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Editor's note: This is a historical chronology principally devoted to marking milestones in human scientific achievement or observation. Detailed information related to the eons, eras, periods, and epochs of geologic time may be found in text and diagrams related to the topic Geologic time.

c.4,600,000,000 B.C.
Origin of Earth: 4,600 million years ago (mya).

c.4,000,000 B.C.
Earliest hominid species appear on Earth.

c.50,000 B.C.
Homo sapiens sapiens emerges as a conscious observer of nature.

c.30,000 B.C.
Stone Age cultures use pigments to color various artifacts.

c.10,000 B.C.
Neolithic Revolution: transition from a hunting and gathering mode of food production to farming and animal husbandry, that is, the domestication of plants and animals.

c.4000 B.C.
Early applied chemistry begins in Egypt with the extraction and working of metals, including copper, tin, and bronze. Egyptians are also familiar with eye paint and plaster of Paris.

c.4000 B.C.
Egyptians astronomically measure time.

c.3500 B.C.
Sumerians describe methods of managing the date harvest.

c.3400 B.C.
Bronze, an alloy of copper and tin, first appears in abundance in Samaria. The Sumerians become expert in working gold, silver, copper, lead, and antimony.

c.3000 B.C.
Iron is forged.

c.2500 B.C.
The earliest known wholly glass objects are beads made in Egypt at this time. Early peoples may have discovered natural glass, which is created when lightning strikes sand. The Egyptians make glass beads by sand (silica), soda, lime, and other ingredients.

c.2000 B.C.
Chinese document experiments with forces of magnetism.

c.1500 B.C.
Use of iron becomes prevalent in the Mediterranean. It appears to have come there from the northeast, possibly beginning with the Hittites, and its use revolutionizes society.

c.900 B.C.
Steel is manufactured in India.

c.700 B.C.
Babylonians and Chinese understand planetary orbits.

c.700 B.C.
Greeks demonstrate force of electric attraction produced by rubbing amber.

c.700 B.C.
The use of anatomical models is established in India.

c.600 B.C.
Anaximander, Greek astronomer, describes the ecliptic plane, and asserts that Earth's surface assumes a curved, cylindrical shape.

c.600 B.C.
Thales of Miletus, Greek philosopher, first notices the electrification of amber (the Greek word for amber is elektron) by friction. Thales also proposes water as the fundamental substance of the Universe. He is the first to systematically study magnetism, and correctly predicts a solar eclipse.

c.550 B.C.
Pythagoras, Greek philosopher and mathematician, advances studies of geometry and geometric form. Pythagoras asserts Earth as a sphere.

c.525 B.C.
Anaximenes, Greek philosopher, proposes that air is the fundamental element of the Universe, and when compressed, it can take the form of water and Earth.

c.500 B.C.
Heraclitus, Greek philosopher, states that fire is the fundamental element of the universe. He also states that all things are in constant motion and that nothing is ever lost.

c.500 B.C.
Parmenides, Greek philosopher, suggests that matter can be neither created nor destroyed.

c.480 B.C.
Oenopides of Chios calculates angle of Earth's polar axial tilt to ecliptic .

c.475 B.C.
Parmenides argues that Earth is a sphere.

c.460 B.C.
Eudoxus of Cnidus, Greek philosopher, corrects faults in Plato's planetary orbital scheme (e.g., that Sun, planets, and stars orbit Earth on celestial spheres).

c.455 B.C.
Philolaus, Greek philosopher, argues that night and day are caused by Earth's rotation.

c.450 B.C.
Anaxagoras, Greek philosopher, offers one of the first atomic theories, saying that all matter consists of atoms or "seeds of life."

c.450 B.C.
Anaxagoras argues that the universe is made entirely of matter in motion.

c.450 B.C.
Empedocles, Greek philosopher, first offers his concept of the composition of matter, postulating that it is made of four elements—earth, air, fire, and water. This notion is adopted by Aristotle and becomes the basis of physical theory for nearly two millennia.

c.450 B.C.
The Greek philosopher Leucippus first states the formal notion of atomism. He argues that upon continuous division of a substance, eventually a point would be reached beyond which further division was impossible. His disciple, the Greek philosopher Democritus ultimately names these small particles atomos, meaning indivisible.

c.450 B.C.
Zeno of Elea, Greek philosopher, formulates paradoxes challenging the discreteness of continuous time and space.

c.425 B.C.
Democritus (470–380), Greek philosopher, states his atomic theory that all matter consists of infinitesimally tiny particles that are indivisible. These atoms are eternal and unchangeable, although they can differ in their properties. They can also recombine to form new patterns. His intuitive ideas contain much that is found in modern theories of the structure of matter.

c.380 B.C.
Plato (427–347), Greek philosopher, teaches a geometrical theory of matter on which he elaborates in his Timaeus.

c.350 B.C.
Aristotle (384–322), Greek philosopher, offers his doctrine of the elements, stating that all things are composed of a basic material in combination with four qualities—hotness, dryness, coldness, and wetness. This theory eventually suggests the idea of transmutation (the changing of ordinary metals into gold or silvers) and gives rise to alchemy.

c.350 B.C.
Aristotle (384–322), Greek philosopher, rejects the atomism of the Greek philosopher Democritus (470–380 B.), thus condemning it to oblivion until modern times. He also states that a vacuum does not exist in nature and that sound travels by a succession of impacts on the air. Aristotle argues correctly that sound is not conducted in the absence of air and incorrectly that a body will move only as long as it keeps being pushed. He also asserts that heavy bodies fall faster than light ones.

c.350 B.C.
Aristotle reasons that Earth is spherical.

c.350 B.C.
Heracleides describes and calculates Earth's rotation.

c.335 B.C.
First description of equinoxes.

c.325 B.C.
Pytheas argues that tides are caused by motions of the Moon.

c.300 B.C.
Arthasastra, an ancient Indian manual on politics, discusses mining, metallurgy, medicine, pyrotechnics, poisons, and fermented liquors.

c.300 B.C.
Aristarchus argues that Earth revolves around the Sun.

c.300 B.C.
Epicurus (341–270), Greek philosopher, elaborates on the atomism of the Greek philosopher Democritus (470–380 B.), but substitutes the notion of chance for the determinism of Democritus.

c.300 B.C.
Euclid, Greek mathematician, writes a treatise on optics in which he makes optics a part of geometry by dealing with light rays as though they are straight lines. He also offers a theory of reflection which he treats geometrically.

c.300 B.C.
Glass blowing first practiced.

c.300 B.C.
Theophrastus, Aristotle's disciple and the founder of botany, attempts to establish a classification system for plants based upon differences between plant and animal morphology.

c.275 B.C.
Herophilus's younger colleague, Eristratus (c. 310–c. 250), asserts that veins and arteries are connected.

c.265 B.C.
Zou Yan asserts the material universe is composed of five elements including water, metal, wood, fire and earth.

c.260 B.C.
Aristarchus of Samos, distance and size of moon from Earth's shadow during lunar eclipse.

c.260 B.C.
Aristarchus of Samos argues for a Sun-centered (heliocentric) cosmology.

c.260 B.C.
Aristarchus of Samos calculates the ratio Sun-Earth distance/Earth-Moon distance from the angle established at half moon.

c.250 B.C.
Chinese, free bodies move at constant velocity.

