid-19th Century, sparked the search for antibiotic agents.  In 1871, Joseph Lister discovered that bacteria would not grow in mold-infected urine.  John Tyndall (Ireland/UK) noted fungal inhibition of bacteria in 1875. In 1887, Louis Pasteur and Jules-François Joubert (France) demonstrated the antibiotic effect. In 1895, Italian physician Vincenzo Tiberio noted that the Penicillum mold killed bacteria. In the 1890s, Rudolf Emmerich and Oscar Löw (Germany) created an antibiotic but it often failed. In 1904 Paul Ehrlich (Germany) sought the ‘magic bullet’ against syphilis and systematically tested hundreds of substances before finding Salvorsan in 1909. Alexander Fleming discovered penicillin in 1928. In 1932, German scientists at Bayer (Josef Klarer, Fritz Mietzsch and Gerhard Domagk) synthesized and tested the first sulfa drug, Prontosil. In 1939, Rene Dubos (France/US) created the first commercially manufactured antibiotic – tyrothricin – although it proved too toxic for systemic usage. In 1943, Selman Waksman (US), derived stretomycin from soil bacteria. In 1955, Lloyd Conover (US) patented tetracycline. In 1957, Nystatin was patented. SmithKline Beecham patented the semisynthetic antibiotic amoxicillin in 1981; it was first sold in 1998. A major concern throughout the history of antibiotics is the development of antibiotic resistant strains of bacteria, which is such a significant problem that researchers are looking for alternatives to antibiotic treatments.
A diagram showing the different mechanisms used by antibiotics to kill bacteria.

THE POSITRON (1932)
The positron (also known as the antielectron) is the antimatter counterpart of the electron that is part of the Standard Model. Paul Dirac (UK) suggested in 1928 that electrons can have both positive and negative charge. In a follow-up paper in 1929, Dirac suggested that the proton might be the negative energy electron. Robert Oppenheimer strongly disagreed with Dirac’s suggestion, which led Dirac in 1931 to predict the existence of an anti-electron with the same mass as an electron, which would be annihilated upon contact with an electron. Ernst Stueckelberg (Germany) and Richard Feynmann (US) developed the theory of the positron, while Yoichiro Nambu (Japan/US) applied the theory to all matter-antimatter pairs of particles. The first to observe the positron was Dmitri Skobelsyn (USSR) in 1929. The same year, Chung-Yao Chao (China/US) conducted similar experiments but results were inconclusive. Carl David Anderson (US) is acknowledged to have discovered the positron on August 2, 1932; he also coined the word.

Carl Anderson (1905-1991) was able to prove the existence of the positron from this 1932 photograph of a trail in a cloud chamber.

SULFA DRUGS (1932)
Sulfonamide (also known as sulphonamide) is the basis for several groups of drugs, some of which are antibacterial.  The first antibacterial sulfa drug was Prontosil, which has the chemical name sulfonamidochrysoidine. Although Paul Gelmo (Austria) had synthesized the chemical in 1909, he did not pursue his findings. Josef Klarer and Fritz Mietzsch (Germany) synthesized it at Bayer, and in 1932, Gerhard Domagk (Germany) discovered it was effective in treating bacterial infections in mice. Results of clinical human studies were published in 1935, but it was treatment of Franklin Roosevelt’s son’s bacterial infection in 1936 that led to widespread acceptance of the drug. In 1935, scientists at the Pasteur Institute (France) discovered that Prontosil is metabolized to sulfanilamide, a much simpler molecule, which, as Prontalbin, soon replaced Prontosil. The chemical nature of sulfanilamide made it easy for chests to link it to other molecules, which led to hundreds of sulfa drugs.

Gerhard Domagk (1895-1964).

DARK MATTER (1933)
Dark matter is a substance that scientists have proposed to explain certain gravitational effects in the universe. Although there is significant indirect evidence for the existence of dark matter, it has not been directly observed or detected. According to the dark matter hypothesis, it cannot be seen with telescopes and does not appear to emit or absorb electromagnetic radiation (including light) at any significant level. Some have suggested that it may be composed of an undiscovered subatomic particle. According to the most recent estimate, the known universe contains 4.9% ordinary matter; 26.8% dark matter and 68.3% dark energy. Jan Oort (The Netherlands) first proposed the existence of unseen matter in 1932 to explain the orbital velocities of stars in the Milky Way. Fritz Zwicky (Switzerland/US) suggested the existence of what he called ‘dark matter’ in 1933 to explain what appeared to be missing mass in measuring the orbital velocities of galaxies in clusters. In 1973, Jeremiah Ostriker and James Peebles (US) calculated mathematically that galaxies would collapse if they only contained the mass we can see. They proposed that an additional mass of three to ten times the size of the visible mass was necessary to explain the observed shapes of the galaxies. At about the same time, Kent Ford and Vera Rubin (US), using new photon detectors, found that the movement of hydrogen clouds in the Andromeda galaxy could not be explained if the majority of the galaxy’s mass was contained in the visible matter, but only if it was contained in invisible matter that existed outside the visible edge of the galaxy. In 2013, a team of scientists said they had discovered a weakly-interacting massive particle (WIMP) that could make up dark matter. In 2014, NASA’s Fermi Gamma-ray Space Telescope recorded high-energy gamma-ray light emanating from the center of the Milky Way that confirmed a prediction about dark matter.

A pie chart showing the composition of the universe.

SYNTHETIC FIBERS (1935)
In 1865, an artificial fiber made from cellulose was used to make acetate. Sir Joseph Swan (UK) invented the first artificial fiber in about 1883 by chemically modifying fibers from tree bark to create a cellulose liquid, from which the fibers were drawn. Swan displayed fabrics made from his material at an 1885 exhibition. Hilaire de Chardonnet (France) produced an artificial silk in the late 1870s from nitrocellulose. He displayed products made from the artificial fiber at an 1889 exhibition, but the material was extremely flammable and was not successful.  Arthur D. Little (US) reinvented acetate from cellulose in 1893. In 1894, Charles Frederick Cross, with Edward John Bevan and Clayton Beadle (UK), produced an artificial fiber from cellulose that they called viscose. Courtaulds Fibers (UK) produced viscose commercially in 1905 and in 1924 renamed it rayon. Camille and Henry Dreyfus (Switzerland) used acetate to make motion picture film and other products beginning in 1910. The Celanese Company (US), Camille Dreyfus founder, used acetate to make textiles beginning in 1924. The first completely synthetic fiber not based on naturally-occurring cellulose, was nylon, which was invented by Wallace Carothers (US) at DuPont in 1935. In 1938, Paul Schlack of I.G. Farben in Germany invented another form of nylon. DuPont began commercial production of nylon for use in women’s stockings as well as parachutes and ropes, among other things, in 1939. Polyester was invented by John Rex Whinfield and James Tennant Dickson (UK) at the Calico Printers’ Association in 1941; they patented their first polyester fiber as Dacron. Also in 1941, DuPont introduced acrylic, a new synthetic fiber that resembled wool, under the brand name Orlon.

Wallace Carothers (1896-1937) demonstrates the strength of his invention, nylon.

THE VIRUS (1935)
A virus is a very small infectious agent that replicates only inside the living cells of other organisms. In the mid-late 19^th Century, when Louis Pasteur (France) could not find a bacterial cause for rabies, he hypothesized that the disease might be caused by a pathogen too small to be seen by a microscope. In 1884, Charles Chamberland (France) created a filter with holes smaller than bacteria, which would become essential for studying viruses. Dmitri Iosefovich Ivanovsky (Russia) used the Chamberland filter to determine that the cause of the tobacco mosaic disease was a pathogen smaller than a bacterium, which he announced in an 1882 article. Martinus Beijerinck (The Netherlands) arrived at similar results in 1898; he coined the term ‘virus’ to describe the unseen pathogen. Also in 1898, Friedrich Loeffler and Paul Frosch (Germany) determined that foot and mouth disease in animals was caused by a virus. In 1914, Frederick Twort (UK) discovered the first bacteriophage, a type of virus that infects bacteria; he published his results in 1915 but they were ignored. French-Canadian microbiologist Félix d’Herelle discovered bacteriophages indendently, and announced his discovery in 1917. Ernst Ruska and Max Knoll (Germany) made the first electron micrographs of viruses in 1931. In 1935, Wendell Stanley (US) succeeded in crystallizing the tobacco mosaic virus in 1935, proving it was a particle, not a fluid, and that it was made largely of protein. Electron micrographs of the tobacco mosaic virus were made in 1939 and X-ray crystallography was performed on it by Bernal and Fankuchen in 1941. Rosalind Franklin (UK) discovered the full structure of the tobacco mosaic virus in 1955. As of 2014, scientists have identified over 2,000 species of virus.

A transmission electron micrograph of bacteriophage viruses attached to a bacterial cell wall.

COMPUTER SCIENCE (1936)
Charles Babbage (UK) is the grandfather of computer science.  Beginning in the 1810s, he developed a theory of computing machines, which he put into practice with progressively more complex designs. Babbage’s 1837 proposal for an Analytical Engine would possibly have been the first true computer, had it actually been built. It had expandable memory, an arithmetic unit, logical processing abilities, and the ability to interpret a complex programming language. Ada Lovelace (UK), who worked with Babbage, furthered his work by designing the first computer algorithm and by predicting that a computer would not only perform mathematical calculations but manipulate symbols of all kinds. Kurt Gödel (Austria/US) established the mathematical foundations of computer science in 1931 with his incompleteness theorem, which showed that every formal system contained limits to what could be proved or disproved within it. If Babbage was the grandfather, Alan Turing (UK) was the father of computer science.  In 1936, Turing (along with American Alonzo Church), formalized an algorithm containing the limits of what can be computed, as well as a purely mechanical model for computing. The Church-Turing thesis of the same year states that, given sufficient time and storage space, a computer algorithm can perform any possible calculation. Turing introduced the ideas of the Turing machine and the Universal Turing machine (which can simulate any other Turing machine) in 1937.  Turing machines are not real objects but mathematical constructs designed to determine what can be computed by any proposed computer.

Alan Turing (1912-1954).

NUCLEAR FISSION (1938)
In induced nuclear fission, the nucleus of an atom is split by bombarding it with a subatomic particle, often a neutron.  The fission process usually releases free neutrons and protons (in the form of gamma rays) and a very large amount of energy. In 1917, Ernest Rutherford (NZ/UK) used alpha particles to convert nitrogen into oxygen, the first nuclear reaction. Ernest Walton (Ireland) and John Cockcroft (UK) used artificially accelerated protons to split the nucleus of a lithium-7 atom into two alpha particles. In 1934, Enrico Fermi (Italy) and his team, bombarded uranium with neutrons, but concluded the experiments created new elements with atomic numbers higher than uranium. In 1934, Ida Noddack (Germany) suggested that Fermi’s experiments had actually broken the nucleus into several large fragments. After reading of Fermi’s results, Otto Hahn, Fritz Strassman (Germany) and Lise Meitner (Austria) began performing similar experiments until Meitner, a Jew, was forced to flee the Nazis to Sweden. In December 1938, Hahn and Strassman proved that bombarding uranium nuclei with neutrons had created barium, an element with 40% less atomic mass than uranium. In 1939, Meitner and Otto Robert Frisch (Austria) interpreted Hahn and Strassmann’s results as proof they had split the uranium nucleus; they coined the term ‘fission’ to describe the reaction. Firsch confirmed this theory experimentally in January 1939. Also in January 1939, a team at Columbia University, including Erico Fermi, replicated the nuclear fission experiment.

A replica of the nuclear fission experiment conducted by Otto Hahn (1879-1968) and Fritz Strassmann (1902-1980) in 1938, located at the Deutsches Museum in Munich.

THE NUCLEAR REACTOR (1942)
A nuclear reactor initiates and controls a sustained nuclear chain reaction. Heat from nuclear fission occurring in a nuclear reactor is used to generate electricity and propel ships. In 1933, Hungarian-American scientist Leó Szilárd recognized that neutron-caused nuclear reactions ould lead to a nuclear chain reaction. In 1934, Szilárd filed the first patent application for the idea of a nuclear chain reaction using neutrons bombarding light elements. After the discovery of nuclear fission of uranium in 1938, Szilárd and Enrico Fermi (Italy) confirmed experimentally in 1939 Otto Hahn and Fritz Strassmann’s prediction that nuclear fission released several neutrons, which were then available to bombard other nuclei. Also in 1939, Francis Perrin (France) and Rudolph Peierls (Germany/US) independently worked out the ‘critical mass’ of uranium needed to sustain the reaction. In 1939, Szilárd proposed that a nuclear chain reaction would work best by stacking alternate layers of graphite and uranium in a lattice, the geometry of which would define neutron scattering and subsequent fission events. In 1942, Ernico Fermi and his team at the University of Chicago (including Szilárd) created the first controlled, self-sustaining nuclear chain reaction (the first nuclear reactor) from ‘piles,’ using Szilárd’s lattice of uranium and graphite. (The term ‘reactor’ has since replaced ‘piles’.) A number of nuclear reactors were built by the US military beginning in 1943 as part of the Manhattan Project to build a nuclear weapon. The first nuclear reactor for civilian use was launched in June 1954 in the USSR.
A black and white photograph of Gary Sheehan’s 1957 painting showing Enrico Fermi (1901-1954) and other scientists observing the world’s first self-sustaining nuclear chain reaction, in the Chicago Pile No. 1 on December 2, 1942.

ONE GENE-ONE ENZYME (1941)
In the 1930s, while experimenting with the genes for eye color in fruit flies, George Beadle (US) and Boris Ephrussi (USSR/France) concluded that each gene was responsible for an enzyme (a type of protein) acting in the metabolic pathway of pigment synthesis. In 1941, Beadle and Edward Lawrie Tatum (US), using the bread mold Neurospora crassa, published their discovery that genes control cells by controlling the specificity of enzymes, i.e., one gene controls one enzyme so a mutation in a gene will change the enzymes available, causing the blockage of a metabolic step. With modifications, the one gene-one enzyme hypothesis remains essentially valid.

George Beadle and Edward Tatum won the 1958 Nobel Prize in Physiology/Medicine for their genetic research.

RADIOCARBON DATING (1947)
Radiocarbon dating uses carbon-14, a radioactive isotope of carbon, to determine the age of organic materials.  Radiocarbon dating works because radioactive carbon in an organism begins to decay at a predictable rate starting at the time of death. Radiocarbon dating is normally accurate for objects that are 50,000 years old or younger.  In a series of experiments beginning in 1939, Willard F. Libby (US) investigated isotopes of elements in organic material, including carbon-14. A 1939 paper by W.E. Danforth and S.A. Korff (US) on carbon-14 sparked Libby’s idea that radiocarbon dating might be possible. In 1946, Libby proposed that living matter might contain carbon-14 and went on to discover carbon-14 in organic methane.  The suggestion of using carbon-14 as a way to date organic materials came in a 1947 paper by Libby and others. Libby and James Arnold (US) announced in 1949 that they had used carbon-14 to date wood samples from the tombs of two Ancient Egyptian kings to 2800 BCE, plus or minus 250 years, which was consistent with independent dates of 2625 BCE, plus or minus 75 years. In the years following, scientists improved and refined the accuracy of radiocarbon dating.

Willard Libby (1908-1980).

TRANSPOSABLE ELEMENTS (1948)
Also known as transposons or jumping genes, transposable elements (TEs) are sequences of DNA that can change position within the genome. In a series of experiments with maize beginning in 1944, American biologist Barbara McClintock at Cold Spring Harbor Laboratory discovered in 1948 that certain parts of the chromosomes had switched positions, which disproved the common belief that genes had fixed positions. McClintock also showed that TEs sometimes reversed earlier mutations and may be responsible for turning genes on and off. McClintock reported her findings in a series of reports and articles between 1950 and 1953, but her work was largely ignored until after TEs were independently discovered in bacteria in 1967 and 1968 by E. Jordan, H. Saedler and P. Starlinger (US). In 1996, Philip SanMiguel (US) estimated that TEs make up a large proportion of the genome of eukaryotic organisms: 50% of human genes and up to 90% of maize genes.

Barbara McClintock (1902-1992).