c.250 B.C.
Philon, Greek engineer, experiments with air and discovers that it expands when heated.

c.240 B.C.
Erastothenes computes the diameter of Earth, suggests it orbits the Sun.

c.220 B.C.
Archimedes (287–212), Greek mathematician and engineer, writes his Treatise on Floating Bodies in which he relates the principle of buoyancy.

c.170 B.C.
Chinese astronomers record observations of Sun spots.

c.150 B.C.
Hipparchus, observes and estimates precession of the equinoxes.

c.130 B.C.
Hipparchus estimates the size of Moon from parallax of eclipse.

c.50 B.C.
Lucretius proposes a materialistic, atomistic theory of nature in his poem On the Nature of Things. He favors the preformation theory of embryological development.

c.10
Cleomedes, Greek astronomer, discusses the optical properties of water and says that in a similar manner, the Sun may be visible when it has actually gone a bit below the horizon. This is the first consideration of atmospheric refraction until Ptolemy.

c.50
Hero, Greek engineer, formulates the principle of the motive power of steam, building many steam-powered devices. He also writes of the five simple machines—lever, pulley, wheel, inclined plane, and wedge—and extends and generalizes the law of the lever. He also maintains correctly that air takes up space and is compressible.

c.70
Roman author and naturalist Pliny the Elder (23–79) writes his influential Natural History, a vast compilation combining observations of nature, scientific facts, and mythology. Naturalists will use his work as a reference book for centuries.

c.150
Ptolemy publishes geocentric model with Earth at the center of the solar system.

c.150
Ptolemy, Greek astronomer, writes a treatise on optics in which he considers the refraction of light. He offers an original approach that is both theoretical and experimental.

415
Library of Alexandria is burned by religious zealots, thus destroying the most comprehensive collection of ancient and classical scholarship in science and the arts. Much of the recorded knowledge of Western Civilization was lost.

517
Johannes Philoponus (c.490–570), Alexandrian philosopher, also called John the Grammarian, rejects Aristotle's idea that a body will only move as long as it pushed and offers his own theory of motion. He says that a body will keep moving in the absence of friction or as long as nothing opposes it. He argues that this is why the stars continue to move.

c.850
Al-Kindi (801–866), Arab physicist, writes a treatise on optics and the reflection of light. He also studies meteorology, the tides, and specific weights.

c.890
Al-Razi identifies Andromeda galaxy.

c.1000
Alhazen (965–1038), Arabian physicist, rejects the idea that people see because their eyes send out a light which reflects back from an object. He argues correctly that light comes from the Sun or another source, and reflects from the object into the eye. He studies all aspects of light, especially reflection and refraction, and also offers an exploration of rainbow formation.

c.1025
Al-Biruni (973–1048), Arab physician, astronomer, and mathematician, makes a fairly accurate calculation of the specific weights of eighteen precious stones and metals. He uses the methods employed by Archimedes.

1054
Supernova of Crab Nebula recorded in China and by Native Americans.

1121
Al-khazini argues that gravity acts towards Earth's center.

1137
Abu Ja'far Alchazin (Al Khazin), Arab mathematician and astronomer, writes a book in which he offers tables of specific densities and a general description of the laws of gravity.

c.1144
First translations of Arabic alchemical manuscripts in Spain, introducing European scholars to alchemy.

c.1225
Robert Grosseteste (1175–1253), English scholar, studies optics and experiments with mirrors and lenses. He attempts to explain the rainbow and argues that light is the basic substance of the universe. He is also the teacher of the English scholar, Roger Bacon (1220–1292).

c.1267
Roger Bacon (1214–1292), English philosopher and scientist, asserts that natural phenomena should be studied empirically.

1269
Pierre de Maricourt experiments with magnets and compass. Discovery that a magnet is encircled by lines which terminate on two magnetic poles.

c.1270
Witelo (1230–1275) of Silesia, also called Vitellio, writes his Perspectiva, a systematic treatment of the optics of the Arabian physicist, Alhazen (965–1038). It deals with refraction and reflection as well as the twinkling of stars (caused by motion).

1304
Theodoric of Freibourg conducts experiments to investigate rainbows. c.1325 William of Ockham (1280–1349), English scholar, argues strongly for the importance of empiricism and lays down the rule called Ockham's razor. According to this rule, when two theories equally fit all observed facts, the one requiring the fewest or simplest assumptions is to be accepted as more valid.

c.1350
Jean Buridan (1300–1385), French philosopher, refutes the Aristotelian notion that an object in motion requires a continuous force, and maintains that only an initial impetus is required. He anticipates Newton's first law of motion by saying that the celestial bodies stay in motion in this manner.

c.1500
Leonardo da Vinci (1452–1519), Italian artist and inventor, experiments with hydrostatics and diffraction and offers a version of the principle of inertia (which will not come until the time of Galileo).

1510
Two German books lay the foundation for industrial chemistry. Bergwerkb'chlein is dedicated to mineralogy and Probierb'chlein focuses on chemical tests and introduces quantitative concepts.

1512
Copernicus advances heliocentric model that includes assertion that the planets orbit the Sun.

c.1525
Paracelsus (1493–1541), Swiss physician and alchemist, uses mineral substances as medicines. Denying Galen's authority, Paracelsus teaches that life is a chemical process.

1543
Andreas Vesalius publishes his epoch-making treatise The Fabric of the Human Body. Although Vesalius generally accepts Galenic physiological doctrines and ideas about embryology, Vesalius is later regarded by many as the founder of modern anatomy because he corrected many of Galen's misconceptions regarding the human body.

1546
Gerardus Mercator describes Earth's magnetic poles.

1563
Bernard Palissy (c.1510–1589), French potter, publishes his Recette veritable, in which he discusses agriculture, geology, mining, and forestry. He discovers the Italian secret of producing majolica (pottery decorated with an opaque tin glaze) and is considered one of the most eminent chemists of France.

1568
Zacharias and Hans Janssen development of the first compound microscope opens new opportunities for the study of structural detail.

1574
Lazarus Ercker (c.1530–1594) of Germany publishes his Beschreibung aller Furnemisten Mineralischen Ertzt und Bergwercks Arten, which is the first manual of analytical, metallurgical chemistry. His text is especially valuable to the practicing assayers.

1574
Tycho Brahe argues that a comet he discovered lies beyond the Moon.

1576
Brahe constructs a planetary observatory to accurately record motions of celestial bodies.

1581
Robert Norman, English navigator and instrument maker, publishes a work on the lodestone called The Newe Attractive. He discusses the known properties of the magnet and is the first to note that steel does not change its weight when magnetized. He also discovers magnetic dipas when he suspends a compass needle to allow vertical movement and notes that it points down toward Earth. This is later used by Gilbert.

1584
Giordano Bruno argues that stars are suns with other planets.

1586
Simon Stevin (1548–1620), Dutch mathematician, publishes a report of his experiment in which he refutes the Aristotelian doctrine that heavy bodies fall faster than light ones. He also founds hydrostatics by demonstrating that the pressure on a liquid varies according to how high above Earth's surface it is and not upon the shape of the container that holds it. His demonstrations also eliminate many standard arguments in favor of the existence of perpetual motion.

1587
Galileo Galilei (1564–1642), Italian astronomer and physicist, begins experiments that lead to his law of falling bodies. He uses a gently sloping inclined plane and shows that the rate of fall of a body is independent of its weight. He eventually states correctly that all objects will fall at the same rate in a vacuum. He also shows that a body can move under the influence of two forces at one time.