POLIO VACCINE (1952)
Poliomyelitis (often called polio) is an acute infectious disease carried by a virus and spread from person to person. Polio, which can cause paralysis, was first identified by Jakob Heine (Germany) in 1840. Karl Landsteiner (Austria) identified the pathogen, poliovirus, in 1908. Failed early attempts to create a vaccine for polio were made by Maurice Brodie and John Kollmer (US), working independently, in 1936. The successful cultivation of human poliovirus in the laboratory in 1948 by Americans John Enders and Thomas Weller was a significant step for vaccine research. Jonas Salk (US) developed a vaccine using inactivated (i.e., dead) poliovirus in 1952, which was approved and released in 1955. Albert Sabin (Poland/US) used live but attenuated poliovirus to create a second, oral vaccine in 1957, which was licensed for use in 1962. The two vaccines have eliminated polio from most of the world.

Jonas Salk (1914-1995). Photo by Yousef Karsh.

GLOBAL WARMING & CLIMATE CHANGE (1958)
Global warming refers to a rise in the average temperature of the Earth’s climate system in recent years. Because 90% of the recent increase in temperature has been absorbed by the oceans, global warming is often used to refer to the average temperature of the air and sea at the Earth’s surface. Climate change, in this context, refers to changes in the climate, including temperature, caused by human activities. The major human activities influencing climate change are fossil fuel combustion, which sends gaseous emissions into the atmosphere, aerosols, carbon dioxide released by cement manufacture, land use, ozone depletion, animal agriculture and deforestation. While humans have long speculated how their activity affects the climate (e.g., 19^th Century Americans debated whether cutting down trees might affect rainfall), the modern science of climate change began in 1896, when Svante Arrhenius (Sweden) predicted the ‘greenhouse effect’ – as humans burned fossil fuels, they would add carbon dioxide to the atmosphere, which would raise the temperature. Arrhenius was not concerned about his conclusions, however, because he believed the warming would take thousands of years and would benefit humanity. Guy Steward Callendar, a British engineer and inventor, followed up on Arrhenius’s predictions in the 1930s, when he published a number of papers on the effects of human-caused carbon dioxide increases on global climate.  His estimates of temperature increases in the half-century before 1938 have been confirmed with modern detectors.  Scientists such as Canadian physicist Gilbert Plass followed up on Callendar’s work in the 1950s. Several important scientific discoveries in the 1950s increased concern in the scientific community about carbon dioxide. First, Hans Suess (Austria/US) found in 1955 that carbon dioxide released by burning of fossil fuels was not immediately absorbed by the ocean. Then, in 1957, work by Roger Revelle (US) showed that the ocean surface layer had a limited ability to absorb carbon dioxide.  Finally, in 1958, Charles David Keeling (US) published detailed, comprehensive measurements showing that the amount of carbon dioxide in the atmosphere was rising. In 1967, Syukuro Manabe (Japan) and Richard Wetherald (US) developed a detailed computer model of the climate incorporating convection, the first of many computer models that scientists used to manage the huge amount of data and many different variables involved in predicting climate over time.

Charles David Keeling (1928-2005).

THE INTEGRATED CIRCUIT (1958)
German physicist Werner Jacobi at Siemens AG designed the first integrated transistor amplifier in 1949. In 1952, Geoffrey Dummer (UK) suggested that a variety of standard electronic components could be integrated in a monolithic semiconductor crystal.  In 1956, Dummer built a prototype integrated circuit. In 1952, American Bernard Oliver invented a method of manufacturing three electrically connected planar transistors on one semiconductor crystal. Also in 1952, Jewell James Ebers (US) at Bell Labs created a four-layer transistor, or thyristor. William Shockley (US) simplified Ebers’s design to a two-terminal, four-layer diode, but it proved unreliable. Harwick Johnson (US) at RCA patented a prototype integrated circuit in 1953. In 1957, Jean Hoerni (Switzerland/US) at Fairchild Semiconductor proposed a planar technology of bipolar transistors. Three breakthroughs occurred in 1958: (1) Jack Kilby (US) at Texas Instruments patented the principle of integration and created the first prototype integrated circuits; (2) Kurt Lehovec (Czech Republic/US) of Sprague Electric Co. invented a method of isolating components on a semiconductor crystal electrically; and (3) Robert Noyce (US) of Fairchild Semiconductor invented aluminum metallization – a method of connecting integrated circuit components. Noyce also adapted Hoerni’s planar technology as the basis for an improved version of insulation.  Hoerni made the first prototype of a planar transistor in 1959. Jay Last and others at Fairchild built the first operational semiconductor integrated circuit on September 27, 1960. Texas Instruments announced its first integrated circuit in April 1960, but it was not marketed until 1961. Texas Instruments sued Fairchild in 1962 based on Kilby’s patent and the parties settled in 1966 with a cross-licensing agreement. The first integrated circuits with transistor-transistor logic instead of resistor-transistor logic were invented by Tom Long (US) at Sylvania in 1962. In 1964, both Texas Instruments and Fairchild replaced the resistor-transistor logic of their integrated circuits with diode-transistor logic, which was not vulnerable to electromagnetic interference.  In 1968, Italian physicist Federico Faggin developed the first silicon gate integrated circuit with self-aligned gates. The same year, Robert H. Dennard (US) invented dynamic random-access memory, a specialized type of integrated circuit. Also in the late 1960s, medium scale integration (MSI), in which each chip contained hundreds of transistors, was introduced.  The specialized integrated circuit known as a microprocessor was introduced by Intel in 1971. Large-scale integration (LSI), which arrived in the mid-1970s, brought chips with tens of thousands of transistors each.  Ferranti (Italy) introduced the first gate-array, the Uncommitted Logic Array (ULA) in 1980, which led to the creation of application-specific integrated circuits (ASICs). Very large-scale integration (VLSI) brought chips with hundreds of thousands of transistors in the 1980s and several billion transistors as of 2009.

A 1959 prototype integrated circuit, made by Jack Kilby (1923-2005).

MOON EXPLORATION (1959)
While man has been observing the Earth’s moon since ancient times, and Galileo Galilei made the first detailed telescopic observations in 1610-1612, physical exploration of the moon did not begin until September 14, 1959, when the USSR’s unmanned probe Luna 2 made a hard landing on the moon’s surface. Luna 3 photographed the far side of the moon for the first time on October 7, 1959.  Luna 9 made a soft landing on the moon and sent the first pictures from the moon’s surface on February 3, 1966.  Frank Borman, James Lovell and William Anders (US), in Apollo 8, became the first humans to enter lunar orbit and see the far side of the moon on December 24, 1968. Neil Armstrong and Edwin “Buzz” Aldrin in Apollo 11 (US) landed on the moon on July 20, 1969. The next day, Armstrong became the first man to walk on the moon. US astronauts Edwin Aldrin and Neil Armstrong land on the moon. The US sent a total of six manned missions to the moon between 1969 and 1972. There were 59 unmanned missions by the US or USSR between 1959 and 1976. Three Luna probes and six Apollo missions returned to Earth with moon rock samples. Japan sent probes into the moon’s orbit in 1990 and 2007. NASA and the Ballistic Missile Defense Organization launched orbiters in 1994 and 1998. A European Space Agency probe began orbiting the moon in 2004, then intentionally crashed in 2006. China send an orbital probe in 2007; it was intentionally crashed on the moon’s surface in 2009. A rover from a second Chinese orbiter soft-landed on the moon on December 14, 2013.  India sent an orbiter to the moon in 2008 and landed an impact probe on November 14, 2008. Many other orbiters and landers are planned for the future.

American astronaut Buzz Aldrin (1930- ) on the surface of the moon on July 20, 1969. Photo by Neil Armstrong (1930-2012).

QUASARS (1963)
Quasars (short for ‘quasi-stellar radio sources’) belong to a class of objects called active galactic nuclei: they are very luminous sources of electromagnetic energy with a high redshift. Most scientists now believe that a quasar is the compact region in the center of a galaxy that surrounds a supermassive black hole and that the quasar’s energy comes from the mass falling onto the accretion disc around the black hole. Allan Sandage (US) and others discovered the first quasars in the early 1960s. In 1960, a radio source named 3C 48 was tied to a visible object. John Bolton (UK/Australia) observed a very large redshift for the object but his claim was not accepted at the time. In 1962, Bolton and Cyril Hazard identified another such radio source, 3C 273. In 1963, Marten Schmidt (The Netherlands) used their measurements to identify the visible object associated with the radio source and obtain an optical spectrum, which showed a very high redshift (37% of the speed of light). Hong-Yee Chiu (China/US) first used the term ‘quasar’ to describe the new type of object in a 1964 article. Scientists debated the distance of quasars until the 1970s, when the mechanisms of black hole accretion discs were discovered. In 1979, images of a double quasar provided the first visual evidence of the gravitational lens effect predicted by Einstein’s general theory of relativity.

An X-ray photograph of quasar PKS 1127-145, taken at the Chandra X-ray Observatory in 2000. The quasar is a highly luminous source of X-rays and visible light that is located about 10 billion light years from Earth. The photo shows an X-ray jet a million light years long that probably resulted from the collision of a beam of high-energy electrons with microwave photons. Photo: NASA.

THE STANDARD MODEL (1973)
Also known as Standard Model of Quantum Field Theory and the Standard Model of Particle Physics, the Standard Model, which is the result of the work of many scientists over the period of 1970-1973 and after, summarizes the forces and particles that make up the universe. According to the Standard Model, there are three classes of elementary particles: fermions, gauge bosons, and the Higgs boson.  There are 12 fermions, all of which have spin ½; they include six leptons (including electrons, muons, and tauons and their neutrino counterparts), and six quarks (including up, charm, and top and their charge complements, down, strange, and bottom).  Leptons and quarks interact by exchanging generalized quanta, particles of spin 1.  Bosons, which have spin 1, are particles involved in the transmission of forces and include gluons, which carry the strong force that binds quarks together.  Thus bound together, the quarks form hadrons, including the protons and neutrons that make up atomic nuclei.  Bosons also include photons, which carry the electroweak force and attract electrons to orbit the nuclei.  Other weak interactions are carried by the W ^–, W⁺, and Z particles.  Additional forces are carried by gravitons and Higgs bosons. The combination of the electromagnetic force and the weak interaction into the electroweak force by Sheldon Glashow (US) and others in 1961 paved the way for the Standard Model.  The muon neutrino was first detected in 1962. In 1964, Murray Gell-Mann and George Zweig propose that hadrons are made of quarks. Steven Weinberg (US) and Abdus Salam (Pakistan) incorporated the Higgs mechanism into the electroweak theory in 1967.  Experimental confirmation of the electroweak theory came in 1973 when the CERN supercollider detected the neutral weak currents that were predicted to result from Z boson exchange. The Standard Model’s explanation of the strong interaction received experimental confirmation in 1973-1974 when it was shown that hadrons are composed of fractionally-charged quarks. In 1983, Carlo Rubbia discovered the W and Z bosons.  In 1995, the final undiscovered quark, the top quark, was discovered. The tau neutrino was detected in 2000 at Fermilab. The Higgs boson was finally discovered in the Large Hadron Collider at CERN in 2012.A diagram of the Standard Model, courtesy of Fermilab.

HYDROTHERMAL VENT BIOLOGY (1977)
A hydrothermal vent is a fissure in the planet’s surface from which geothermally heated water issues.  Hydrothermal vents are found near volcanic activity, in ocean basins and hotspots and in areas where tectonic plates are moving apart. Deep sea hydrothermal vents often form large features called black smokers. Although they have no access to sunlight, some hydrothermal vents are biologically active and host dense and complex communities based on chemosynthetic bacteria and archaea. A deep water survey of the Red Sea in 1949 revealed hot brines that could not be explained. In the 1960s, the hot brines and muds were confirmed and found to be coming from an active subseafloor rift. No biological activity was found in the highly saline environment. A team from Scripps Intitution of Oceanography led by Jack Corliss (US) found the first evidence of chemosynthetic biological activity surrounding underwater hydrothermal vents that formed black smokers along the Galapagos Rift in 1977; they returned in 1979 to use Alvin, a deep-water research submersible, to observe the hydrothermal vents directly.  Peter Lonsdale published the first paper on hydrothermal vent biology in 1979. Neptune Resource NL discovered a hydrothermal vent off the coast of Costa Rica in 2005. Among the deepest hydrothermal vents are the Ashadze hydrothermal field on the Mid-Atlantic Ridge (-4200 meters), a vent at the Beebe site in the Cayman Trough (-5000 meters), discovered in 2010 by scientists from NASA and Woods Hole Oceanographic Institute; and a series of hydrothermal vents in the Caribbean found in 2014 (-5000 meters). By 1993, more than 100 species of gastropods had been found in hydrothermal vent communities. Scientists have discovered 300 new species at hydrothermal vents, including the Pompeii worm, discovered by Daniel Desbruyères and Lucien Laubier (France) in 1980 and Craig Gary (US) in 1997 and the scaly-foot gastropod in 2001.

A hydrothermal vent with black smokers and a biological community with large numbers of tube worms.

INFLATION THEORY (1980)
According to inflation theory, the universe underwent an exponential expansion of space from 10 ‾36 seconds after the Big Bang to between 10 ‾33 and 10 ‾32 seconds post-Big Bang. Inflation theory purports to explain the origin of the large-scale structure of the universe. The origins of inflation theory go back to 1917, when Albert Einstein invoked the cosmological constant to prove that the universe was static. At about the same time, Dutch scientist Willem de Sitter, analyzing general relativity, discovered a formula that described a highly symmetric inflating empty universe with a positive cosmological constant. Some believe that inflation theory was proposed by Erast Gliner (USSR) in 1965, who was not taken seriously at the time. In the early 1970s, Yakov Zeldovich (USSR) noted that the Big Bang model had serious problems with flatness and horizon. Vladimir Belinski (USSR), Isaak M. Khalatnikov (USSR), and Charles Misner (US) tried to solve the problems.  American physicist Sidney Coleman’s study of false vacuums in the late 1970s raised important questions for cosmology. In 1978, Zeldovich drew attention to the monopole problem, a version of the horizon problem. In 1979, Alexei Starobinsky (USSR) predicted that the early universe went through a de Sitter phase, or inflationary era. In January 1980, Alan Guth (US) proposed scalar driven inflation to solve Zeldovich’s problem of the nonexistence of magnetic monopoles. In October 1980, Demosthenes Kazanas (Greece/US) suggested that exponential expansion might eliminate the particle horizon. Martin Einhorn (US) and Katsuhiko Sato (Japan) published a model similar to Guth’s in 1981. Guth’s theory and other early versions of inflation had a problem: bubble wall collisions. Andrei Linde (USSR/US) solved the problem in 1981, as did Andreas Albrecht and Paul Steinhardt (US), independently, with new inflation, or slow-roll inflation.  Linde revised the model in 1983, calling the new version chaotic inflation. Numerous scientists worked on calculating the tiny quantum fluctuations in the inflationary universe that led to the structure we see today, particularly at a 1982 workshop at Cambridge University. Predictions of inflation theory were experimentally confirmed in 2003-2009 by the Wilkinson Microwave Anisotropy Probe’s findings of the flatness of the universe. The first direct evidence of gravitational waves, announced by Harvard-Smithsonian Center astronomers on March 17, 2014, provides additional support for inflation.

Alan Guth (1947- ).

CRETACEOUS-PALEOGENE MASS EXTINCTION (1980)
At the end of the Cretaceous Period 66 million years ago, a mass extinction eliminated 75% of all animal and plant species, including the dinosaurs. Although many hypotheses have been offered to explain this mass extinction (one of several in Earth’s history), the predominant theory is that of Luis Alvarez, who proposed in 1980 that an asteroid impact resulted in the extinctions. In 1980, Alvarez, an American physicist, his son geologist Walter Alvarez and chemists Frank Asaro and Helen Michel (US) reported that the sedimentary rocks at the border between the Cretaceous and Paleogene (formerly Tertiary) periods contained an abnormally high amount of the rare element iridium, which is common in asteroids and comets. They suggested an asteroid impact occurred about 66 million years ago. The theory has been supported by additional evidence, including the finding of rock spherules formed by the impact and shocked minerals from intense pressure. The presence of thicker sedimentary layers and giant tsunami beds in southern US and Central America supported the idea that the asteroid impact site was nearby, a prediction confirmed by the discovery of a giant crater (110 miles in diameter) at Chicxulub along the coast of the Yucatan in Mexico in 1990. Some scientists believe that the asteroid was only one of several factors in the mass extinction.