1591
Thomas Harriot (1560–1621), English mathematician, is the first Westerner to note that snowflakes are hexagonal (six-sided). He does not publish his findings. The Chinese however, document snow flake crystal shapes from the second centuryB.C.

1592
Galileo develops primitive thermometer.

1592
Galileo argues that the physical laws of Cosmos are the same as those on Earth.

1600
William Gilbert (1544–1603), English physician and physicist, publishes his De magnete which is a full account of his extensive investigations on magnetic bodies and electrical attraction. He suggests that the Earth itself is a great magnet, and he is the first to use the terms electric attraction, electrical force, and magnetic pole.

1604
Galileo observes that distance for falling object increases as square of time.

1604
German astronomer and mathematician Johannes Kepler (1571–1630) writes a treatise on optics.

1608
Hans Lippershey develops optical telescope.

1609
Using a telescope, Galileo observes craters and mountains on the Moon.

1609
Kepler theorizes that a force of gravity exists that can exert itself through empty space, and that its strength is related to the size of the bodies involved.

1609
Kepler offers 1st and 2nd laws of planetary motion.

1610
Galileo observes moons of Jupiter.

1610
Jean Beguin (1550–1620) publishes the first textbook on chemistry.

1613
Galileo documents existence and movements of Sunspots.

1630
Jean Rey (1582–1645), French physician, writes on the nature of air and its role in combustion, and lays the foundation for future chemical discoveries. He suggests a possible experiment for weighing air that Galileo actually performs.

1632
Henry Gellibrand (1597–1636), English astronomer and mathematician, publishes his findings that offer the first indication that the Earth's magnetic field slowly changes over time.

1637
Rene Descartes offers physical explanations of refraction, rainbows and clouds.

1638
Galileo publishes his Discorsi e dimostrazioni mathematiche intorno a due nuovescienze which lays the foundations of modern mechanics. In it, he formulates what becomes known as the first law of motion (or the law of inertia), as well as the laws of cohesion and strength of materials, and of the pendulum. It also provides a definition of momentum and details the steps or stages of what becomes known as the experimental method. This work marks the end of Aristotelian physics.

1640
Evangelista Torricelli (1608–1647), Italian physicist, writes his De motu gravium in which he applies Galileo's laws of motion to fluids and founds the study of hydrodynamics.

1643
Torricelli is the first to create a sustained vacuum when he invents the barometer. He fills a four-foot-long glass tube with mercury and inverts it onto a dish. He observes that not all the mercury flows out and that over time, the level remaining in the tube varies. He concludes correctly that these changes are caused by atmospheric pressure.

1644
Kenelm Digby (1603–1665), English natural philosopher, observes magnetic and electrical attractions as well as acoustic resonance.

1646
Johann Rudolf Glauber (1604–1670), German chemist, publishes the first of his five-volume Furni novi philosophici. This work gives his recipes for mineral acids and salts, including "sal mirabile"—the sodium sulfate residue that formed by the action of sulfuric acid on ordinary salt—that becomes known as "Glauber's salt."

1648
Ortus medicinae by Johann Baptista van Helmont (1580–1644), Flemish physician and alchemist, is published posthumously. He is the first to use quantitative methods in connection with a biological problem. He is also the first to recognize that one air-like substance exists, and he names this vapor, or non-solid, "chaos," which in Flemish sounds like "gas."

1648
Blaise Pascal (1623–1662), French mathematician, physicist, and philosopher, conducts his famous experiment on the Puy-de-Dôme mountain and not only verifies Torricelli's experiments, but goes beyond them to demonstrate that air pressure decreases as altitude increases.

1648
Johannes Marcus Marci von Kronland (1595–1667), Bohemian physician, discovers the diffraction of light, but it does not become a recognized fact until Newton's time.

1651
William Harvey publishes a landmark treatise on embryology entitled On the Generation of Animals, stating that all living things come from eggs. Harvey demonstrates that oviparous and viviparous generations are analogous to each other. Although Harvey discovers many errors in Aristotle's ideas, he supports the Aristotelian doctrine that generation occurs by epigenesis.

1653
Blaise Pascal (1623–1662), French mathematician, physicist, and philosopher, studies fluids and formulates what comes to be known as Pascal's principle— that the pressure at any point in a liquid is the same in all directions. Pascal's principle forms the basis of the hydraulic press that he also describes in theory. This information is not published until a year after his death.

1660
Vincenzo Viviani (1622–1703), Italian mathematician, and Giovanni Alfonso Borelli (1608–1679), Italian mathematician and physiologist, collaborate on an experiment in which they measure the velocity of sound by using the cannon-flash-and-sound method.

1661
Robert Boyle (1627–1691), English physicist and chemist, publishes his book, The Sceptical Chymist. Boyle espouses the experimental method and breaks from the Greek notion of elements.

1662
Boyle announces what becomes known as Boyle's Law, stating that when an ideal gas is under constant pressure, its volume and pressure vary inversely.

1665
Newton experiments with gravity, spectrum of light, invents differential calculus.

1665
Nicolaus Steno (1638–1686), Danish anatomist and geologist, briefly states what is now called the first law of crystallography. Also called the law of the constancy of crystalline angles, it states that crystals of a specific substance have fixed characteristic angles at which the faces, however distorted, always meet.

1665
Robert Hooke publishes Micrographia, an account of observations made with the new instrument known as the microscope. Hooke presents his drawings of the tiny box-like structures found in cork and calls these tiny structures "cells." Although the cells he observes are not living, the name is retained. He also describes the streaming juices of live plant cells.

1666
Isaac Newton begins work on laws of mechanics and gravitation.

1666
Robert Boyle (1627–1691), English physicist and chemist, publishes his Hydrostatical Paradoxes in which he details his experiments with fluids and refutes the old doctrine that a light liquid can exert no pressure against a heavier fluid.

1669
Johann Joachim Becher (1635–1682), German chemist, publishes his Physica subterranea, in which he is the first to attempt the formulation of a general theory of chemistry. His concept of "terra pinguis" as the substance in air that burns forms the basis of the later phlogiston theory.

1671
Robert Boyle (1627–1691), English physicist and chemist, produces hydrogen by dissolving iron in hydrochloric or sulfuric acid, but he is unaware of his achievement.

1672
Isaac Newton (1642–1727), English scientist and mathematician, publishes his letter on light in the Royal Society's Philosophical Transactions. This letter, which is his first scientific publication, details his prism experiments of 1666 and offers findings that reveal for the first time the nature of light. He recounts how he let a ray of sunlight enter a darkened room through a small hole and then passed the ray through a prism onto a screen. The ray was refracted and a band of consecutive colors in rainbow order appeared. He then passed each separate color through another prism and noted that although the light was refracted, the color did not change. From this, he deduced that sunlight (or white light) consists of a combination of these colors. Later he elaborates further on this ground-breaking experiment.

1673
Christiaan Huygens (1629–1695), Dutch physicist and astronomer, publishes his Horologium oscillatorium in which he details his invention of the pendulum, or grandfather, clock. He employs Galileo's principle of isochronicity and ingeniously adapts it to the inner workings of a clock, beginning the era of accurate timekeeping that is so important to the advancement of physics. This highly original work not only demonstrates great mechanical ability but superior mathematical theorizing as well.