A view of the Chicxulub impact crater in the Yucatan based on seismic readings.  Image courtesy of the Canada Geological Survey.

HIV (1983)
The human immunodeficiency virus (HIV) causes acquired immunodeficiency syndrome (AIDS), a highly fatal disease that cripples the immune system, allowing opportunistic infections and cancers to wreak havoc. AIDS was first observed in the US in 1981 in patients with a rare form of pneumonia and later a rare skin cancer called Kaposi’s sarcoma. In May 1983, a French research group led by Luc Montagnier (with Françoise Barré-Sinoussi) isolated a new retrovirus they called LAV (lymphadenopathy-associated virus), that appeared to be the cause of AIDS. In May 1984, an American team led by Robert Gallo discovered the same virus, which they named HTLV-III (human T lymphotropic virus type III). By March 1985, it was clear that LAV and HTLV-III were the same virus and in May 1986, the International Committee on Taxonomy of Viruses named the virus discovered by both groups HIV, for human immunodeficiency virus. Further study indicated that two types of HIV originated in primates in west-central Africa and transferred to humans in the early 20^th Century.

Luc Montagnier (1932- ) (left) and Robert Gallo (1937- ) in 2000.

FULLERENES/BUCKYBALLS (1985)
A fullerene is a molecule made entirely of carbon in the form of a hollow sphere, ellipsoid, tube or certain other shapes. Spherical fullerenes are also known as buckyballs. Cylindrical fullerenes are called carbon nanotubes or buckytubes. Sumio Iijima (Japan) had predicted the existence of the C 60 molecule (which became the first fullerene) in 1970 and identified it in a electron micrograph in 1980. R.W. Henson (US) had proposed the structure of C 60 in 1970 and made a model of it, but his results were not accepted. In 1973, Professor Bochvar (USSR) made a quantum-chemical analysis of C 60’s stability and calculated its electronic structure, but the scientific community rejected his conclusions. In 1985, Harold Kroto (UK) and Americans Richard Smalley, Robert Curl, James Heath and Sean O’Brien and at Rice University, in the course of experiments designed to mimic carbon clusters, discovered and prepared C-60, the first fullerene, which they named buckminsterfullerene, by firing an intense pulse of laser light at a carbon surface in the presence of helium and then cooling the gaseous carbon to near absolute zero.

A diagram of a buckyball.

THE WORLD WIDE WEB (1989)
In 1980, Tim Berners-Lee (UK), working at CERN in Switzerland, built a personal database of people and software models called ENQUIRE, that used hypertext.  In March 1989, Berners-Lee proposed a large hypertext database with typed links.  He began implementing his proposal on a NeXT workstation, calling it the World Wide Web.  Berners-Lee’s collaborator Robert Cailliau (Belgium) rewrote the proposal in 1990. By Christmas 1990, Berners-Lee had created the HyperText Transfer Protocol (HTTP), the Hypertext Markup Language (HTML), the first Web browser, the first HTTP server software, the first Web server and the first Web pages.  Nicola Pellow (UK) created Line Mode Browser, that allowed the system to run on any computer.  In January 1991, the first non-CERN servers came online. The Web became publicly available after August 23, 1991. The first American Web server was established at the Stanford Linear Accelerator Center by Paul Kunz and Louise Addis in September 1991. In 1993, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, under the lead of Marc Andreessen (US), introduced the Mosaic graphical web browser, which evolved into Netscape Navigator in 1994.  Also in 1993, Microsoft also released Cello, written by Thomas R. Bruce (US) at Cornell, a browser for Microsoft Windows.  As of 2008, there were one trillion public web pages on the World Wide Web.

Tim Berners-Lee (1955- ) in 2008.

EXTRASOLAR PLANETS (1992)
In 1584, Giordano Bruno (Italy) speculated that other stars had planets circling them. Although a number of 19th and early 20th Century astronomers claimed to have discovered planets around other stars, they have all been discredited. A 1988 claim by Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang that they had discovered a planet orbiting the star Gamma Cephei was tentative at the time but confirmed in 2003 after advances in technology.  In 1992, Aleksander Wolszczan (Poland) and Dale Frail (Canada/US) discovered two Earth-sized planets orbiting the pulsar PSR 1257+12, which is generally considered the first definitive detection of exoplanets, or extrasolar planets. The team found a third planet in 1994. In 1995, Michel Mayor and Didier Queloz (Switzerland) observed a giant planet in a four-day orbit around 51 Pegasi, the first detection of a planet circling a standard, or main sequence star. In 2009, the US launched the Kepler Space Telescope (KST), which has the mission of discovering Earth-like extrasolar planets. As of August 14, 2014, KST had facilitated the discovery of 1815 confirmed exoplanets in 1130 planetary systems, of which 466 are multiple planetary systems.  The smallest planet known is about twice the size of the moon, while the largest is 29 times the size of Jupiter. Some planets are so near their stars they their orbits take only a few hours, while some are so distant that they take thousands of years to complete one orbit. In March 2014, KST identified the first exoplanet that is similar to Earth in size with an orbit within the habitable zone (i.e., the area that would support life) of another star. The planet is Kepler-186f and the star is a red dwarf (Kepler 186) about 500 light years from Earth.

In 2005, scientists captured the first visual image (using false colors) of a planet outside our solar system.  The large bluish-white object is a brown dwarf star called 2MASSWJ1207334-3932, which is 230 light years from Earth.  The smaller red object is a planet called 2M1207 b.  The exoplanet has five times the mass of Jupiter and travels in an orbit that is five billion miles from its star, about twice the distance of Neptune from the Sun.

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AGRICULTURE & PLANT DOMESTICATION (11,000 BCE)
The move from hunter-gatherer to agriculture was gradual and occurred at different times in different places.  Evidence of humans exerting some control over wild grain is found in Israel in 20,000 BCE.  There is evidence of planned cultivation and trait selection of rye at a Syrian site dating to 11,000 BCE.  Domesticated lentils, vetch, pistchios and almonds were also found in Franchthi Cave in Greece at about 11,000 BCE.  Humans domesticated the eight founder crops of agriculture (emmer wheat, einkorn wheat, barley, peas, lentils, bitter vetch, chickpeas and flax) some time after 9500 BCE at various sites throughout the Levant (Syria, Lebanon, Palestine, Jordan, Cyprus and part of Turkey). The oldest known settlement associated with human agriculture is in Cyprus, dating from 9100-8600 BCE.  Rice and millet were domesticated in China by 8000 BCE. Farming was fully established along the Nile River by 8000 BCE (although there is evidence of agriculture in Egypt as early as 10,000 BCE) and in Mesopotamia by 7000 BCE. The first evidence of agriculture in the Indus Valley dates from 7000-6000 BCE. The first signs of agriculture in the Iberian peninsula date from 6000-4500 BCE.  The oldest field systems, including stone walls, are found in Ireland and date to 5500 BCE.  By 5500 BCE, the Sumerians had developed large-scale intensive cultivation of land, mono-cropping, organized irrigation and use of a specialized labor force.  By 5000 BCE, humans in Africa’s Sahel region had domesticated rice and sorghum. Maize was domesticated in Mesoamerica around 3000-2700 BCE.

A map showing the origin of various domesticated crops.

WRITING (3200 BCE)
Written language evolved from pictures and other symbols into proto-writing, and then true writing. Sumerian archaic cuneiform script was invented independently around 3200 BCE, although the first true written texts do not appear until about 2600 BCE. The first Egyptian hieroglyphics date to 3400-3200 BCE; the Indus script of Ancient India dates to 3200 BCE; and Chinese characters date to 1600-1200 BCE, but there is debate about whether these were independent discoveries or derived from pre-existing scripts. The Phoenicians began to develop the first phonetic writing system between 2000 and 1000 BCE. Mesoamerican cultures developed writing systems independently. The Olmecs of Mexico developed the first Mesoamerican writings, beginning about 900-600 BCE.

An inscription on a clay tablet written in the archaic cuneiform script and dating to c. 26th Century BCE.

IRON WORKING (3000-2700 BCE)
Iron, a common element in the Earth’s crust and in meteorites, is easily corroded, so few ancient iron artifacts remain. The earliest man-made iron objects that still exist were found in Iran and date to 5000-4000 BCE. They were made from iron-nickel meteorites, as were the earliest iron artifacts from Egypt and Mesopotamia, dating from 4000-3000 BCE, and China, from 2000-1000 BCE. The earliest evidence of smelting of native iron ore to create  wrought iron comes from Mesopotamia and Syria about 3000-2700 BCE and India in 1800-1200 BCE. The Hittites of Anatolia (present-day Turkey) created iron artifacts as early as 2500 BCE, but beginning in 1500-1200 BCE, they developed a sophisticated iron working industry that involved bellows-aided furnaces called bloomeries. The products of iron smelting also become more common in Mesopotamia, Egypt and Niger starting about 1500. By 1100-1000 BCE, the technology of smelting iron ore to make wrought iron had spread to Greece, sub-Saharan Africa and China and the Iron Age had begun. Wrought iron working reached central Europe in the 8th Century BCE and was common in Northern Europe and Britain after 500 BCE. Meanwhile in the 5th Century BCE, Chinese ironworkers produced the first cast iron, which was cheaper to produce than wrought iron. Cast iron production reached Europe in the Middle Ages. In 1709, Abraham Darby (UK) built a coke-fired blast furnace to make cast iron more efficiently. The next major improvement in wrought iron technology did not arrive until 1783, Englishman Henry Cort introduced the puddling process for refining iron ore.

The remains of one of the oldest bloomery iron smelting operations, located at Tell Hammeh in Jordan.

THE SHAPE OF THE EARTH (1500 BCE)
The notion that humans believed the Earth was flat until Christopher Columbus’s voyages in the 1490s is simply untrue.  The earliest suggestion that the Earth is a sphere comes from the Rig Veda, the ancient Hindu scripture, which was composed in India about 1500 BCE, although the oldest existing texts are much later. The Ancient Greeks also believed that the Earth was a sphere, but it is not clear who first made the discovery: Pythagoras in the 6^th Century BCE or Parmenides or Empedocles in the 5^th Century. Plato (Ancient Greece) asserted the roundness of the Earth in the early 4^th Century BCE. Later in the 4^th Century, Aristotle (Ancient Greece) reasoned that the Earth was a sphere because some stars are visible in the south that are not visible in the north, and vice versa. In 240 BCE, Eratosthenes (Ancient Greece) experimentally determined that the Earth was curved.

A diagram of Eratosthenes’ measurements of the Earth’s circumference.

THE ALPHABET (1400 BCE)
The alphabet may have its origins in the Proto-Sinaitic scripts that date to 1700 BCE, but there are not enough examples to be sure. The Ugarit writing of about 1400 BCE in Syria appears to use the first known alphabet. The Proto-Canaanite alphabet, precursor to the Phoenician alphabet, is known from about 1300 BCE. Scholars date the Phoenician alphabet to 1050 BCE. The Phoenician alphabet, which had no vowels, was the precursor to many of the writing systems in use today and throughout history.  It led directly to Aramaic, which led to Arabic and Hebrew.  When the Hellenic Greeks adopted the Phoenician alphabet in about 800 BCE, they converted some Phoenician letters to vowels, and the resulting language became the basis for the Latin, Cyrillic and Coptic scripts used in so many Western languages today.

The Phoenician alphabet and the alphabets derived from it.

THE CROSSBOW (600 BCE)
Crossbows were first used in China as weapons of war between 600 and 500 BCE.  Greek soldiers began using crossbows between 500-400 BCE.  Romans used crossbows in war and hunting between 50 and 150 CE.  There is evidence of crossbow use in Scotland between the 6th and 9th Centuries CE. In the early 11^th Century, crossbows with sights and mechanical triggers were developed. The invention of pushlever and ratchet drawing mechanisms in the Middle Ages allowed the use of crossbows on horseback.  The Saracens invented composite bows, made from layers of different material, and the Crusaders adopted the design upon their return to Europe.  By 1525, the military crossbow had mostly been replaced by firearms.

A reconstructed Greek crossbow, or gastraphetes, from the 5th Century BCE.

THE MAGNETIC COMPASS (206 CE)
Magnetic compasses were invented after humans discovered that iron could be magnetized by contact with lodestone and once magnetized, would always point north. There is some evidence that the Olmecs, in present-day Mexico, used compasses for geomancy (a type of divination) between 1400-1000 BCE. The first confirmed compasses, made in China about 206 BCE, were also used for divination and geomancy and used a lodestone or magnetized ladle. The first recorded use of a compass for navigation is 1040-1044 CE, but possibly as early as 850 CE, in China; 1187-1202 in Western Europe and 1232 in Persia.  Later (by 1088 in China), iron needles that had been magnetized by a lodestone replaced the lodestone or other large object as directional arm of the compass.  In many early compasses, the iron needle would float in water. The first dry needle compasses are described in Chinese documents dating from 1100-1250. Another form of dry compass, the dry mariner’s compass, was invented in Europe around 1300, possibly by Flavio Gioja (Italy). Further developments included the bearing compass and surveyor’s compass (early 18th Century); the prismatic compass (1885); the Bézard compass (1902); and the Silva orienteering compass (Sweden 1932). Liquid compasses returned in the 19th Century, with the first liquid mariner’s compass invented by Francis Crow (UK) in 1813. In 1860, Edward Samuel Ritchie invented an improved liquid marine compass that was adopted by the U.S. Navy. Finnish inventor Tuomas Vohlonen produced a much-improved liquid compass in 1936 that led to today’s models.

The first Chinese compasses used a spoon on a flat board. It is not clear if this image shows an actual Han Dynasty compass or a reproduction.

THE WINDMILL (800 CE)
Precursors to the windmill include the windwheel of Heron of Alexandria, a Greek engineer, and the prayer wheels that have been used in Tibet and China since the 4th Century. A windmill uses the power of the wind to create energy; the first windmills were used to mill grain. The first practical windmills were made in Persia in the 9th Century and had ‘sails’ that rotated in a horizontal plane, not vertical as we normally see in the West. This technology spread throughout the Middle East and Central Asia and later to China and India. A visitor to China in 1219 remarked on a horizontal windmill he saw there. Vertical windmills first appeared in an area of Northern Europe (France, England and Flanders) beginning in about 1175. The earliest type of European windmill was probably a post mill. The oldest known post mill, dating to 1191, is located in Bury St. Edmunds, England. By the late 13th Century, masonry tower mills were introduced; the smock mill was a 17th Century variation. Hollow-post mills arose in the 14th Century.

The horizontal windmills at Nashtifan in Iran, some of which are still working, were built during the Safavid Dynasty (1501–1736).

EYEGLASSES (c. 1286)
References to the use of lenses, jewels or water-filled globes to correct vision go back as far as the 5th Century BCE, but it was only after Arab scholar Ibn al-Haytham’s Book of Optics was translated into Latin between 1275 and 1280 that the stage was set for the invention of true eyeglasses in Italy by an unnamed individual in 1286 who was “unwilling to share them” according to a historian.  Alessandro di Spina (Italy) followed soon afterwards and “shared them with everyone.” The first eyeglasses used convex lenses to correct both farsightedness (hyperopia) and presbyobia. They were designed to be held in the hand or pinched onto the nose (pince-nez).  Some have speculated that eyeglasses originated in India prior to the 13th Century. The earliest depiction of eyeglasses is Tommaso da Modena’s 1352 portrait of Cardinal Hugh de Provence. A German altarpiece from 1403 also shows the invention. The first glasses that extended over the ears were made in the early 18th Century. Concave lenses to cure shortsightedness, or myopia, were not developed until c. 1450.

Detail of Tommaso da Modena’s 1352 portrait of Cardinal Hugh de Provence wearing eyeglasses.

THE GREGORIAN CALENDAR (1582)
In 45 BCE, Sosigenes of Alexandria developed a calendar and presented it to Roman emperor Julius Caesar, who adopted it for the Roman Empire as the Julian Calendar. In the Julian calendar, each year consisted of 365 days divided into 12 months, with a leap year every four years, creating an average year of 365.25 days.  Because a true solar year is slightly less than 365.25 days, the Julian calendar became out of sync with the seasons and religious holidays over the centuries. This led Pope Gregory XIII in 1582 to revise the calendar to skip three leap years every four centuries, which keeps the calendar in line with the seasons to this day. The average year is now 365.2425 days. Although some non-Western countries and religious groups maintain their own calendars, the Gregorian calendar is used universally for trade and international relations, including by the United Nations.

A copy of Pope Gregory’s 1582 proclamation of the new calendar.

THE THERMOMETER (1611)
Ancient Greek scientists had invented primitive thermoscopes, based on the principle that certain substances expanded when heated.  Scientists in Europe, including Galileo Galilei (Italy) in c. 1593, developed more sophisticated thermoscopes in the 16th and 17th centuries. The thermometer was born in 1611-1613, when either Francesco Sagredo or Santorio Santorio (Italy) first added a scale to a thermoscope. Daniel Gabriel Fahrenheit (Netherlands/Germany/ Poland) invented an alcohol thermometer in 1709 and the first mercury thermometer in 1718. Each inventor used a different scale for his thermometer until Fahrenheit suggested the scale that bears his name in 1724. That scale was becoming the standard when, in 1742, Anders Celsius (Sweden) suggested a different scale. The two scales have been in competition ever since. William Thomson, Lord Kelvin (UK) developed the absolute zero scale, known as the Kelvin scale, in 1848.

One of the original thermometers made by Daniel Fahrenheit (1686-1736), dating to between 1714 and 1724.

LOGARITHMS (1614)
The exponent that a fixed value, called the base, must be raised to in order to produce a number is called the logarithm of that number.  (E.g., the logarithm of 1000 in base 10 is 3 because 1000 = 10³.)  Precursors to logarithms were invented by the Babylonians in 2000-1600 BCE, Indian mathematician Virasena in the 8th Century CE; and Michael Stifel (Germany) in a 1544 book. In 1614, John Napier of Merchiston (Scotland) announced the discovery of logarithms, which highly simplified multiplication and addition calculations. Henry Briggs (England) created the first table of logarithms in 1617. Joost Bürgi (Switzerland) discovered logarithms independently before Napier, but did not publish until 1620. Alphonse Antonio de Sarasa (Flanders) related logarithms to the hyperbola in 1649. Natural logarithms were first identified by Nicholas Mercator (Germany) in 1668, but John Speidell (England) had been using them since 1619. Swiss mathematician Leonhard Euler vastly expanded the theory and applications of logarithms in the 18th Century by using them in analytic proofs, expressing logarithmic functions using power series, and defining logarithms for negative and complex numbers.

John Napier (1550-1617).

THE RINGS OF SATURN (1655)
Galileo Galilei (Italy) observed Saturn’s rings through a telescope in 1610, but did not identify them as rings, but as ears or a ‘triple form’. Galileo noted but could not explain the disappearance of the rings when Saturn was oriented directly at the Earth in 1612 and their reappearance in 1613. Christiaan Huygens (The Netherlands), using a 50-power refracting telescope, definitively identified Saturn’s rings in 1655. In 1666, Robert Hooke (England) also identified the rings and noted that Saturn cast a shadow on the rings. Giovanni Domenic Cassini (Italy) noted in 1675 that Saturn had multiple rings with gaps between them. In 1787, Pierre-Simon Laplace (France) suggested that the rings consisted of many solid ringlets, a theory that James Clerk Maxwell (UK) disproved in 1859 by showing that solid rings would become unstable and break apart. Maxwell proposed instead that the rings were made of numerous small particles, which was experimentally confirmed by James Keeler (US) and Aristarkh Belopolsky (Russia) in 1895 using spectroscopy.

A color enhanced photograph of Saturn’s rings.

BINARY NUMBERS & BINARY CODE (1679)
Binary numbers are numbers expressed in a binary or base-2 numeral system, which normally represents numeric values with the symbols zero and one. A binary code is text or computer processor instructions using the binary number system. An early form of binary system is used in the ancient Chinese book, the I Ching (2000 BCE?).  Between the 5th and 2nd Centuries BCE, Indian scholar Pingala invented a binary system.  Shao Yong (China) developed a binary system for arranging hexagrams in the 11th Century. Traditional African geomancy such as Ifá used binary systems and French Polynesians on the island of Magareva used a hybrid binary-decimal system before 1450.  Francis Bacon invented an encoding system in 1605 that reduced the letters of the alphabet to binary digits. Gottfried Leibniz (Germany), who was aware of the I Ching, invented the modern binary number system in 1679 and presented it in his 1703 article Explication de l’Arithmétique Binaire.  In 1875, Émile Baudot (France) added binary strings to his ciphering system.  In 1937, Claude Shannon (US), in his MIT master’s thesis, first combined Boolean logic and binary arithmetic in the context of electronic relays and switches. He showed that relay circuits, being switches, resembled the operations of symbolic logic: two relays in series are and, two relays in parallel are or, and a circuit which can embody not and or can embody if/then.  This last meant that a relay circuit could make a choice.  Since switches are either on or off, binary mathematics was therefore possible. George Stibitz (US), at Bell Labs, demonstrated a relay-based computer in 1937 that calculated using binary addition.  Stibitz and his team made a more complex version called the Complex Number Computer in 1940. Common binary coding systems include ASCII (American Standard Code for Information Interchange) and BCD (binary-coded decimal).

A page from Gottfried Leibniz’s 1703 article Explication de l’Arithmétique Binaire.

THE KINETIC THEORY OF GASES (1738)
According to the kinetic theory of gases, a gas consists of a large number of atoms or molecules in constant, random motion that constantly collide with each other and the walls of a container.  Lucretius (Ancient Rome) proposed in 50 BCE that objects were composed of tiny rapidly moving atoms that bounced off each other.  Daniel Bernoulli (Switzerland) proposed the kinetic theory of gases in 1738. He proposed that gas pressure is caused by the impact of gas molecules hitting a surface and heat is equivalent to the kinetic energy of the molecules’ motion. Other advocates of the kinetic theory included: Mikhail Lomonosov (Russia, 1747), Georges-Louis Le Sage (Switzerland, ca. 1780, published 1818), John Herapath (UK, 1816); John James Waterston (UK, 1843), August Krönig (Germany, 1856) and Rudolf Clausius (Germany, 1857).  James Clerk Maxwell (UK) formulated the Maxwell distribution of molecular velocities in 1859, and Ludwig Boltzmann (Austria) formulated the Maxwell-Boltzmann distribution in 1871. In papers on Brownian motion, Albert Einstein (Germany, 1905) and Marian Smoluchowski (Poland, 1906) made testable predictions based on kinetic theory.

The kinetic theory of gases states that the average kinetic energy of the atoms in an ideal gas can be determined by the temperature of the gas, while pressure is related to the impacts of the atoms on the walls of their enclosure.  This animation shows helium atoms under a pressure of 1950 atmospheres, drawn to scale, at room temperature.  Their movements have been slowed down by two trillion times.

THE LAW OF CONSERVATION OF MASS (1748)
According to the law of conservation of mass, the mass of any system that is closed to all transfers of matter and energy must remain constant over time. The law has ancient roots. The Jains in 6^th Century BCE India believed that the universe and its constituents cannot be created or destroyed.  In Ancient Greece, Empedocles said in the 5^th Century BCE that nothing can come from nothing and once something exists it can never be completely destroyed, a belief echoed by Epicurus in the 3^rd Century BCE. Persian philosopher Nasir al-Din al-Tusi stated a version of the law in the mid-13^th Century. The first modern scientific statement of the law came from Mikhail Lomonosov (Russia) in 1748. Although Antoine Lavoisier (France) is often credited with discovering the law in 1774, precursors include Jean Rey (France, 1583-1645), Joseph Black (Scotland, 1728-1799) and Henry Cavendish (UK, 1731-1810).

A diagram explaining the law of conservation of mass.

THE MARINE CHRONOMETER (1761)
A marine chronometer is a clock that is accurate enough to be a portable time standard, which can be used to determine longitude by using celestial navigation. Until the 18th Centuries, navigators were able to determine the latitude of a ship at sea, but not its longitude.  Gemma Frisius (The Netherlands) suggested in 1530 that a highly accurate clock could be used to calculate longitude.  In the 17th Century, Galileo Galilei (Italy), Edmund Halley (England), Tobias Mayer (Germany) and Nevil Maskelyne (England) proposed observations of astronomical objects as the solution, but the deck of a ship at sea proved too unstable for accurate measurements.  Recognizing that his pendulum clock would not be effective at sea, Christiaan Huygens (The Netherlands) invented a chronometer in 1675 with a balance wheel and a spiral spring, but it proved too inaccurate in nautical conditions.  Similar problems plagued the chronometers made by Jeremy Thacker (England) in 1714 and Henry Sully (France) in 1716.  In 1714, the British government offer a large cash reward for anyone who could invent an accurate chronometer. John Harrison (England) submitted versions in 1730, 1735 and 1741, although they were all sensitive to centrifugal force.  A 1759 version, with a bi-metallic strip and caged roller bearings, was even more accurate, but it was the much smaller 1761 design that won Harrison the £20,000 prize in 1765.  French clockmaker Pierre Le Roy’s 1766 chronometer, with a detent escapement, temperature-compensated balance and isochronous balance spring, was the first modern design. Thomas Earnshaw and John Arnold developed an improved version with Le Roy’s innovations in 1780, which led to the standard chronometer used for many years afterwards.

The 1761 ‘sea watch’ created by John Harrison (1693-1776) to solve the problem of determining longitude, which won him a prize.

THE SPINNING FRAME (1769)
In the mid-18^th Century, the English textile industry was growing, and its machines were becoming faster. The flying shuttle had doubled loom speed, and the invention of the spinning jenny in 1764 had also increased speed and production. John Kay and Thomas Highs (England) had designed a new machine called the spinning frame, which produced a stronger thread than the spinning jenny. The spinning frame used the draw rollers invented by Lewis Paul to stretch the yarn. In 1769, Sir Richard Arkwright (England) asked John Kay to produce the spinning frame for him. Because the spinning frame was too large to be operated by hand, Arkwright experimented with other power sources, trying horses first and then switching to the water wheel. Unlike the spinning jenny, which was inexpensive but required skilled labor, the spinning frame required considerable capital outlay but little skill to operate.

A 1790 spinning frame made by Slater, now in the Smithsonian Institution.

PLANT RESPIRATION & PHOTOSYNTHESIS (1779)
Flemish scientist Jan van Helmont discovered in the mid-17^th Century that the mass of the soil used by a plant changed very little as the plant’s mass increased. He hypothesized that the additional mass came from the added water. In 1774-1777, Joseph Priestley (England) published the results of experiments in which he burned a candle in a sealed jar, it quickly stopped burning, and that a mouse trapped in a jar would soon stop breathing, but he found that if he added a plant to the jar, both mouse and candle would continue to flourish. Priestley concluded that plants make and absorb gases. Following up on Priestley’s experiments, Jan Ingenhousz (The Netherlands) discovered that when light is present, plants give off bubbles from their green parts, which he identified as oxygen, but not in the shade, and that it was the oxygen that revived Priestley’s mouse. He also discovered that plants give off carbon dioxide in the dark, but that the amount of oxygen given off in the light is greater than the amount of carbon dioxide given off in the dark. In 1796, Jean Senebier (Switzerland) confirmed Ingenhousz’s finding that plants release oxygen in the light, and also found that they consume carbon dioxide in the light. Calculations by Nicolas-Théodore de Saussure (Switzerland) in the late 1790s showed that the increase in the plant’s mass was due to both carbon dioxide and water. Charles Reid Barnes (US) proposed the term ‘photosynthesis’ in 1893. In 1931, Cornelis Van Niel (The Netherlands/US) studied the chemistry of photosynthesis and demonstrated that photosynthesis is a light-dependent reaction in which hydrogen reduces carbon dioxide. Also in the 1930s, scientists proved that the oxygen liberated in photosynthesis comes from water.

Jan Ingenhousz (1730-1799).

THE CARNOT CYCLE (1824)
The Carnot cycle is a theoretical thermodynamic cycle that is the most efficient cycle for converting a given amount of thermal energy into work, or conversely, for creating a temperature difference by doing a given amount of work.  Nicolas Léonard Sadi Carnot (France) proposed the Carnot cycle in 1824, when he also proposed the Carnot engine, a hypothetical machine that converts heat into work or vice versa.  In the 1850s, Rudolf Clausius (Germany) and William Thomson, Lord Kelvin (UK) realized that the Carnot engine converts only part of the heat into work. A sizable amount of remaining heat is given off to the cold reservoir and the first law of thermodynamics states that the remaining heat plus delivered work are equal to the input heat. Although Carnot’s ideas were in contradiction to the first law, nevertheless they proved to be very useful to Clausius and Thomson, when they postulated the second law of thermodynamics.
A diagram of the Carnot cycle.

THE CELL NUCLEUS (1831)
Nearly all cells of living organisms have an organelle called the nucleus that contains the cell’s DNA in chromosomes.  Antonie van Leeuwenhoek (The Netherlands) observed nuclei, which he called ‘lumen’, in the red blood cells of salmon in 1719. Franz Bauer (Austria) described cell nuclei in more detail in 1804.  More detailed still was the 1831 report of Robert Brown (UK) regarding what he called the areola, or nucleus, in the cells of orchids.  Matthias Schleiden (Germany) called the nucleus the ‘cytoblast’ because he believed that the organelle played a role in generating cells. Oscar Hertwig showed in 1877-1878 that during fertilization of an egg, the nucleus of the sperm fuses with the egg’s nucleus.  It took the discovery of mitosis and the rediscovery of Mendel’s laws of heredity in the 20th Century before scientists understood the true importance of the nucleus.

A diagram of the cell nucleus.

A scanning electron micrograph of a cell nucleus.

ENZYMES (1833)
Enzymes are macromolecular biological catalysts, usually proteins, that greatly accelerate the rate and specificity of thousands of metabolic reactions. In 1833, Anselme Payen, with Jean-François Persoz (France) discovered the first enzyme, diastase, which they isolated from barley malt.  Jon Jakob Berzelius (Sweden) studied enzymes in 1835 and was the first to describe their action as ‘catalytic.’  Wilhelm Kühne (Germany) first used the term ‘enzyme’ in 1877.  In 1897, Eduard Buchner discovered that yeast extracts could ferment sugar in the absence of living yeast cells and he identified the enzyme responsible as ‘zymase.’  James B. Sumner (US) at Cornell was able to isolate and crystallize the enzyme urease from the jack bean in 1926 and for catalase in 1937.  John H. Northrop and Wendell M. Stanley (US) invented a precipitation technique that allowed them to isolate pepsin in 1930, and later trypsin and chymotrypsin. Using X-ray crystallography, a team led by David Chilton Phillips discovered the structure of lysozyme in 1965.

A simplified diagram showing one of the mechanisms used by enzymes.

VULCANIZATION (1839)
Vulcanization is a chemical process that converts natural rubber into a more durable material by adding sulfur or other curatives or accelerators, which modify the polymer by forming bridges between individual polymer chains. Vulcanized rubber is less sticky than natural rubber and has superior mechanical properties. Natural latex rubber from rubber trees was known to Mesoamericans since ancient times.  As early as 1600 BCE, Mesoamericans created processed rubber by mixing the latex with the juice of a local vine.  Unprocessed rubber was used for some products in Europe and America prior to vulcanization, but was of limited practicality. According to Charles Goodyear (US), he discovered vulcanization in 1839 through a series of experiments. Thomas Hancock (UK), who may have seen Goodyear’s early samples, was the first to patent vulcanization in the UK in 1844. Goodyear received a US patent the same year. An important development occurred in 1905, when George Oenslager (US) at Goodrich discovered that addition of thiocarbanilde accelerated the sulfur-rubber reaction and reduced consumption of energy.

Charles Goodyear (1800-1860).

OZONE (1840)
Ozone is an inorganic molecule made of three oxygen atoms that is a pale blue gas at room temperature with a sharp, pungent smell. Ozone is an allotrope of the element oxygen. Christian Friedrich Schönbein (Switzerland/Germany) isolated ozone and identified it as a distinct substance in 1840. Jacques-Louis Soret (Switzerland) determined the chemical formula for ozone in 1865. In 1913, French physicists Charles Fabry and Henri Buisson discovered a layer of ozone in the atmosphere. This ozone layer absorbs 97-99% of dangerous medium-frequency ultraviolet light from the sun. In 1930, Sydney Chapman (UK) determined the photochemical process that created the ozone layer. David Bates (Ireland) and Marcel Nicolet (Belgium) demonstrated in the 1950s that free radicals reduced the amount of ozone in the atmosphere. Paul Crutzen (The Netherlands) showed in 1970 that human activity (such as use of nitrogen fertilizers) could reduce the ozone in the atmosphere. Frank Sherwood Rowland (US) and Mario J. Molina (Mexico/US) hypothesized in 1974 that organic halogen compounds such as chlorofluorocarbons (CFCs) could deplete the ozone layer; the theory was confirmed by experiment within three years. This led to the banning of CFCs in aerosol spray cans in 1978.