1674
Hennig Brand (c.1630–c.1692), German chemist, discovers phosphorus, which he finds in urine. This is the first discovery of an element that was not known in any earlier form.

1674
Robert Hooke attempts to explain planetary motion as a balance of centrifugal force and gravitational attraction.

1675
Giovanni Cassini, Saturn has separated rings which must be composed of small objects.

1675
Nicolas Lémery (1645–1715), French chemist and physician, publishes his Cours de chymie, which becomes the authoritative textbook on chemistry for the next 50 years. He is an adherent of Boyle's, and advocates the experimental method.

1676
Edmé Mariotte (1620–1684), French physicist, independently formulates Boyle's law and adds an important qualification to it. Like Boyle, he notes that air expands with rising temperature and contracts with falling temperature, but he adds that the inverse relationship between temperature and pressure only holds if the temperature is kept constant. Because of this, Boyle's law is called Mariotte's law in France.

1676
Gottfried Wilhelm Leibniz (1646–1716), German philosopher and mathematician, criticizes Descartes' ideas of motion and formulates his own theory of dynamics which substitutes kinetic energy for the conservation of movement.

1676
Olaus Roemer measures the speed of light by observing Jupiter's moons.

1676
Robert Hooke, law of elasticity and springs.

1678
Christiaan Huygens writes about wave theory of light.

1678
Huygens discovers polarization of light.

1680
Isaac Newton demonstrates that inverse square law implies elliptical orbits.

1681
Johann Joachim Becher (1635–1682), German chemist, obtains tar from the distillation of coal. He also suggests that sugar is necessary for fermentation.

1683
Edmund Halley (1656–1742), English astronomer, states that Earth's magnetism is caused by four poles of attraction, two of them being in each hemisphere near each pole of Earth.

1684
Isaac Newton (1642–1727), English scientist and mathematician, provides the first summary exposition of his theory of gravitation in a memoir entitled Demotu corporum. He expands on it later in his 1687 Principia.

1686
Edmund Halley (1656–1742), English astronomer, develops a reliable formula that links the altitudes of various localities with the atmospheric pressure measured there. His altimetric formula is one of the first practical applications of the new barometric discoveries.

1687
Denis Papin (1647–1712), French physicist, publishes a work in which he offers details on the use of steam to drive a piston in a cylinder which eventually becomes the basic design for an early steam engine. He never built one of his own design.

1687
English physicist Sir Isaac Newton (1642–1727) publishes a law of universal gravitation in his important and influential work Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). Newton articulates three laws of motion. Still widely regarded as the greatest scientific work ever written, Principia states that the entire world is subsumed under a single set of laws. They are: (1) a body remains at rest unless it is compelled to change by a force impressed upon it; (2) the change of motion (the change of velocity times the mass of the body) is proportional to the forces impressed; and (3) to every action there is an equal and opposite reaction. From these laws he then deduces his law of universal gravitation. In its simplest form, Newton's law of universal gravitation states that bodies with mass attract each other with a force that varies directly as the product of their masses and inversely as the square of the distance between them.

1688
John Clayton (1657–1725), English cleric, obtains methane and recognizes its flammable nature.

1690
Christiaan Huygens (1629–1695), Dutch physicist and astronomer, publishes his Traité: de la lumiére in which he states his wave theory of light. This unpopular theory sees light as a longitudinal wave that undulates in the direction of its motion much as a sound wave does. The worth of this theory is not understood foray full century.

1690
Christiaan Huygens, principle of Huygens, secondary waves.

1690
Huygens publishes his (wave) theory of light.

1690
John Locke, knowledge comes only from experience and sensations.

1699
Guillaume Amontons (1663–1705), French physicist, publishes his observations on gases. In his work on different gases, he shows that each gas changes in volume by the same amount for a given change in temperature. Implied in this is the notion of absolute zero at which gases can contract no further.

1702
Wilhelm Homberg (1652–1715), German physician, discovers boric acid, which he calls "sedative salt."

1704
Isaac Newton (1642–1727), English scientist and mathematician, publishes his Opticks which is a comprehensive work containing his main discoveries on the nature of light and color as well as his particle, or corpuscular, theory flight.

1709
John Freind (1675–1728), English physician and chemist, publishes Praelectiones chemicae, one of the earliest attempts to use Newtonian principles to explain chemical phenomena.

1714
Gabriel Daniel Fahrenheit (1686–1736), German-Dutch physicist, invents the mercury thermometer, the first accurate thermometer. His use of mercury instead of alcohol means that temperatures far above the boiling point of water and well below its freezing point can be measured (since mercury has a higher boiling point than alcohol). He also invents the Fahrenheit temperature scale in which the freezing point of water is 32 degrees and the boiling point is 212 degrees. He arrives at these numbers by adding salt to water to find its lowest freezing point which he calls zero.

1718
Edmund Halley measures motion of stars.

1718
Etienne-François Geoffroy (1672–1731), French apothecary, publishes his Tables des différens rapports, which offers a table of affinities between various acids and alkalis or metals. Chemistry eventually accepts his prophetic concept of affinity.

1729
Pierre Bouguer (1698–1758), French physicist, publishes his Essai d'optique sur la gradation de la lumièere in which he makes some of the earliest measurements in astronomical photometry. He also investigates the absorption of light in the atmosphere and formulates what comes to be known as Bouguer's law. This concerns the attenuation of a light beam upon passage through a transparent medium.

1730
René Antoine Ferchault de Réaumur (1683–1757), French naturalist and physicist, develops a thermometer independently of Fahrenheit and establishes what comes to be known as the Reaumur temperature scale. This system has zero degrees as the freezing point of water and 80 degrees as the boiling point of water at normal atmospheric pressure.

1731
Rene Reaumur, alcohol/water thermometer.

1732
Herman Boerhaave (1668–1738), Dutch physician, publishes his Elementa chemicae, whose comprehensiveness makes it the most popular chemical textbook for many decades. It serves chemistry as a great teaching book and presents a concise outline of all chemical knowledge.

1732
Pierre Louis Moreau de Maupertuis (1698–1759), French mathematician, publishes his Discours sur la figure des astres in which he predicts the shape of Earth using Newtonian mechanics.

1733
Charles François de Cisternay Du Fay (1698–1739), French physicist, discovers that two electrified objects sometimes attract and sometimes repel each other. He notes that their means of being charged seems to be the difference and states that there are two types of electricity, resinous and vitreous. He formulates the basic electrical law that "like charges repel and unlike charges attract." Later, American statesman and scientist, Benjamin Franklin (1706–1790) calls these two types of electricity positive and negative.

1733
Georg Brandt (1694–1768), Swedish chemist, publishes the first accurate and complete study of arsenic and its compounds.

1735
Carl Linnaeus publishes his Systema Naturae, or The Three Kingdoms of Nature Systematically Proposed in Classes, Orders, Genera, and Species, a methodical and hierarchical classification of all living beings. He develops the binomial nomenclature for the classification of plants and animals. In this system, each type of living being is classified in terms of genus (denoting the group to which it belongs) and species (its particular, individual name). His classification of plants is primarily based on the characteristics of their reproductive organs.

1738
Daniel Bernoulli (1700–1782), Swiss mathematician, publishes his Hydrodynamica containing his kinetic theory of gases. This treatise becomes a work of major importance in both physics and chemistry. This is the first attempt at an explanation of the behavior of gases, which, he assumes, are composed of a vast number of tiny particles.