A diagram of the ozone cycle: (1) the sun’s radiation splits an oxygen molecule into two oxygen atoms; (2) a single oxygen atom bonds with an oxygen molecule to make an ozone molecule; (3) solar radiation splits an ozone molecule into an oxygen molecule and an oxygen atom.

THE DOPPLER EFFECT (1842)
The Doppler effect is the apparent change in the frequency of a wave that occurs when an observer is moving relative to the source of the wave (or when the source of the wave is moving relative to the observer). For example, as a wave-emitting object approaches an observer, the observer receives the waves at a higher frequency than the waves actually being emitted; when the object recedes from the observer, the waves are received at a lower frequency. The Austrian physicist Christian Doppler first proposed the effect in 1842. Buys Ballot (The Netherlands) confirmed Doppler’s hypothesis for sound waves in 1845. Hippolyte Fizeau (France) independently discovered the phenomenon in electromagnetic waves in 1848. John Scott Russell (UK) confirmed the Doppler effect in a series of experiments published in 1848.

Christian Doppler (1803-1853).

THE ROTATION OF THE EARTH (1851)
While most ancient scientists believed that the Earth was stationary, some suggested that the apparent movement of the sun, moon, and planets was the result of the Earth rotating on its axis. The first was Ancient Greek scientist Philolaus in the 5th Century BCE, followed by Hicetas, Heraclides and Ecphantus in the 4th Century BCE.  In the 3rd Century, Aristarchus of Samos proposed that the Earth rotated on its axis and revolved around the sun.  In the opposite camp were Aristotle (4th Century BCE) and Ptolemy (2nd Century CE), who suggested a rotating Earth would create horrific winds.  In 499 CE, Indian astronomer Aryabhata theorized that the Earth rotated each day.  William Gilbert (England) supported the idea of a rotating Earth in 1600 and in 1687, Sir Isaac Newton (England) calculated the extent that the poles would flatten and equator would bulge if the Earth was rotating.  Seventeenth Century scientists also recognized that, if the Earth was rotating, there should be a slight deflection (the Coriolis effect) to falling bodies.  Attempts to measure any such effect by Giovanni Riccioli (Italy) and Robert Hooke (England) in the 17th Century failed, but Giovanni Battista Guglielmini (Italy) in 1791-1792, Johann Friedrich Benzenberg (Germany) in 1802 and 1804, and Ferdinand Reich (Germany) in 1831 all found small deviations that supported the rotation hypothesis.  Newton’s predictions about the flattening of the Earth’s poles was proven by Pierre Louis Maupertuis (France) in 1736-1738.  In 1851, French physicist Léon Foucault definitively demonstrated the rotation of the Earth by his pendulum, which slowly turned with the Earth, with the rate depending on the latitude where the pendulum is located.

An exact replica of the original 1851 pendulum made by Léon Foucault (1819-1868).  A 28 kilogram brass-coated lead bob hung at the end of a 67-meter long wire, which itself was attached to the dome of the Panthéon in Paris.  The replica remained in place from 1995 to 2014, when it was removed temporarily to allow for repairs to the building.

CELLULOID (1855)
Celluloid is a chemical compound made from nitrocellulose and camphor with added dyes and other agents. It is considered the first thermoplastic. Alexander Parkes (UK) created the first celluloid, called Parkesine, in 1855, although his business soon went bankrupt. Daniel Spill (UK), who had worked with Parkes, formed the Xylonite Company with the intention of taking over Parkes’ patents and making Parkesine under the name Xylonite. John Wesley Hyatt (US) claimed to have acquired Parkes’ patent in the 1860s and began experimenting with the intention of making billiard balls. Isaiah Hyatt (US), brother of John Wesley, dubbed the new product celluloid in 1872. A patent dispute between the Hyatts and Spill between 1877 and 1884 resulted in a ruling that both men could continue to made celluloid. In 1888 and 1889, celluloid was adapted for use as photographic film, and all movie and photography films were made of celluloid until acetate film replaced it in the 1950s. The biggest drawback of celluloid film was its extremely high flammability.

Celluloid film.

ATOMIC LIGHT SIGNATURES & SPECTROSCOPY (1859)
Joseph Fraunhofer (Germany) discovered in 1814 that the sun’s light was not distributed evenly throughout the spectrum, but no one pursued this finding at the time. In the 1850s, Gustav Kirchhoff (Poland) began systematically studying the colors made by different elements when placed in flame, known as their atomic light signature. In 1859, Kirchhoff joined with Robert Bunsen (Germany); together, they placed different substances in flame (using a new burner that produced little interfering light), placed the light from the flames through a prism and noted the spectrum. The result was the identification of the unique spectrum for each known element, and the discovery of two previously-unknown elements: cesium and rubidium. Kirchhoff and Bunsen also performed spectroscopy on the light of the sun, which created a new tool for astronomers to understand the make-up of stars.

Gustav Kirchhoff (1824-1887) (left) and Robert Bunsen (1811-1899).

CATHODE RAYS (1869)
Cathode rays are now understood to be streams of electrons observed in vacuum tubes to which two electrodes (the negative cathode and positive anode) are attached and a voltage is applied. Early vacuum tubes still contained so much gas that electrons collided with the gas and made it glow (leading to the invention of neon lights).  In the late 1860s and early 1870s, William Crookes (UK) invented the Crookes Tube, which contained almost no gas, so the inside of the tube was dark and the electrons struck the back of the tube near the anode instead. In 1869, Johann Hittorf (Germany) realized that the anode was casting a shadow on the back wall of the tube, meaning that something was traveling in straight lines from the cathode. In 1876, Eugen Goldstein (Germany) coined the term ‘cathode rays’ for Hittorf’s beams. In the next decades, scientists debated about the nature of cathode rays. Crookes and Arthur Schuster (Germany/UK) believed they were electrically-charged atoms; Goldstein, Eilhard Wiedermann and Heinrich Hertz (Germany) believed they were a new form of electromagnetic radiation. Experiments by Philipp Lenard (Germany) eventually led to J.J. Thomson’s discovery that cathode rays (and electricity generally) consisted of a beam of electrons, the first subatomic particle discovered.

A diagram showing how cathode rays are formed.

THE PHONOGRAPH (1877)
The phonograph had many precursors. In 1857, Édouard-Léon Scott de Martinville (France) invented the phonautograph, which could record sound as lines on paper, but could not reproduce the sounds. Charles Cross (France) invented the paleophone in 1877, which had the capacity to both record and play sounds. Thomas Edison invented the first true phonograph in late 1877. The first model embossed sounds on a tin foil cylinder; a later device used a wax-covered cardboard cylinder.  In 1886, Chichester Bell and Charles Sumner Tainter (US) invented the Graphophone, which engraved recordings on wax coated cylinders. German-born Emile Berliner (US) invented the Gramophone in 1887 – it traced a spiral on a zinc disc coated with beeswax. Discs were first offered to the public in 1892, and by 1908 had become the dominant format.  Edison began producing discs in 1912 and ended cylinder production in 1929. The first discs were made of hard rubber; in 1895, Berliner switched to shellac; more flexible vinyl discs became the standard during World War II.  Throughout the 20th Century, various improvements and changes occurred (e.g., 78 rpm led to 45 rpm singles and then 33 1/3 rpm long playing records, or LPs) until the 1980s, when the compact disc became the dominant format for listening to recorded music for a period of time. In the 21^st Century, many if not most listen to music in various digital formats, such as mp3 files, on computers and other electronic devices.

Thomas Edison’s original phonograph, which made recordings on tin foil.

LARGE-SCALE ELECTRIC SUPPLY NETWORK (1882)
In 1882, Thomas Edison (US) built the first electrical supply network, which provided 110 volts of direct current to 59 homes in Manhattan.  In the late 1880s, George Westinghouse (US) set up a rival system using alternating current, using an induction motor and transformer invented by Nikola Tesla (Serbia/US). AC eventually prevailed over DC.

A sketch of the Edison Company’s original power generation plant at Pearl Street in New York City, which began supplying direct current energy to light 59 homes.

TUBERCULOSIS BACTERIUM (1882)
Some scientists believe that tuberculosis has affected humans for 40,000 years. Evidence of tuberculosis infection was found in human remains from 9,000 years ago in the eastern Mediterranean, even though specimens of the tuberculosis bacterium from human skeletons in Peru and Africa indicate that its DNA is less than 6,000 years old.  Signs of the disease were discovered in Egyptian mummies from 3000-2400 BCE. Tuberculosis (called consumption or phthisis) is mentioned in texts in ancient India, China and Greece. In 1810, French physician Gaspard Laurent Bayle studied 900 corpses and identified six types of tuberculosis. René Laennec (France) studied the disease from 1816-1826, eventually dying from it, and invented the stethoscope to identify respiratory symptoms. In the 1820s, Pierre Charles Alexandre Louis (France) followed up on Laennec’s work using numerical analysis.  Jean Antoine Villemin (France) demonstrated in 1869 that the disease was contagious. In 1882, Robert Koch (Germany) identified Mycobacterium tuberculosis as the cause of the disease. Albert Calmette and Camille Guérin (France) developed a vaccine in 1906. The antibiotic streptomycin, discovered by Albert Schatz (US) in 1943, was found to be effective against tuberculosis in a 1946-1947 double-blind, placebo-controlled trial at the Medical Research Council Tuberculosis Unit (UK).

A photograph of tuberculosis bacteria infecting a human lung (20 X magnification).

CHOLERA BACTERIUM (1883)
The bacterium that causes cholera is Vibrio cholerae, which infects the small intestine and causes diarrhea and vomiting. Transmission occurs by eating food or water that has been contaminated by the feces of an infected person. Cholera first arose in the Indian subcontinent and has spread through pandemics, with the first from 1817-1824, the second from 1827-1835, the third from 1839-1856, the fourth from 1863-1875, killing millions. Filippo Pacini (Italy) isolated the bacterium in 1854 but its nature was not known. The same year, John Snow (UK) connected cholera with contaminated drinking water. After studying cholera during an outbreak in Egypt, then moving to an outbreak in India, Robert Koch (Germany) identified Vibrio cholerae as the cause of cholera in 1883. Cholera vaccines were developed by Jaume Ferran i Clua (Spain) in 1885 and Waldemar Haffkine (Russia) in 1892. In the 1940s and 1950s, Robert Allan Phillips (US) and the US Naval Medical Research Unit 2 conducted extensive research on cholera prevention and treatment.

A scanning electron microscope image of Vibrio cholerae, the bacteria that causes cholera.

THE ALTERNATING CURRENT MOTOR (1885-1887)
Hippolyte Pixii (France) created a crude form of alternating current (AC) in 1832 when he designed and built the first alternator. In 1879, Walter Baily (UK) demonstrated a battery operated polyphase motor aided by a commutator. Marcel Deprez (France) described a similar motor, with a rotating magnetic field produced by a two-phase AC system of currents, in an 1880 paper. In 1881, Lucien Gaulard (France) and John D. Gibbs (UK) demonstrated an AC transformer in London, which they sold to Westinghouse. Elihu Thomson (UK) built an AC motor in 1886 by using the induction-repulsion principle. In 1887, Charles Schenk Bradley (US) patented a two-phase AC power transmission with four wires. Working independently, Galileo Ferraris (Italy), in 1885, and Nikola Tesla (Serbia/US), in 1887, built commutatorless AC induction motors. Ferraris made a single phase motor, while Tesla made a two-phase motor. In 1890, Mikhail Dolivo-Dobrovolsky (Russia) made the first three-phase induction motor. In 1891, he combined the motor with a three-phase generator and transformer to create the first three-phase AC system. Charles Eugene Lancelot Brown (Switzerland) further developed the three-phase motor design. Other three-phase AC systems were developed by Friedrich August Haselwander (Germany) and Jonas Wenstrom (Sweden).

One of original alternating current motors made by Nikola Tesla (1856-1943), now at the British Museum in London.

DISPROVING AETHER (1887)
The Ancient Greeks believed that the gods did not breathe air, but aether, a pure essence that filled up the space where they lived.  Plato believed aether was ‘the most translucent kind’ of air, but Aristotle thought it was the fifth element (or quintessence), after air, water, fire and earth, and it did not follow the rules that applied to other substances.  According to Aristotle, the sun, moon, planets and stars were held in circular orbits by spheres made of crystallized aether.  By the Middle Ages, scholars believed that celestial bodies traveled in dense aether, while ’empty’ space was filled with aether that was ‘subtler than light.’  Beginning in the late 17th Century, the notion of aether was revived to explain certain scientific phenomena. So, proponents of the wave theory of light, such as Christiaan Huygens (The Netherlands) in 1678, invoked luminiferous aether as the medium to propagate the waves, just as air propagated sound waves.   Sir Isaac Newton (England) was a big fan of aether: he invoked it to support his particle theory of light in 1675, and also to explain how gravity operated in the Principia in 1687.  Similarly, Johann Bernoulli (Switzerland), a proponent of the particle theory of light, also called upon the aether in 1736 as the medium in which the particles traveled.  A type of aether composed of tiny unseen particles formed the basis for Le Sage’s theory of gravitation, proposed by Nicolas Fatio de Duillier in 1690 and Georges-Louis Le Sage in 1748.  By the 19th Century, the luminiferous aether theory was predominant, yet no one had been able to confirm that aether existed.  It was hypothesized that the moving Earth would drag the aether either partly (Augustin-Jean Fresnel, France 1818) or completely (George Gabriel Stokes, Ireland/UK, 1844) as it revolves around the sun and scientists began to design experiments to detect the aether wind.  An experiment by Albert A. Michelson and Edward W. Morley (US) conclusively found no stationary aether, although it left open the possibility of the less-popular theory that the Earth completely dragged the aether along with it.  Further experiments between 1893 and 1935 by numerous scientists with ever more sophisticated equipment failed to turn up any evidence for aether.  The last gasp of the aether theory came from Hendrik Lorentz (The Netherlands) in 1892-1895, who developed a theory of a completely motionless aether, which eventually morphed into a variation of Einstein’s special theory of relativity, which assumed no aether whatsoever.

The experimental apparatus of the 1887 Michelson-Morley experiment at Case Western Reserve in Cleveland, Ohio.

MITOCHONDRIA (1890)
A mitochondrion is a membrane-bound organelle found in most eukaryotic cells that generates most of a cell’s supply of energy through a reaction with adenosine triphosphate. Mitochondria are also involved in signaling, cell differentiation, cell death, and cell growth. The majority view supports the theory of endosymbiosis, that mitochondria were originally prokaryotic cells (related to Rickettsia bacteria) that became endosymbionts living inside the eukaryotic cells. In the 1850s, Albert von Kölliker (Switzerland) described granules in the sarcoplasm of muscle cell nuclei that were later identified as mitochondria. Gustaf Retzius (Sweden) named these granules ‘sarcosomes’ in 1890. In 1894, Richard Altmann (Germany) identified that mitochondria, which he called ‘bioblasts’, were organelles. Carl Benda (Germany) coined the term ‘mitochondria’ for the organelles in 1898. Friedrich Meves (Germany) observed mitochondria in plants in 1904. In 1908, Meves and Claudius Regaud (France) suggested that mitochondria contain proteins and lipids. Benjamin F. Kingsbury (UK) linked mitochondria and respiration in 1912. Early evidence of the respiratory function of mitochondria was made by Otto Heinrich Warburg and Heinrich Otto Wieland (Germany) in 1913, but the actual respiratory chain was not revealed until 1925, when David Keilin (USSR) rediscovered cytochromes. In 1957, Philip Siekevitz (US) described mitochondria as the ‘powerhouse of the cell.’ In 1967, scientists discovered that mitochondria contained ribosomes and had their own DNA. A map of the mitochondrial genome was completed in 1976. As early as 1918, Paul Portier became convinced that mitochondria were direct descendents of bacteria. Ivan Wallin (US) proposed that mitochondria had an endosymbiotic origin in the 1920s, but the theory was ignored at the time. In a 1967 paper, Lynn Margulis (US) advanced the endosymbiotic theory, which she elaborated on in a 1981 book.

A diagram of a mitochondrion.

A photograph of a mitochondrion.

MOTION PICTURES (1894)
Prior to motion pictures, numerous devices were invented to take advantage of the phenomenon of persistence of vision, in which the brain continues to perceive an image for a short period after it has been removed, which allows a series of still images to create the illusion of motion. In the late 1870s, photographic technology had advanced enough to capture moving objects, as demonstrated by Eadweard Muybridge’s photos of human and animal locomotion. In 1882, Etienne-Jules Marey (France) invented a precursor to the movie camera that could take 12 pictures per second and eventually 30 frames per second. The first true movie camera, which could rapidly photograph a series of images, was invented by Louis Le Prince (France/UK) in 1888.  William Friese-Greene (UK) invented a movie camera in 1889, with a public demonstration in 1890, but its 10 frames per second rate and unreliability were serious drawbacks.  George Eastman (US) invented celluloid roll film in 1889, which would become the film used to make movies. In late 1890, Thomas Edison and his assistant William Dickson (US) invented the Kinetographic Camera, a motor-driven movie camera that became the first commercially successful movie camera after it was introduced in 1892.  Dickson and Edison also invented the Kinetoscope, which allowed individual viewers to watch motion pictures through a peephole. The Kinetoscope was demonstrated publicly on May 20, 1891. Dickson and Edison produced an improved version in 1892 and debut it at the Chicago World’s Fair in 1893. Edison’s movie studio, the Black Maria, opened in February 1893 in West Orange, New Jersey. Early films include Fred Ott’s Sneeze (1894), Carmencita (1894), Annabelle Butterfly Dance (1894) (one of the first color tinted films) and The Kiss (1896). Georges Demenÿ (France) built his Beater Movement camera in 1893. Polish inventor Kazimierz Prószyński made the Pleograph, which combined camera and projector, in 1894.  Charles Moisson (France) made the Domitor camera for Auguste and Louis Lumière (France) in 1894. Edison released the Kinetoscope for commercial use in 1894. Using his Phantoscope projector, inventor Charles Francis Jenkins (US) projected a motion picture he filmed and hand tinted with color onto a screen for an audience in Richmond, Indiana on June 6, 1894. Jenkins and Thomas Armat (US) improved the Phantoscope and demonstrated it at two exhibitions in the last half of 1895. After a patent dispute, the Phantascope was sold to Edison, who renamed it the Vitascope. Inspired by Edison, the Lumières invented the Cinematographe in 1895 – a combination movie camera (hand cranked), film printer and projector. They demonstrated the system in their basement on March 22, 1895 with their first film, Workers Leaving the Lumiere Factory. On December 28, 1895, the Lumières presented the first public, commercial exhibition of projected motion pictures to a paying audience in Paris’s Salon Indien at the Grand Café. The ten short films in the program included The Gardener, or The Sprinkler Sprinkled, the first comedy. Perhaps the Lumieres’ most famous short film is Arrival of a Train at La Ciotat (1895), which reportedly frightened some moviegoers. The first theatrical exhibition of Edison’s Vitascope projector occurred at Koster and Bial’s Music Hall in New York City on April 23, 1896. French filmmaker Georges Melies’ A Trip to the Moon of 1902 introduced special effects to the movies. In 1903, Edison employee Edwin S. Porter (US) made the 12-minute film The Great Train Robbery, the first Western and the most sophisticated motion picture to date, with 14 shots cutting between simultaneous events. The first permanent theater dedicated to showing motion pictures was The Nickelodeon, which opened in Pittsburgh, Pennsylvania in 1905. The first two-reel film was D.W. Griffith’s Enoch Arden, from 1911. The first sound film was The Jazz Singer, from 1927.

An 1897 patent application for a Kinetographic Camera made by William Dickson (1860-1935) and Thomas Edison (1847-1931). The original camera was made in 1890-1892.

THE AIRSHIP (1895)
An airship (also known as a dirigible) is a lighter-than-air aircraft that can navigate through the air under its own power. There are three types: (1) a non-rigid airship consists of a gas-filled envelope; (2) a semi-rigid airship is a pressurized gas balloon or envelope attached to a lower metal keel; and (3) a rigid airship has an internal frame and gas-filled bags. In 1670, Francesco Lana de Terzi (Italy) designed an ‘Aerial Ship’ supported by four copper spheres from which air was evacuated, but the design was unsound and it was never built. A more practical design was proposed by Jean Baptiste Marie Meusnier (France) in 1783: a 260-foot long ship with internal balloons to regulate life, attached to a carriage that doubled as a boat in the unlikely event of a water landing. When Jean-Pierre Blanchard (France) fitted a hand-powered propeller to a hot-air balloon in 1784, he created the first powered airship; he used wings for propulsion and a tail for steering to navigate a balloon across the English Channel in 1785. A design of a balloon with a steam engine driving twin propellers was proposed by William Bland (Australia) in 1851. Henry Giffard (France) was the first to make an engine-powered flight when he flew 17 miles in 1852. In 1872, Dupuy de Lome (France) designed and flew a balloon driven by a large propeller turned by eight men. The same year, Paul Haenlein (Germany) flew an airship with an internal combustion engine that ran on coal gas. Charles F. Ritchel (US) created a hand-powered one man rigid airship in 1878. Gaston Tissandier (France) made the first electric-powered flight in 1883, using a 1.5 horsepower Siemens electric motor. Fully controllable free flight was achieved by Charles Renard and Arthur Constantin Krebs (France) in La France in 1884, using an 8.5 hp electric motor and 435 kilogram battery.  The Campbell Air Ship, designed and built by Peter C. Campbell (US) in 1888, was lost at sea in 1889. Frederich Wölfert (Germany) built three airships in 1888-1897 powered by gasoline engines, the last of which caught fire in flight and killed both occupants. Augusto Severo de Albuquerque Maranhão (Brazil) designed and built semi-rigid airships in 1894 and 1902. In 1895, Count Ferdinand von Zeppelin (Germany) patented a rigid airship that combines balloon air cells with a structural framework. An aluminum airship was built by David Schwarz (Hungary) in 1897. The Luftschiff Zeppelin LZ1, a rigid airship, flew in July 1900. An improved LZ2 was built in 1906. The Zeppelin airships held the crew and engines in a gondola that hung beneath the hull driving propellers attached to the sides of the frame. Alberto Santos-Dumont (Brazil/France) designed 18 balloons and dirigibles beginning in 1901 with the Number 6, which flew around the Eiffel Tower. Thomas Scott Baldwin (US), built and flew airships beginning in 1904, and created the first US military airship in 1908. Walter Wellman (US) and Melvin Vaniman (US) unsuccessfully attempted airship flights to the North Pole in 1907 and 1909 and across the Atlantic in 1910 and 1912. An innovative new three-lobed design was proposed by Leonardo Torres Quevedo (Spain) in 1902; he and Captain A. Kindelan (Spain) built the España in 1905, then designed an improved version in 1909, which was mass produced in 1911. Hans Gross (Germany) developed one of the first successful semi-rigid airships in 1907. Airships were used as bombers in World War I. Goodyear (US) launched its first helium-filled blimp in 1925. In the 1930s, rigid airships were used for luxury passenger transport until the world’s largest passenger airship, the Hindenburg, caught fire and burned in New Jersey in 1937, killing 36.

A 1910 photograph of the Lebaudy Morning Star Airship, which crossed the English Channel.

PSYCHOANALYSIS (1895)
Psychoanalysis is a series of techniques intended to cure mental and emotional disturbances. The premises of psychoanalytic theory include: (1) psychological development is determined by genetic inheritance and early childhood experiences; (2) attitude, mannerism, experience and thought are largely influenced by unconscious irrational drives; (3) the mind resists attempts to become aware of the irrational drives through defense mechanisms; (4) mental and emotional disturbances such as neuroses and mental illness are the result of conflicts between the conscious and unconscious mind; and (5) to liberate the self from the harmful effects of the unconscious mind, the psychoanalyst must assist the patient to bring the unconscious material into the conscious mind.  The foundation was laid for psychoanalysis in the 1880s, when Austrian physician Josef Breuer began his ‘talking cure’ with a patient named Anna O.  Breuer discussed the case and the theory behind it with his protege Sigmund Freud, and together they wrote Studies on Hysteria, which set out many of the principles of psychoanalysis, in 1895.  Freud continued to develop the theory in a series of publications between 1900 and 1925.  Other contributors were Austrians Otto Rank (1924), Robert Waelder (1936) and Freud’s daughter Anna (1936). Later theorists included Heinz Hartmann (Austria/US), Karen Horney (Germany/US), Charles Brenner (US), Erik Erikson (Germany/US), Heinz Kohut (Austria/UK), Jacques Lacan (France), Harry Stack Sullivan (US), Robert Langs (US), Stephen Mitchell (US) and Robert Stolorow (US), who have taken Freud’s original theory in many different directions.

A 1938 photograph of Sigmund Freud (1856-1939).

AIR CONDITIONING (1902)
Air cooling techniques have existed since Ancient Egypt, when people hung reeds in windows and moistened them with trickling water – the evaporating water cooled the air blowing through the window.  Ancient Roman houses had cool water circulating through the walls.  Ding Huan (China) invented a manually-powered rotary fan in 180 CE, and by 747 CE, there are references to water-powered fan wheels in China.  Medieval Persians used cisterns and wind towers to cool their buildings. Cornelius Drebbel (The Netherlands) developed an evaporation-based cooling system in the 17th Century.  Benjamin Franklin and John Hadley (US) conducted important evaporation experiments in 1758.  In 1820, Michael Faraday (UK) discovered the cooling power of compressed, liquefied ammonia.  In 1842, American physician John Gorrie created ice using compression and then used fans to circulate the cool air, but financial woes prevented him from developing the invention.  In 1851, James Harrison (Australia) developed an ice-making machine.  Willis Carrier (US) invented the first modern electrical air conditioner in 1902 in order to control temperature and humidity in a printing plant.  Stuart Cramer (US) developed a similar machine in 1906 for a textile mill.  Cramer coined the term ‘air conditioning’, which Carrier adopted.  An air conditioning unit was installed in the home of Charles Gates (US) in 1914.  Thomas Midgley, Jr. (US) invented Freon, the first non-flammable, non-toxic refrigerant, in 1928.  (Unfortunately, Freon and other chlorofluorocarbon gases destroy the ozone layer and are being phased out.)  In 1931, H.H. Schultz and J.Q. Sherman (US) created a very expensive individual room air conditioner.  The DuBose house in Chapel Hill, NC (US) became fully air conditioned in 1933.  Packard introduced the first air conditioned automobile in 1939.  In 1945, Robert Sherman (US) invented an affordable, portable, in-window air conditioner.  In the 1970s, central air conditioning was developed.

Willis Carrier (1876-1950) and his air conditioner.

CLASSICAL CONDITIONING (1902)
Classical conditioning is a type of learning in which a conditioned stimulus is paired with an unconditioned stimulus, which leads to an unlearned reflex response. After the pairing is repeated to the organism, it begins to exhibit the reflex response in the presence of the conditioned stimulus without the unconditioned stimulus. In Russian scientist Ivan Pavlov’s famous experiments in the early 20th Century, he noticed that dogs salivated in the presence of meat. He began to ring a bell (or use other stimuli) whenever the meat was brought to them. Eventually, the dogs would salivate upon the stimulus, even when no meat was present. Pavlov announced his results in 1903 at conferences in Sweden and Spain, and he is generally credited with the discovery. American psychologist Edwin Twitmyer independently discovered the conditioned reflex in 1902, when he associated the use of a hammer to induce the knee jerk reflex with the sound of a bell. Eventually his human subjects would jerk their knees at the sound of the bell, without the hammer. Twitmyer’s discovery was barely noticed when he presented it at a conference in 1904. Russian psychologist Vladimir Bekhterev set out a rival theory of conditioned reflexes in a 1903 book. American psychologist John B. Watson used Pavlov’s experiments as the basis for his new behaviorist model of psychology in 1913. His controversial 1921 “Little Albert” experiment involved conditioning a human infant to associate a white rat with a frightening noise, so that eventually he feared the rat.

A diagram showing how classical conditioning works.

RADIOMETRIC DATING (1905)
Radiometric dating (also called radioactive dating) is a technique used to date materials by comparing the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates. Ernest Rutherford (NZ/UK) first proposed the possibility of radiometric dating in 1905. Following up on Rutherford’s suggestion, Bertram Boltwood (US) demonstrated that radiometric dating was possible in 1907. Radiometric dating has permitted scientists to date rocks and fossils and determine the age of the Earth. Scientists use a specific form of radiometric dating called radiocarbon dating to find the age of organic materials less than 50,000 years old.

A diagram showing how radiometric dating works.

THE EARTH’S MANTLE (1909)
The mantle of the Earth is a layer between the crust and the outer core. The mantle is a silicate rocky shell about 1800 miles thick that constitutes about 84% of the Earth’s volume. The mantle is divided into four layers: (1) lithosphere; (2) asthenosphere; (3) upper mantle; and (4) lower mantle. Although it is mostly solid, over periods of geologic time the mantle behaves like a viscous liquid. In 1909, Andrija Mohorovičić (Croatia) discovered that there is a sudden increase in seismic activity at the top of the mantle; the boundary of such activity is known as the Mohorovičić discontinuity or “Moho”.

A cross-section diagram of the Earth.

THE BURGESS SHALE (1909)
The Burgess Shale is a series of fossil-bearing rock formations in the Canadian Rockies of British Columbia.  The fossils date to the Middle Cambrian Period (505 million years old) and contain many unusual and unique life forms, many preserved with impressions of soft body parts.  The first person to notice the Burgess Shale was Richard McConnell of the Geological Survey of Canada in 1886.  His discovery came to the attention of American paleontologist Charles D. Walcott, who first explored the area in 1907 but did not discover the main fossil-bearing area until his 1909 visit.  By 1910, Walcott had opened a quarry.  He returned each year until 1913, and again in 1917, 1919, 1921 and 1924.  He brought back 65,000 specimens on 30,000 rock slabs to the Smithsonian before his death in 1927.  Harvard professor Percy Raymond (US) began collecting fossils from the area in 1924 and into the 1930s.  British scientist Harry B. Whittington returned to the Burgess Shale in the 1960s and his team reexamined Walcott’s original fossils.  They determined that many of the fossils were previously unknown types of animals and some belonged to entirely new phyla.  UNESCO named the Burgess Shale a World Heritage site in 1981.

A fossil of Anomalocaris canadensis from the Burgess shale.

X-RAY DIFFRACTION & X-RAY CRYSTALLOGRAPHY (1912)
Early work with the symmetry of crystals was done by Nicholas Steno (Denmark) in 1669; René-Just Haüy (France) in 1784 and 1801; William Hallowes Miller (UK) in 1839; William Barlow (UK) in 1894 and others.  Paul Peter Ewald (Germany/UK) and Max von Laue (Germany) raised the idea that crystals could be used as a diffraction grating for X-rays in 1912 and the same year, Von Laue performed the first X-ray diffraction using a copper sulfate crystal.  William Henry Bragg (UK) and his son William Lawrence Bragg (Australia/UK) followed up on Von Laue’s experiments in 1912-1913 to determine the structures of molecules and minerals.  Ralph Walter Graystone Wyckoff (US) used X-ray crystallography to determine the structures of sodium nitrate and caesium dichloroiodide in 1919.  Dorothy Hodgkin (UK) used X-ray crystallography to determine the three-dimensional structures of cholesterol (1937), penicillin (1946), vitamin B12 (1956), and insulin (1969).  In addition to X-ray crystallography, other methods of X-ray diffraction include powder diffraction, SAXS and X-ray fiber diffraction.  Powder diffraction was invented by Peter Debye (The Netherlands/US) and Paul Scherrer (Switzerland) in 1916, and, independently, Albert Hull (US), in 1917.  Rosalind Franklin (UK) used X-ray fiber diffraction in 1952 to take a photograph that helped determine the double helix structure of DNA.

The X-ray diffraction pattern of a crystallized enzyme.