1739
Georg Brandt, element cobalt.

1742
Anders Celsius (1701–1744), Swedish astronomer, applies a new scale to his thermometer by dividing the temperature difference between the boiling and freezing points of water into an even 100 degrees (with zero at the boiling point, but eventually this is reversed). His system becomes known as the centigrade scale and eventually is adopted internationally by scientists.

1742
Anton Svab (1703–1768) of Sweden, also known as Swab, distills zinc from the alloy calamine.

1743
Alexis Claude Clairaut (1713–1765), French mathematician, publishes his Theorie de la figure de la terre in which he definitively discusses a rotating body in the shape of the Earth and how it acts under the influence of gravity and centrifugal force.

1746
Leonhard Euler (1707–1783), Swiss mathematician, argues against the particle theory of light and suggests correctly that light has a wave form and that color depends on the length of that wave.

1750
Thomas Wright, Milky Way could be due to slab like distribution of stars.

1752
Benjamin Franklin (1706–1790), American statesman and scientist, flies a kite carrying a pointed wire in a thunderstorm and attempts to test his theory that atmospheric lightning is an electrical phenomenon similar to the spark produced by an electrical frictional machine. To the kite he attaches a silk thread with a metal key at the end, and as lightning flashes, he puts his hand near the key that sparks, just as a Leyden jar would. He proves his point in this extremely dangerous experiment.

1754
Joseph Black's discovery of carbon dioxide establishes that there are gases other than air.

1754
Pierre-Louis Moreau de Maupertuis suggests that species change over time, rather than remaining fixed.

1756
John Canton (1718–1772), English physicist, begins three years of careful weather observations and finds that on days when the aurora borealis is very noticeable, a compass needle becomes irregular. This is the first observation of what become known as magnetic storms.

1757
Albrecht von Haller 1757–1766), publishes the first volume of his eight-volume Elements of Physiology of the Human Body, subsequently to become a landmark in the history of modern physiology.

1758
Axel Fredrick Cronstedt (1722–1756), Swedish chemist, initiates the classification of minerals by their chemical structure as well as by their appearance. He notes four kinds of minerals: earths, metals, salts, and bitumens.

1760
Lagrange formulates principle of least action.

1766
Henry Cavendish (1731–1810), English chemist and physicist, publishes a paper on "Factitious Airs" in the Royal Society's Philosophical Transactions, which relates his discovery of hydrogen, or what he calls "inflammable" air.

1772
Joseph Priestley (1733–1804), English chemist, experiments with "fixed air" and writes his "Directions for Impregnating Water with Fixed Air," in which he details the production of seltzer water by using carbon and water. The distinctive taste of the seltzer, or soda, water brings Priestley much fame.

1774
Antoine-Laurent Lavoisier (1743–1794), French chemist, discovers that oxygen is consumed during respiration.

1774
Manganese is discovered by Swedish mineralogist Johann Gottlieb Gahn (1745–1818) and Karl Wilhelm Scheele (1742–1786), Swedish chemist.

1775
Antoine-Laurent Lavoisier (1743–1794), French chemist, publishes his "Memoire," which contains his first major disavowal of the phlogiston theory, as well as a revision of his combustion theory.

1777
Lavoisier argues that chemical compounds are composed of discrete elements.

1777
Coulomb invents torsion balance (measuring charge).

1780
Lazzaro Spallanzani carried out experiments on fertilization in frogs and attempted to determine the role of semen in the development of amphibian eggs.

1781
William Herschel discovers Uranus.

1782
Torbern Olaf Bergman (1735–1824), Swedish mineralogist, publishes his Skiagraphia regni mineralis, in which he classifies minerals into four main groups: salts, earths, metals, and inflammable bodies.

1783
Antoine-Laurent Lavoisier (1743–1794), French chemist, and Pierre-Simone de Laplace (1749–1827), French astronomer and mathematician, jointly publish a paper, "Memoire sur la chaleur," which lays the foundations of thermochemistry. They demonstrate that the quantity of heat required to decompose a compound into its elements is equal to the heat evolved when that compound was formed from its elements.

1783
Antoine-Laurent Lavoisier (1743–1794), French chemist, repeats the experiment conducted by Cavendish in 1766 and realizes that he is dealing with a separate gas. He calls this flammable gas "hydrogen," from the Greek phrase meaning "giving rise to water."

1783
Horace Benedict de Saussure (1740–1799), Swiss physicist, publishes his Essais sur l'hygrometrie in which he details his invention of a hygrometer that uses human hair for measuring humidity. He also discusses the general principles of hygrometry.

1783
Juan Jose d'Elhuyar (1754–1796) and his younger brother, Don Fausto d'Elhuyar (1755–1833), both Spanish mineralogists, analyze a mineral called wolframite and discover a new metal, tungsten.

1785
James Hutton (1726–1797) proposes uniformitatianism as theoretical basis to interpret geological history of the earth.

1785
Charles Augustin de Coulomb (1736–1806), French physicist, publishes the first of seven papers in which he establishes the basic laws of electrostatics and magnetism. Using his newly invented torsion balance, he determines that Newton's law of inverse squares also applies to electrical and magnetic attraction and repulsion. He states that the degree of attraction or repulsion depends on the amount of electric charge or the magnetic pole strength.

1787
Antoine-Laurent Lavoisier (1743–1794), French chemist, publishes Methode de la nomenclature chimique in collaboration with French chemist, Louis-Bernard Guyton de Morveau (1737–1816). This book gives the new chemistry a modern terminology and changes chemical nomenclature to correspond to the new antiphlogiston theory.

1787
Ernst Florens Friedrich Chladni (1756–1827), German physicist, publishes his Theorie des Klanges in which he is the first to discover the quantitative relationships that rule the transmission of sound. He also creates Chladni's figures by spreading sand on thin plates and vibrating them, producing complex patterns from which much is learned about vibrations. He is considered the father of acoustics.

1787
Jacques-Alexandre Charles (1746–1823), French physicist, demonstrates that different gases all expand by the same amount with a given rise in temperature if the pressure is held constant. This becomes known as Charles's law, and also Gay-Lussac's law.

1789
Martin Heinrich Klaproth (1743–1817), German chemist, discovers uranium in pitchblende. The Curies will refine this same substance in 1898. Later the same year, Klaproth discovers the element zirconium in the mineral zircon.

1790
France introduces the metric system.

1795
James Hutton publishes Theory of the Earth (in Scotland).

1795
Martin Heinrich Klaproth (1743–1817), German chemist, isolates a new metal and names it titanium, after the Titans of Greek mythology. He gives full credit to English mineralogist William Gregor (1761–1817), who first discovered it in 1791.

1796
Erasmus Darwin, grandfather of Charles Darwin and Francis Galton, publishes his Zoonomia. In this work, Darwin argues that evolutionary changes are brought about by the mechanism primarily associated with Jean-Baptiste Lamarck, that is, the direct influence of the environment on the organism.

1798
Henry Cavendish (1731–1810), English chemist and physicist, is the first to calculate Earth's mass. He does this by obtaining what Isaac Newton had not provided—a value for the gravitational constant. He builds a model with light balls and large, heavy ones and uses a sensitive wire to calculate the strength of the attraction between the two. Once he obtains this constant value, he calculates Earth's mass close to modern value.

1801
William Hyde Wollaston (1766–1828), English chemist and physicist, establishes that frictional and galvanic electricity are identical. He also discovers, independently of Ritter, the existence of invisible light beyond violet light (ultraviolet light).

1802
John Dalton introduces modern atomic theory into the science of chemistry.

1802
Thomas Thomson (1773–1852), Scottish chemist, publishes his System of Chemistry and introduces a system of symbols for individual minerals using the first letters of their names.

1802
Thomas Young (1773–1829), English physicist and physician, performs his classic experiment on interference in which sunlight is made to pass through two pinholes in an opaque screen. With a wave interpretation of his observations, he is able to provide the first quantitative values for the length of light waves.

1803
John Dalton (1766–1844), English chemist, states the law of multiple proportions which applies to two elements that could combine in more than one way. He also states his atomic theory that says that all elements are composed of extremely tiny, indivisible, indestructible atoms, and that all the known substances are composed of some combination of these atoms. He finally states that these atoms differ from each other only in mass, and that this difference can be measured.

1803
Louis Poinsot (1777–1859), French mathematician, publishes his Éléments de statique which contains his theory of couples (two parallel forces of equal magnitude but opposite direction form a couple). He also introduces the concept of torque.

1803
Smithson Tennant, elements osmium and iridium.

1803
William Henry (1774–1836), English physician and chemist, proposes what becomes Henry's law, stating that the amount of gas absorbed by a liquid is in proportion to the pressure of the gas above the liquid, provided no chemical reaction occurs.

1805
John Dalton (1766–1844), English chemist, publishes the first table of atomic weights and invents a new system of chemical symbols.

1805
Joseph-Louis Gay-Lussac (1778–1850), French chemist, establishes that precisely two volumes of hydrogen combine with one volume of oxygen to form water.

1811
Amedeo Avogadro (1776–1856), Italian physicist, first proposes his theory of molecules, which is confirmed much later by modern chemistry. He states that equal volumes of all gases contain the same number of molecules if they are under the same pressure and temperature.

1813
Sowerby publishes Sowerby's Mineral Conchology.

1815
William Smith publishes first geological map of UK.

1818
Jöns Jakob Berzelius (1779–1848), Swedish chemist, discovers selenium, publishes a table of atomic weights, and offers a system of chemical symbols. His weight table is based on a standard 100 for oxygen and becomes accepted in the twentieth century. His symbols use one or two letters of the Latin name and are essentially also retained in the twentieth century.

1819
Eilhardt Mitscherlich (1794–1863), German chemist, discovers isomorphism. This chemical theory states that compounds of similar composition tend to crystallize together, or, conversely, that compounds with the same crystal form are analogous in chemical composition. This law becomes useful in establishing atomic weights.

1819
Øersted discovers electromagnetism.

1820
André Marie Ampère (1775–1836), French mathematician and physicist, extends Ørsted's work and formulates one of the basic laws of electromagnetism. He discovers that two parallel wires each carrying a current attract each other if the currents are in the same direction, but they repel each other if in the opposite direction. He concludes that magnetism is the result of electricity in motion.

1820
Hans Christian Ørsted (1777–1851), Danish physicist, experiments with a compass and electricity and demonstrates that a current of electricity creates a magnetic field. He announces his discovery in a short article. This is the first time a real connection can be shown between electricity and magnetism, and it founds the new field of electromagnetism.

1821
Faraday proposes flux line picture for electricity and magnetism.

1821
Friedlieb Ferdinand Runge (1795–1867), German chemist, discovers caffeine.

1822
Omalius d'Holloy names the Cretaceous.

1823
Mary Anning discovers first complete pterodactyl in Lyme Regis in Dorset UK.

1824
Nicolas Léonard Sadi Carnot (1796–1832), French physicist, publishes his Réflexions sur la puissance motrice du feu in which he is the first to consider quantitatively the manner in which heat and work are interconverted. This highly original work also introduces the important concept of cyclic operations and the principle of reversibility. Although this work founds the science of thermodynamics, or the movement of heat, it is neglected for ten years.

1825
Hans Christian Ørsted (1777–1851), Danish physicist, isolates aluminum through a four-step process that involves a vacuum.

1826
André Marie Ampère (1775–1836), French mathematician and physicist, publishes his Mémoire sur la théoriemathématique des phénomàmes électrodynamiquesin which he offers the mathematical laws that govern the new field of electricity in motion and founds the study of electrodynamics.

1826
James Cowles Prichard presented his views on evolution in the second edition of his book Researches into the Physical History of Man (first edition 1813). These ideas about evolution were suppressed in later editions.

1827
Brown discovers "Brownian Motion".

1830
Charles Lyell publishes Principles of Geology and argues Earth is a least millions of years old.

1830
Joseph Henry (1797–1878), American physicist, discovers the principle of electromagnetic induction, showing how an electric current in one coil may set up a current in another through the development of a magnetic field. He puts off further work on this discovery until the following summer, and in the intervening time, the English physicist and chemist, Michael Faraday (1791–1867), publishes his discovery first.

1831
Charles Robert Darwin begins his historic voyage on the H.M.S. Beagle (1831–1836). His observations during the voyage lead to his theory of evolution by means of natural selection.

1834
F. Von Alberti names the Triassic Period.

1840
Friedrich Gustav Jacob Henle publishes the first histology textbook, General Anatomy. This work includes the first modern discussion of the germ theory of communicable diseases.

1840
Germain-Henri Hess (1802–1850), Swiss-Russian chemist, formulates the law that states that the quantity of heat evolved in a chemical change is the same no matter what chemical route the reaction takes (through a single stage or through many stages. This becomes known as Hess's law.

1840
Jöns Jakob Berzelius (1779–1848), Swedish chemist, first introduces the term "allotropy" to describe the existence of different varieties of an element. He converts charcoal into graphite and declares that the same element may have in different forms.

1840
Karl Bogislaus Reichert introduces the cell theory into embryology. He proves that the segments observed in fertilized eggs develop into individual cells, and that organs develop from cells.

1840
Louis Agassiz publishes his Etudes sur les glaciers. Also discovers glacial feature in Scotland away from an ice covered area and advances the theory of glaciation.

1840
Murchison and Sedgwick name the Devonian Period after the county of Devon in UK.

1840
Rudolf Albert von Kölliker establishes that spermatozoa and eggs are derived from tissue cells. He attempts to extend the cell theory to embryology and histology.

1841
Murchison names the Permian Period.

1842
Charles Robert Darwin writes out an abstract of his theory of evolution, but he does not plan to have this theory published until after his death.

1842
Christian Johann Doppler (1803–1853), Austrian physicist, discovers the effect of motion of the source or observer on the observed frequency of sound waves. This mathematical relationship that relates their pitch to the relative motion of the source or observer is called the Doppler effect.

1846
Johann Gottfried Galle discovers Neptune, accounting for observed perturbations in the motion of Uranus.

1848
William Thomson (1824–1907), later known as Lord Kelvin, Scottish mathematician and physicist, explores the concept of absolute zero at which the volume of a gas would be reduced to zero and explains it by stating that it is not that the volume reaches zero but rather that the motion of the gas's molecules stops. He then proposes a new temperature scale with its zero mark at absolute zero and its degrees equal to those on the centigrade scale. It becomes known as the kelvin scale. He also coins the term thermodynamics.