ISOTOPES (1913)
Isotopes of a chemical element all have the same number of protons in their nuclei (and therefore the same atomic number) but different numbers of neutrons (and therefore varying atomic masses). Isotopes of an element may have varying chemical properties and some may be radioactive. In 1902, Ernest Rutherford (NZ/UK) and Frederick Soddy (UK) set the stage for the discovery of isotopes with their study of the way in which radioactive decay changed one element into another, eventually reaching a stable element, as uranium eventually decayed into lead. Work by H.N. McCoy and W.H. Ross (US) in 1907 further explored the nature of decay products and devised a method for separating them. In 1913, Soddy predicted the existence of isotopes based on the results of experiments on the radioactive decay of uranium to lead: there are only 11 elements in the decay chain, yet Soddy found 40 separate decay products, which implied that multiple elements must occupied the same spaces in the periodic table. Margaret Todd (UK) suggested the Greek term ‘isotope’ meaning ‘in the same place’ to Soddy. Also in 1913, Polish-American chemist Kazimierz Fajans reached essentially the same conclusion as Soddy. J.J. Thomson (UK) found the first evidence of multiple isotopes for a stable, non-radioactive element (the inert gas neon) in 1913. In 1914, Theodore William Richards (US) found that different radioactive forms of the same element had different atomic weights. In 1919, Francis W. Aston (UK) used a mass spectrograph to identify multiple isotopes for a number of stable elements. He also formulated the whole number rule, which states that the atomic masses of isotopes are integers and a deviation from an integral atomic mass is usually the result of a mixture of isotopes. Harold Urey and G.M Murphy (US) discovered deuterium, an isotope of hydrogen, in 1931.

A 1915 photograph of Frederick Soddy (1877-1956).

ATOMIC NUMBER (1913)
The atomic number of a chemical element (Z) is equal to the number of protons in its nucleus. Each element has a unique atomic number and each atomic number identifies a unique element. In an atom with no charge, the atomic number also indicates the number of electrons in the atom. When Dmitri Mendeleev (Russia) created his periodic table in 1869, before atomic number was understood, he used atomic weight to organize the elements, although he made some exceptions (e.g., putting tellurium ahead of iodine), which took account of elements in which the atomic number was not half the atomic weight. According to Ernest Rutherford’s 1911 theory of the atomic structure, he focused on the positively charged nucleus and the negatively charged electrons. He hypothesized (incorrectly) that the atomic weight equaled twice the number of electrons, if each electron weighed as much as a hydrogen atom (i.e., a proton). Following up on Rutherford, Antonius van den Broek (The Netherlands) suggested in 1911 that the number of electrons was exactly equal to the element’s place in the periodic table, thus anticipating the concept of atomic number. Niels Bohr (Denmark) used van den Broek’s theory in his 1913 model of the atom, where he predicted that the frequency of atomic spectra should be proportional to the square of Z, the atomic number. After discussions with Bohr, Henry Moseley (UK) in 1913 obtained spectra for elements from aluminum (Z = 13) to gold (Z = 79) and found that the results confirmed Bohr’s prediction, thus providing conclusive evidence that atomic number, not atomic weight, was the defining characteristic of a chemical element.

Henry Moseley (1887-1915).

THE ASSEMBLY LINE (1913)
Elements of the assembly line include division of labor, interchangeable parts, and a moving, linear start-to-finish assembly process. A very early example of a division of labor comes from China in the 3rd Century BCE, when workers created the Terracotta Army for the tomb of Chinese Emperor Qin Shi Huangdi.  Another early example was the Venetian Arsenal (Italy) in the early 16th Century, which employed 16,000 workers and used standardized parts to build ships.  The notion of interchangeable parts was championed in the mid-18th Century by Honoré Blanc (France), who inspired Eli Whitney (US) to use some of Blanc’s ideas in making muskets in 1798.  Oliver Evans (US) built an automatic flour mill in 1785 using conveyors, elevators and other devices.  An early example of a linear and continuous assembly process was Porstmouth Block Mills (UK), built by Marc Isambard Brunel (France/UK) between 1801 and 1803. Another assembly line factory was the Bridgewater Foundry, built by James Nasymth and Holbrook Gaskell (UK) in 1836. Starting in 1867, Chicago meatpackers began to use assembly lines in which workers would stand at fixed stations and a pulley system would move the meat along the line. Ransom Olds (US) built a modern assembly line in 1901 to mass-produce the Oldsmobile Curved Dash automobile.  The assembly line idea was brought to Henry Ford by his employee William “Pa” Klann, after visiting a Chicago slaughterhouse, and was implemented by a team of Ford employees.  The Ford assembly line to build the Model T began operating on December 1, 1913.  Soon, the assembly line method had spread throughout automobile manufacturing.

A portion of the Ford Motor Company Model T assembly line in 1913.

THE PROTON (1917)
The proton is a subatomic particle with a positive electric charge. Every atomic nucleus includes one or more protons. The number of protons in an atom’s nucleus is its atomic number. In 1815, William Prout (UK) suggested that all atoms are composed of one or more hydrogen atoms. In the Standard Model, the proton is a hadron composed of three quarks. In 1886, Eugen Goldstein (Germany) discovered positively charged particles that were produced from gases, although the different values of charge-to-mass ratio prevented Goldstein from reducing the theory to a single particle. In 1898, Wilhelm Wien (Germany), while studying streams of ionized gas, identified a positive particle equal in mass to the hydrogen atom. Ernest Rutherford (NZ/UK) conducted experiments in 1917 (and announced results in 1919) that showed the hydrogen nucleus was present in other nuclei. Rutherford named the particle in the hydrogen nucleus the ‘proton’ (a combination of Prout’s name and Prout’s term ‘protyle’) in 1920.

A diagram of a proton containing two up quarks and one down quark.

STELLAR NUCLEOSYNTHESIS (1920)
Stellar nucleosynthesis is the process by which fusion reactions inside stars create the heavier elements found in the universe, which are then distributed throughout space when the star explodes.  Scientists believe that the Big Bang alone only created hydrogen, helium and a few of the lighter elements, while the remainder were created in stars or in exploding stars.  The idea was first proposed by Arthur Eddington (UK) in the 1920s.  Fred Hoyle (UK) developed the theory in the late 1940s.  In 1951, Ernst Öpik (Estonia/Northern Ireland) and, independently, the following year, Edwin E. Salpeter (Austria/Australia/US) explained how helium could become carbon inside the cores of red giant stars through the triple alpha process.  In 1957, Hoyle’s team of physicists published a paper (known as the B2FH paper) that organized nucleosynthesis into complementary nuclear process and explained the synthesis of heavy elements in detail.

A cutaway diagram of a red giant prior to exploding in a supernova.

NEUROTRANSMITTERS (1921)
Neurotransmitters are chemicals found in the nervous systems of living organisms that transmit signals across a synapse from one neuron to another. Prior to the discovery of chemical neurotransmitters, most scientists believed neurons communicated exclusively through electric impulses. In the late 19^th and early 20^th Century, Santiago Ramón y Cajal (Spain) discovered a gap between neurons known as the synaptic cleft, which suggested that some neuronal communication took place via chemicals. T.R. Elliott (US) suggested in 1904 that adrenaline acted as a neurotransmitter in nerves, helping the nerve signal across the synapse. Otto Loewi (Germany) conducted experiments on the vagus nerve of a frog in 1921 that provided direct evidence that neurons communicate by releasing chemicals. Loewi also identified the first known neurotransmitter, acetylcholine. Ulf von Euler (Sweden) discovered the neurotransmitter norepinephrine in 1946 and Arvid Carlsson (Sweden) disovered dopamine in the 1950s. Vittorio Erspamer (Italy) discovered serotonin in the 1930s; Irvine Page (US) rediscovered it and named it in 1948, and Betty Twarog and John Welsh (US) identified serotonin as a neurotransmitter in 1952 and 1954, respectively.
A diagram containing a description of seven different neurotransmitter processes.

THE CYCLOTRON (1931)
A cyclotron is a particle accelerator in which charged particles accelerate outwards along a spiral path.  A rapidly varying electric field accelerates the particles and a static magnetic field holds the particles to a spiral trajectory.  The idea of the cyclotron came to both Leó Szilárd (Hungary) and Ernest O. Lawrence (US).  Szilárd applied for a patent in 1929.  Lawrence had precedence; his cyclotron, which he built with student M. Stanley Livingston, began operating at the University of California at Berkeley in 1932.  The first European cyclotron was proposed in 1932 by George Gamow and Lev Mysovskii (USSR) and began operating at the Radium Institute in Leningrad in 1937.  In Nazi Germany, Walther Bothe and Wolfgange Gentner created a cyclotron in Heidelberg, where it began running in 1943.  Some of the largest cyclotrons are those at the RIKEN laboratory in Japan and TRIUMF at the University of British Columbia in Vancouver, Canada.

M. Stanley Livingston (1905-1986) (left) and Ernest Lawrence (1901-1958) standing with the 27″ cyclotron.

THE STRONG NUCLEAR FORCE & THE STRONG INTERACTION (1932)
The strong interaction is the mechanism by which the strong nuclear force works. The strong nuclear force only operates at a distance of a femtometer (10‾15 meters) but it is the strongest force, 137 times stronger than magnetism. The strong nuclear force, which is carried by gluons, holds protons and neutrons together in the nucleus and binds quarks into hadrons. Most of the mass-energy of protons and neutrons consists of the strong force field energy. The model of the atom prior to the 1970s contained a number of contradictions; according to the existing physics, the positive charges of the protons should cause the nucleus to fly apart, which did not occur. Scientists then hypothesized a new force, the strong force, that held the protons and neutrons together.  When the Standard Model was developed, it became clear that the strong interaction causes quarks with unlike color charge to attract one another. In about 1932, Eugene Wigner (Hungary/US) and Werner Heisenberg (Germany) independently theorized that protons and neutrons were held together by a force separate from the electromagnetic force. Hideki Yukawa (Japan) attempted an early hypothesis in 1934-1935. Major discoveries were made by Murray Gell-Mann (US) and Yuval Ne’eman (Israel) in 1962 and Gell-Mann and George Zweig (US) in 1964.

A diagram showing how the strong force keeps nuclei together.

RADAR (1935)
In 1886, Heinrich Hertz (Germany) showed that radio waves could be bounced off solid objects. In 1897, while testing an early radio communication device (the spark-gap transmitter) between two ships at sea, Alexander Popov (Russia) noted that the passage of a third ship caused interference in the signal. Christian Hülsmeyer (Germany) was the first to use radio waves to detect the presence of distant objects in a 1904 experiment in dense fog, but the device could not measure the distance to the object. In 1922, U.S. Navy scientists Albert Taylor and Leo Young noticed that that ships reflected radio signals. In 1930, Taylor and Young, with Lawrence Hyland (US), detected a plane using the same method, but without information about distance or speed. The team, with new member Robert Page (US), then developed a pulse-interference device and successfully used in to identify the range and speed of an airplane in December 1934. Earlier in 1934, a team of German scientists led by Rudolf Kühnhold used Doppler-beat interference to detect ships and airplanes, including their range. On January 3, 1934, Russian scientists M.M. Lobanov and Y.K. Korovin detected an airplane at 600 meters range and 100-150 meters altitude using a Doppler signal; later the same year, the Bistro device was introduced. Maurice Ponte (France) developed a short wavelength device in the 1930s that did not measure distance. Meanwhile, by 1935, the Germans had developed a much more accurate pulse-modulated system. In 1935, Robert Watson-Watt, Arnold “Skip” Wilkins and Edward Bowen (UK) demonstrated a device that could detect radio waves reflected off a flying airplane 17 miles away and determine its range. By 1936, the U.S. Navy had a prototype radar system that could detect aircraft at 25 miles distance. Also by 1936, German companies Lorenz and Telefunken had developed accurate radar systems. The U.S. Army developed its own radar system by 1937.  By 1938, the U.S. Navy system could detect aircraft at 100 miles and the first radar was placed on an American ship. In 1939, the USSR developed a radar system capable of determining range and velocity. In 1940, the word ‘radar’ was coined from the phrase ‘Radio Detection and Ranging.’ In 1940 John Randall and Harry Boot (UK) invented the cavity magnetron, which made short wavelength radar a reality. Robert Page invented monopulse radar in 1943. Luis Alvarez (US) invented phased-array radar during World War II. Goodyear Aircraft Corp. (US) invented synthetic-aperture radar in the early 1950s.

The U.S. Navy installed the experimental XAF radar on the USS New York in late 1938.

THE ECOSYSTEM (1935)
An ecosystem is a community of living organisms in conjunction with the non-living components of their environment, interacting as a system. The biotic and abiotic components of an ecosystem are linked through nutrient cycles and energy flows. In 1924, Alfred Lotka, in Elements of Physical Biology, compared the global ecosystem to “a great world engine” in which “plants and animals act as coupled transformers of energy” in “the mill-wheel” that is driven by “solar energy.” Arthur Tansley (UK) coined the term ‘ecosytem’ in a 1935 publication in which he emphasized the transfers of materials between organisms and their environment. Early developers of the ecosystem concept were G. Evelyn Hutchinson (UK), Raymond Lindeman (US) and Howard T. and Eugene P. Odum (US), who developed a systems approach to studying ecosystems.

A 1913 photograph of Arthur Tansley (1871-1955) and his wife Edith.

THE JET ENGINE (1937)
Ancient Greek scientist Archytas is reputed to have invented an artificial, self-propelled flying device that flew 200 meters propelled by a jet of steam between 400 and 350 BCE. In 150 BCE, Hero of Alexandria described a device called an aeolipile that used steam to cause a sphere to spin rapidly on its axis. Chinese engineers invented rockets in the 13th Century and Lagari Hasan Çelebi of the Ottoman Empire reputedly launched himself into the air on a homemade rocket in 1633. John Barber (UK) patented a turbine design in 1791. Charles Parsons (UK) invented the steam turbine in 1884.  In 1903, Aegidius Elling (Norway) built the first gas turbine with a centrifugal compressor. Between 1903 and 1906, Armengaud and Lemale (France) built an inefficient gas turbine engine. Hans Holzwarth (Germany) began work on an explosive cycle gas turbine in 1908 and reached 13% efficiency by 1927. In 1908, René Lorin (France) patented a ramjet engine, which was modified by Georges Marconnet (France) in 1909 to create the pulsejet. In 1910, Henri Coandă (Romania) built and briefly flew the Coandă-1910, the first motorjet. Sanford Alexander Moss (US) began work on turbochargers at General Electric in 1917. In 1921, Maxime Guillaume (France) designed the first axial-flow turbine engine. In a seminal 1926 paper, Alan Arnold Griffith (UK) explained how jet engines are possible. A single-shaft turbocompressor based on Griffith’s theory was tested in the UK in 1927. Frank Whittle (UK) presented a jet engine design to the UK Air Force in 1928 but it was rejected.  He submitted a patent for the design in 1930. Also in 1930, Paul Schmidt (Germany) patented a pulsejet engine. In 1931, Secondo Campini (Italy) patented a motorjet engine.  In 1934, Hans Von Ohain (Germany) patented a jet propulsion engine.  In April 1937, Whittle bench tested an engine with a single-stage centrifugal compressor coupled to a single-stage turbine, a prototype of the turbojet engine.  In September 1937, Von Ohain and Ernst Heinkel (Germany) bench tested a jet engine.  Also in 1937, György Jendrassik (Hungary) designed and built the first working turboprop engine, although it was never installed in a plane.  Heinkel built an airplane to test Von Ohain’s engine – the Heinkel He178 – which flew for the first time on August 27, 1939.  Von Ohain then improved his design and flew it in the He S.8A aircraft on April 2, 1941.  On May 15, 1941, the Pioneer aircraft flew with a Whittle engine (the W1) for the first time.  A centrifugal jet engine designed by Frank Halford (Scotland) called the de Havilland Goblin flew in 1942. Anselm Franz (Austria) improved on the centrifugal jets by creating the axial-flow compressor, with the first test in 1940.  His engine was used in the Messerschmitt Me 262 in 1942. The first axial-flow engine in the UK, the Metrovick F.2, was tested in 1941 and flown in 1943. By the 1950s, almost all combat aircraft used jet engines. In 1952, the first commercial jet airliner, the de Havilland Comet, entered the market, and by the 1960s, almost all large civilian aircraft were jet-powered. By the 1970s, the high bypass jet engine’s fuel efficiency had surpassed that of piston and propeller engines.

Frank Whittle (1907-1996) stands next to the prototype of his jet engine.