1850
Michael Faraday experiments to find link between gravity and electromagnetism fail.

1850
Rudolph Julius Emanuel Clausius (1822–1888), German physicist, publishes a paper which contains what becomes known as the second law of thermodynamics, stating that, "heat cannot, of itself, pass from a colder to a hotter body." He later refines the concept.

1850
Thomas Graham (1805–1869), Scottish physical chemist, studies the diffusion of a substance through a membrane (osmosis) and first distinguishes between crystalloids and colloids. He becomes the founder of colloidal chemistry.

1851
Armand Hippolyte Fizeau (1819–1896), French physicist, measures the speed of light as it flows with a stream of water and as it goes against the stream. He finds that the velocity of light is higher in the former.

1851
Jean Bernard Léon Foucault (1819–1868), French physicist, conducts his spectacular series of experiments associated with the pendulum. He swings a heavy iron ball from a wire more than 200 feet long and demonstrates that the swinging pendulum maintains its plane while Earth slowly twists under it. The crowd of spectators who witnesses this demonstration come to realize that they are watching Earth rotate under the pendulum — experimental proof of a moving Earth.

1851
William Thomson (1824–1907), later known as Lord Kelvin, Scottish mathematician and physicist, publishes On the Dynamical Theory of Heat in which he explores Carnot's work and deduces that all energy tends to rundown and dissipate itself as heat. This is another form of the second law of thermodynamics and is advanced further by Clausius at about the same time. Kelvin's work is considered the first nineteenth-century treatise on thermodynamics.

1852
Abraham Gesner (1797–1864), Canadian geologist, prepares the first kerosene from petroleum. He obtains the liquid kerosene by the dry distillation of asphalt rock, treats it further, and calls the product kerosene after the Greek word keros, meaning oil.

1852
Alexander William Williamson (1824–1904), English chemist, publishes his study which shows for the first time that catalytic action clearly involves and is explained by the formation of an intermediate compound.

1852
Edward Frankland (1825–1899), English chemist, announces the theory of valence, in which he states that each type of atom has a fixed capacity for combining with other atoms. This concept will lead eventually to Mendeleev's Periodic Table.

1852
James Joule (1818–1889) and William Thomson (1824–1907), both English physicists, show that when a gas is allowed to expand freely, its temperature drops slightly. This becomes known as the Joule–Thomson effect and is evidence that molecules of gases have a slight attraction for other molecules. Overcoming this attraction uses energy and causes a drop in temperature.

1852
Jean Bernard Léon Foucault (1819–1868), French physicist, learns from his pendulum experiment and invents the gyroscope. He sets a wheel within a heavy rim in rotation and sees that when tipped, it is set right again by the force of gravity.

1853
Anders Jonas Ångström (1814–1874), Swedish physicist, demonstrates that the rays emitted by an incandescent gas have the same refrangibility (ability to be refracted) as the rays absorbed by the same gas.

1853
Hans Peter Jorgen Julius Thomsen (1826–1901), Danish chemist, works out a method of manufacturing sodium carbonate from the mineral cryolite. This mineral will soon become important to the production of aluminum.

1853
Johann Wilhelm Hittorf (1824–1914), German chemist and physicist, offers the notion of the transport number as he suggests that ions in a solution with a current running through it travel at different speeds.

1853
William John Macquorn Rankine (1820–1872), Scottish engineer, introduces into physics the concept of potential energy, also called the energy of position.

1854
George Airy, Estimate of Earth mass from underground gravity.

1854
Gregor Mendel begins studying 34 different strains of peas. He selects 22 kinds for further experiments. From 1856 to 1863, Mendel grows and tests over 28,000 plants and analyzes seven pairs of traits.

1855
Charles-Adolphe Wurtz (1817–1884), French chemist, develops a method of synthesizing long-chain hydrocarbons by reactions between alkyl halides and metallic sodium. This method is called the Wurtz reaction.

1855
James Clerk Maxwell, mathematics of Faraday's lines of force.

1855
Johann Heinrich Wilhelm Geissler (1814–1879), German inventor, devises a mercury pump that produces a much better vacuum than old piston pumps. Called Geissler tubes, they make possible a more advanced study of electricity and eventually of the atom. It is with Geissler tubes that the English physicist, Joseph John Thomson (1856–1940), performs his famous experiments elucidating the nature of electrons.

1855
William Parsons, spiral galaxies.

1856
Neanderthal fossil identified.

1857
Louis Pasteur demonstrates that lactic acid fermentation is caused by a living organism. Between 1857 and 1880, he performs a series of experiments that refute the doctrine of spontaneous generation. He also introduces vaccines for fowl cholera, anthrax, and rabies, based on attenuated strains of viruses and bacteria.

1857
Rudolf Julius Emmanuel Clausius (1822–1888), German physicist, offers a new explanation of evaporation in terms of molecules and their velocities. He shows that evaporation produces a loss of energy in the liquid and a decrease in temperature.

1858
Charles Darwin and Alfred Russell Wallace agree to a joint presentation of their theory of evolution by natural selection.

1858
Friedrich August Kekulé von Stradonitz (1829–1896), German chemist, and Archibald Scott Couper (1831–1892), Scottish chemist, first develop symbols to represent the atom is always tetravalent (meaning always combines with four other atoms).

1858
Hermann Ludwig Ferdinand von Helmholtz (1821–1894), German physiologist and physicist, publishes his study on the integrals of hydrodynamic equations that express whirling motion. This becomes the point of departure for new ideas on the structure of matter that eventually replaces the old atomistic concepts.

1858
Julius Plücker (1801–1868), German mathematician and physicist, sends an electric current through a vacuum and describes fluorescent effects in detail. He also discovers that the glow shifts position when placed in the field of an electromagnet. This is the very beginning of an awareness of subatomic particles.

1859
Gustav Robert Kirchhoff (1824–1887), German physicist, after discovering the relation between emission and absorption spectra, concludes that the ratio of the emissive and absorptive powers of a body at each wave length is the same for all bodies at the same temperature. This becomes known as Kirchhoff's law.

1859
James Clerk Maxwell (1831–1879), Scottish mathematician and physicist, studies the rings of Saturn and produces the first extensive mathematical development of the kinetic theory of gases. He shares this discovery of the distribution of molecular speeds in a gas with Ludwig E. Boltzmann (1844–1906), Austrian physicist, who accomplishes the same independently. It comes to be known as the Maxwell–Boltzmann theory of gases.

1859
Urbain Le Verrier, anomolous perihelion shift of Mercury.

1860
Cesium is the first element discovered using the newly developed spectroscope. Robert Wilhelm Bunsen (1811–1899), German chemist, and Gustav Robert Kirchhoff (1824–1887), German physicist, name their new element cesium after its "sky blue" color in the spectrum.

1860
Gustav Robert Kirchhoff (1824–1887), German physicist, introduces the concepts of black bodies and emissivity. A black body is a surface that absorbs all radiation of any wavelength falling on it. Such a body would emit all wavelengths if it were heated to incandescence. This concept later becomes important to quantum theory.

1860
Jean-Servais Stas (1813–1891), Belgian chemist, begins work that leads to an accurate method of determining atomic weights. By 1865, he produces the first modern table of atomic weights using oxygen as a standard.