THE KREBS CYCLE (1937)
The Krebs Cycle (also known as the citric acid cycle) is the series of chemical reactions used by aerobic organisms to obtain energy by oxidizing acetate from carbohydrates, fats and proteins into carbon dioxide and adenosine triphosphate (a source of energy).  Several components of the cycle were discovered by Albert Szent-Györgyi (Hungary) in the early 1930s, but the cycle itself was identified by Hans Adolf Krebs (Germany/UK) in 1937.  Because the Krebs Cycle is so central to biochemisty, some scientists believe that it was one of the first elements of living cells and may even have existed before life originated.
A diagram of the Krebs Cycle.

FUSION (1938)
Nuclear fusion occurs when two or more atomic nuclei collide at high speed and join to form the nucleus of a different element, usually releasing energy in the process.  Arthur Eddington (UK) suggested in 1920 that stars obtain their energy by fusing hydrogen into helium.  In 1929, Robert Atkinson (UK) and Fritz Houtermans (The Netherlands/Austria /Germany) predicted that fusing small nuclei would release large amounts of energy. Mark Oliphant (Australia) obtained fusion of hydrogen isotopes in 1932.  In 1938, Hans Bethe (Germany/US) explained how nuclear fusion provided the source of energy in stars.  In 1951, the US carried out a test of nuclear fusion, and in 1952 it tested a hydrogen bomb, which was based on fusion.

A diagram of the fusion reaction in the sun described by Hans Bethe (1906-2005).

POLYETHYLENE (1939)
Polyethylene is the most common type of plastic in use today and is used to make plastic bags, plastic bottles and many other items. German chemist Hans von Pechmann was the first to synthesize polyethylene, albeit accidentally, while heating diazomethane in 1898. His colleagues Eugen Bamberger and Friedrich Tschirner (Germany) analyzed the resulting substance and named it polymethylene. In 1933, Eric Fawcett and Reginald Gibson (UK) of ICI, accidentally synthesized polyethylene using a very high pressure method that was industrially practical (unlike von Pechmann’s). ICI chemist Michael Perrin was able to reproduce the synthesis in 1935 and industrial production of the plastic began in 1939. Its first use was for insulation of radar cables during World War II. Large scale production began at Bakelite and DuPont in 1944. In 1951, Robert Banks and J. Paul Hogan (US) at Philips Petroleum discovered a catalyst that allowed synthesis of polyethylene at milder temperatures and pressures. In 1953, Karl Ziegler (Germany) and Giulio Natta (Italy) discovered a catalyst that worked in milder conditions but was more expensive. Another catalytic system, using soluble catalysts, was invented by Walter Kaminsky (Germany) and Hansjörg Sinn in 1976. Polyethylene now comes in two basic types: (1) hard, or HDPE or (2) soft, or LDPE.

A 3-D model of a polyethylene molecule.

ORAL CONTRACEPTIVES (1951)
Also known as the birth control pill, or just “The Pill”, the combined oral contraceptive pill includes a combination of estrogen (estradiol) and progestogen (progestin). Margaret Sanger (US) began advocating for access to birth control in 1914. In 1916, she opened the first US birth control clinic, which led to her arrest. In 1921, Sanger formed the American Birth Control League, which later became Planned Parenthood. In the following decades, Sanger fought to overturn laws banning birth control methods and sought to educate the public about birth control. By the 1930s, scientists knew that high doses of certain steroid hormones inhibited ovulation in rabbits. European scientists had synthesized hormones but they were too expensive to import elsewhere. In 1939, Russell Marker (US) at Penn. State University learned to synthesize progesterone from sarsaparilla, and then Mexican yams. In 1944, he and two partners started Syntex in Mexico and began producing synthetic steroid hormones. In 1951, Gregory Pincus (US), a hormone specialist, attended a dinner with Sanger and Planned Parenthood medical director Abraham Stone. They urged Pincus to research the birth control possibilities of hormones. In 1951, Pincus’s colleague Min Chueh Chang (China/US) repeated a 1937 experiment showing that progesterone suppressed ovulation in rabbits. Also in 1951, Carl Djerassi (Austria/US), Luis Miramontes (Mexico) and George Rosenkranz (Hungary/Mexico) at Syntex synthesized the first orally highly active progestin, norethindrone. Further research was stalled by lack of funding support for this controversial area. Then, Sanger found a donor – Katherine Dexter McCormick – who contributed large sums beginning in 1953. Pincus brought in Harvard gynecologist John Rock (US) to do clinical research with women. Rock had been using progesterone and estrogen with infertility patients since 1952. Meanwhile, Frank B. Colton (US) at Searle had synthesized two orally highly active progestins in 1952 and 1953. John Rock started a clinical trial of three different oral progestins in 1954 and concluded that Colton’s norethynodrel worked best. After more experimentation, Pincus and Rock decided they should add a small amount of estrogen, to prevent bleeding. They named the resulting pill Enovid. Trials were conducted by Edris Rice-Wray and Edward T. Tyler (US) in 1956. The results indicated that they could reduce the estrogen content by a third. The FDA approved Enovid in 1957, but only for menstrual disorders. The FDA approved a 10 mg dose of Enovid for contraception in 1960 and a 5 mg dose in 1961. Legal barriers still existed, however, until Supreme Court decisions that made access to birth control a right for married women (in 1965) and unmarried women (in 1972).

The first birth control pill was Enovid, by Searle. It was approved by the FDA for contraception in 1960.

RNA (1955)
RNA (ribonucleic acid) refers to a number of macromolecules that play multiple roles in genetic chemistry.  Many viruses have a genome made with RNA. In cellular organisms, messenger RNA (mRNA) conveys genetic information that directs synthesis of proteins on ribosomes, transfer RNA (tRNA) brings amino acids to the ribosomes and ribosomal RNA (rRNA) connects the amino acids to form proteins.  Friedrich Miescher (Germany) discovered the nucleic acids in 1868. Between 1930 and 1950, scientists distinguished the different chemical properties of DNA and RNA. Ribosomes were discovered by George Emil Palade (Romania/US) in 1955 and by the end of the 1950s, it was evident that ribosomes were the site of protein synthesis.  In 1956, Mahlon Hoaglund and Paul Zamecnik (US) discovered tRNA and its role in protein synthesis.  By 1957, Francis Crick (UK) announced that genes make proteins and hypothesized that DNA was a template for a messenger RNA molecule that would bring the information to an ‘adaptor’ molecule to match the mRNA code to amino acid and then a ‘ribonucleic-protein complex’ to assemble the protein. Crick’s predictions were accurate.  By 1960, experiments by Arthur Pardee (US), François Jacob and Jacques Monod (France) led to the identification of mRNA. Marshall Nirenberg (US) deciphered the genetic code by 1961 and Har Gobind Khorana (India/US) confirmed and extended his work.  In 1965, Robert W. Holley (US) sequenced the tRNA of yeast.  In 1967, Carl Woese hypothesized that RNA was also an enzyme and could have been a component of the earliest forms of life.  David Baltimore (US), Renato Dulbecco (Italy/US) and Howard Temin (US) discovered retroviruses and reverse transcriptase in 1970.  In 1977, Philip Sharp (US) and Richard Roberts (UK) discovered introns and RNA splicing.  Thomas Cech (US) and Sidney Altman (Canada/US) discovered ribozymes (RNA molecules that catalyze reactions) in the early 1980s.

A diagram of protein synthesis using tRNA, mRNA and ribosomes.

THE VAN ALLEN BELTS (1958)
While pursuing the work of Kristian Birkeland (Norway) on auroras, Carl Størmer (Norway) proposed that particles might be trapped within the Earth’s magnetic field, and he worked out the orbits of these trapped particles between 1907 and 1913.  To  test the theory, American scientist James Van Allen placed Geiger–Müller tube experiments on three 1958 satellites: Explorer 1, Explorer 3 and Pioneer 3.  The experiments confirmed the existence of two radiation belts containing trapped particles.  The inner belt consists mostly of energetic protons, which are the product of the decay of neutrons created by cosmic ray collisions in the upper atmosphere. The outer belt consists mostly of electrons; they are injected from the geomagnetic tail after geomagnetic storms and are energized through wave-particle interactions.  Two Van Allen probes were launched by NASA in 2012.  They discovered a temporary third Van Allen belt.
A diagram of the Van Allen belts.

SEAFLOOR SPREADING (1960-1962)
Seafloor spreading is a process in which new oceanic crust if formed through volcanic activity at mid-ocean ridges and then gradually moves away from the ridge. Seafloor spreading helps to explain continental drift in the theory of plate tectonics. Between 1947 and 1953, William Maurice Ewing, Bruce Heezen, David Ericson and Marie Tharp (US) at Lamont Geological Observatory collected enormous amounts of data about the nature of the seafloor and in so doing discovered the global mid-ocean ridge and its volcanic nature, particularly a rift valley that ran down the center of the ridges. In 1960, Harry Hess, with Robert Dietz (US), hypothesized that the seafloor was spreading, and that was the cause of continental drift. In 1962, Hess furher proposed that volcanic activity along the mid-ocean ridges caused seafloor spreading. Evidence of seafloor spreading came in 1963 from Frederick Vine and Drummond Matthews (UK) who discovered that as the crust along the mid-ocean ridges consisted of parallel crust with alternating magnetic polarity. (Although it was not until 1966 that scientists proved the Earth’s magnetic field periodically reversed itself.) This showed that new crust was created at the ridges and then spread out from the ridges over time. In 1965, J. Tuzo Wilson (Canada) combined the continental drift and seafloor spreading hypotheses into plate tectonics theory.

A diagram of the seafloor spreading process.

MANNED SPACE FLIGHT (1961)
The idea of humans traveling outside the Earth’s atmosphere has a long history in literature, but the first scientific proposal came from Konstantin Tsiolkovsky (Russia) in 1903. A 1919 paper by Robert Goddard (US) combining the de Laval nozzle with rockets using liquid fuel was the first practical proposal, and influenced Hermann Oberth and Wernher Von Braun (Germany). The unmanned German V-2 rocket was the first to reach space in 1944. The first unmanned satellite was placed into orbit by the USSR in 1957. The first man to leave the atmosphere and orbit the Earth was Yuri Gagarin (USSR) on April 12. 1961.  On May 5, 1961, the US sent Alan Shepherd into a suborbital flight, while John Glenn became the first American to orbit the Earth on February 20, 1962. The USSR put a number of men and the first woman (Valentina Tereshkova) in space in 1962 and 1963. Alexei Leonov (USSR) made the first spacewalk on March 8. 1965.  On July 20, 1969, an American crew landed on the moon.

Soviet rocket Vostok 1 lifts Yuri Gagarin (1934-1968) into orbit in April 1961.

THE PERSONAL COMPUTER (1977)
Between 1948 and 1957, John Lentz (US) developed the Personal Automatic Computer at Columbia University; it later became the IBM 610, which sold for $55,000.  Only 180 were made. (Minicomputers such as the LINC (1962) and PDP-8 (1965) and similar models from DEC, Data General and Prime could also be classified as personal computers, although they were refrigerator-sized and very costly.) In 1965, Olivetti (Italy) introduced the Programma 101 – the first commercially produced desktop computer. Olivetti sold 44,000 units at $3,200 each. Victor Glushov (USSR) produced the MIR from 1965-1969. John Blankenbaker (US) of Kenbak Corp. invented the Kenbak-1 in 1970, but only 40 were made. Also in 1970, CTC (now Datapoint) created the Datapoint 2200 – the first machine to resemble modern PCs.  Polish scientist Jacek Karpiński and his team developed the K-202 in 1971-1973, the first 16-bit non-kit desktop, but only 30 were sold.  R2E (France) made the Micral N in 1973.  Also introduced in 1973 was the Xerox Alto, which had a mouse and a graphical user interface. The Mark-8 was a 1974 microcomputer build-it-yourself kit.  IBM brought out the IBM 5100 in 1975. The Altair 8800, created by MITS (US) in 1975, was a very popular and inexpensive kit that spawned many imitators. In 1976, Gary Ingram and Bob Marsh (US) at Processor Technology Corporation, designed the Sol-20 Personal Computer, the first all-in-one PC, which sold well from 1976-1979. Three important personal computers were released in 1977: (1) the Commodore PET, created by Chuck Peddle (US), sales less than one million units; (2) the Apple II, created by Steve Wozniak/Apple (US), 2.1 million sold by 1985; and (3) the TRS-80/Model I, created by Tandy/Radio Shack, sales of 1.5 million units by 1981. The IBM 5100 was announced in 1978 and withdrawn in 1982.  Atari released its first home computers, the 400 and 800, in 1978-1979. The same year, Texas Instruments made the TI-99/4A home computer. In 1980, two UK companies released home computers: the Sinclair ZX80, by Science of Cambridge and the Acorn Atom by Acorn Computers. Also in 1980, Commodore released the VIC-20.  In 1981, Xerox introduced the Xerox Star workstation, with many modern features. The IBM PC was also introduced in 1981, as was the BBC Micro. In 1982, the Commodore 64 was introduced – it would sell 17 million units. The most popular personal computer in Japan was NEC’s PC-9801, which was released in 1982.  IBM followed up the PC with the XT in 1983 and the PC/AT in 1984. Many companies created clones of the IBM products. In 1983, Apple brought out the Lisa, a mass-marketed microcomputer with a graphical user interface but it was too slow and expensive to succeed. Apple’s 1984 design, the mouse-driven Macintosh, was much more successful.

The Commodore PET personal computer, from 1977.

CLONING (1996)
Cloning (also known as reproductive cloning) is a biotechnological technique in which scientists create biological organisms from the DNA of another organism, usually by taking the DNA from an adult somatic cell and transferring it to an egg cell from which the DNA has been removed. In 1924, German embryologists Hans Spemann and Hilde Mangold first discovered embryonic induction, in which parts of the embryo direct the development of groups of cells into particular tissues or organs. Spemann and Mangold performed the first somatic-cell nuclear transfer in 1928.  Somatic-cell nuclear transfer is one form of cloning; another form is embryo-splitting, which uses an existing embryo to create artificial identical twins.  Robert Priggs and Thomas J. King (US) cloned northern leopard frogs in 1952 using nuclear transfer of embryonic cells  In 1958, John Gurdon (UK) further advanced the work of Priggs and King.  In 1963, Tong Dizhou (China) cloned a carp.  Soviet scientists Chaylakhyan, Veprencev, Sviridova and Nikitin cloned a mouse in 1986.  A sheep was cloned by Steen Willadsen (Denmark) using early embryonic cells in 1984.  In 1995, scientists at the the Roslin Institute in Edinburgh, Scotland cloned two sheep from differentiated embryonic cells in 1995.  Ian Wilmut and his team at the Roslin Institute cloned a sheep (named Dolly) from a somatic cell in 1996 (although not announced until 1997) – this was the first mammal cloned from an adult somatic cell.  Scientists have also cloned: cow (1997); rhesus monkey (1999, 2007); pig (2000); cat (2001); gaur (2001); goat (2001); mule (2003); horse (2003); deer (2003); rabbit (2003); ferret (2004); dog (2005); fruit fly (2005); wolf (?), water buffalo (2005); camel (2009); zebrafish (2009); and Pyrenean ibex (extinct) (2009).  Humans have not yet been cloned; some nations have laws prohibiting human cloning.

Dolly the cloned sheep and her first lamb Bonny, at the Roslin Institute in 1998-1999.

THE ACCELERATING UNIVERSE (1998)
While scientists have known since 1929 that the universe is expanding, they believed that the rate of expansion was constant. In 1998, Saul Perlmutter (US), leader of the Supernova Cosmology Project along with Gerson Goldhaber (Germany/US), Rich Muller and Carl Pennypacker (US), and Brian P. Schmidt and Adam G. Riess (US), heads of the High-Z Supernova Search Team, discovered that the expansion of the universe is accelerating, through observations of Type Ia supernovae. This finding has since been corroborated by: the cosmic microwave background radiation, the large scale structure of the universe, the size of baryon acoustic oscillations, measurements of the age of the universe, X-ray properties of galaxy clusters and Hubble constant data. Most models proposing explanations of the accelerating expansion invoke dark energy.

The winners of the 2011 Nobel Prize in Physics were (from left): Adam Reiss (1969- ), Saul Perlmutter (1959- ) and Brian Schmidt (1967- ).

To see a longer version of the Most Important Scientific Discoveries list, organized chronologically, go here.

To see the Timeline of Science and Technology, go here.