1860
Robert Wilhelm Bunsen (1811–1899), German chemist, collaborates with Gustav Robert Kirchhoff (1824–1887), German physicist, and they develop the first spectroscope.

1860
Stanislao Cannizzaro (1826–1910), Italian chemist, publishes the forgotten ideas of Italian physicist Amedeo Avogadro (1776–1856)—about the distinction between molecules and atoms—in an attempt to bring some order and agreement on determining atomic weights.

1861
Alexander Mikhailovich Butlerov (1828–1886), Russian chemist, introduces the term "chemical structure" to mean that the chemical nature of a molecule is determined not only by the number and type of a atoms but also by their arrangement.

1861
Friedrich August Kekulé von Stadonitz (1829–1896), German chemist, publishes the first volume of Lehrbuch der organischen Chemie, in which he is the first to define organic chemistry as the study of carbon compounds.

1861
Robert Wilhelm Bunsen (1811–1899), German chemist, and Gustav Robert Kirchhoff (1824–1887), German physicist, discover the metal rubidium, using their new spectroscope.

1861
William Crookes (1832–1919), English physicist, discovers the element thallium by using the newly invented spectrum analysis. The following year, it is isolated by French chemist Claude-August Lamy (1820–1878).

1861
William Thomson (1824–1907), later known as Lord Kelvin, Scottish mathematician and physicist, publishes his Physical Considerations Regarding the Possible Age of the Sun's Heat which contains the theme of the heat death of the Universe. This is offered in light of the principle of dissipation of energy stated in 1851.

1862
Anders Ångström observes hydrogen in the sun.

1863
Ferdinand Reich (1799–1882), German mineralogist, and his assistant Hieronymus Theodor Richter (1824–1898), examine zinc ore spectroscopically and discover the new, indigo-colored element iridium. It is used in the next century in the making of transistors.

1863
William Huggins, stellar spectra indicate that stars made of same elements found on Earth.

1864
James Clerk Maxwell publishes equations of electromagnetic wave propagation in the ether.

1865
Alexander Parkes (1813–1890), English chemist, produces celluloid, the first synthetic plastic material. After working since the 1850s with nitrocellulose, alcohol, camphor, and castor oil, he obtains a material that can be molded under pressure while still warm. Parkes is unsuccessful at marketing his product, however, and it is left to the American inventor, John Wesley Hyatt (1837–1920), to make it a success.

1865
Johann Joseph Loschmidt (1821–1895), Austrian chemist, is the first to attempt to determine the actual size of atoms and molecules. He uses Avogadro's hypothesis to calculate the number of molecules in 22.4 liters of gas, and calls the resulting number Avogadro's number (6.02x10²³).

1865
Rudolf Julius Emanuel Clausius (1822–1888), German physicist, refines the second law of thermodynamics and first introduces the term entropy, stating that the energy in a closed system will always eventually rundown.

1866
August Adolph Eduard Eberhard Kundt (1839–1894), German physicist, invents a method by which he can make accurate measurements of the speed of sound in the air. He uses a Kundt's tube whose inside is dusted with fine powder which is then disturbed by traveling sound waves.

1866
Ernst Heinrich Haeckel publishes his book A General Morphology of Organisms. Haeckel summarizes his ideas about evolution and embryology in his famous dictum "ontogeny recapitulates phylogeny." Haeckel suggests that the nucleus of a cell transmits hereditary information. He introduces the use of the term "ecology" to describe the study of living organisms and their interactions with other organisms and with their environment.

1866
Johann Gregor Mendel (1822–1884), Austrian botanist and monk, discovers the laws of heredity and writes the first of a series of papers on heredity (1866–1869), which formulate the laws of hybridization. His work is disregarded until 1900, when de Vries rediscovers it. Unbeknownst to both Darwin and Mendel, Mendelian laws provide the scientific framework for the concepts of gradual evolution and continuous variation.

1868
Pierre-Jules-César Janssen (1824–1907), French astronomer, studies a total eclipse of the Sun and observes an unknown spectral line. He forwards the data to the English astronomer Joseph Norman Lockyear (1836–1920), who concludes it is an unknown element that he names helium, after the Sun.

1868
William Huggins, Doppler shifts of stellar spectra.

1869
Dimitri Ivanovich Mendeleev (1834–1907), Russian chemist, and Julius Lothar Meyer (1830–1895), German chemist, independently put forth the Periodic Table of Elements, which arranges the elements in order of atomic weights. However, Meyer does not publish until 1870, nor does he predict the existence of undiscovered elements as does Mendeleev.

1871
Charles Robert Darwin published The Descent of Man, and Selection in Relation to Sex. This work introduces the concept of sexual selection and expands his theory of evolution to include humans.

1871
John William Strutt Rayleigh (1842–1919), English physicist, discovers that the degree of scattering of light by very fine particles is a function of the wavelength of light. His equation offers a solution to the question of why the sky is blue.

1871
Ludwig Eduard Boltzmann (1844–1906), Austrian physicist, begins work on his mathematical treatment of the second law of thermodynamics, interpreting it in terms of the statistics of molecular motions. His work lays the foundations of statistical mechanics.

1872
Ferdinand Julius Cohn publishes the first of four papers entitled "Research on Bacteria," which establishes the foundation of bacteriology as a distinct field. He systematically divides bacteria into genera and species.

1873
Franz Anton Schneider describes cell division in detail. His drawings includes both the nucleus and chromosomal strands.

1873
James Clerk Maxwell (1831–1879), Scottish mathematician and physicist, publishes Treatise on Electricity and Magnetism in which he identifies light as an electromagnetic phenomenon. He determines this when he finds his mathematical calculations for the transmission speed of both electromagnetic and electrostatic waves are the same as the known speed of light. This landmark work brings together the three main fields of physics— electricity, magnetism, and light.

1873
Johannes Van der Waals (1837–1923), Dutch physicist, offers an equation for the gas laws which contains terms relating to the volumes of the molecules themselves and the attractive forces between them. It becomes known as the Van der Waals equation.

1873
Walther Flemming discovers chromosomes, observes mitosis, and suggests the modern interpretation of nuclear division.

1876
Henry Augustus Rowland (1848–1901), American physicist, establishes for the first time that a moving electric charge or current is accompanied by electrically charged matter in motion and produces a magnetic field.

1876
James Clerk Maxwell (1831–1879), Scottish mathematician and physicist, publishes his Matter and Motion in which he considers the categories of space and time and states with great prescience that, "all our knowledge, both of time and place, is essentially relative."

1876
Josiah Willard Gibbs (1829–1903), American physicist, discovers the phase rule. He arrives at an equation that relates the variables (like temperature and pressure) to different phases (solid, liquid, gas). His work helps lay the foundation for chemical thermodynamics and generally for modern physical chemistry. The rule is later put into practical application by Dutch physical chemist Hendrik Willem Bakhuis Roozeboom (1854–1907).

1877
Astonomer Asaph Hall identifies two moons of Mars.

1880
Carl Oswald Viktor Engler (1842–1925), German chemist, begins his studies on petroleum. He is the first to state that it is organic in origin.

1882
Robert Koch (1843–1910), German bacteriologist, discovers the tubercle bacillus and enunciates "Koch's postulates," which define the classic method of preserving, documenting, and studying bacteria.

1883
August F. Weismann begins work on his germplasm theory of inheritance. Between 1884 and 1888, Weismann formulates the germplasm theory that argues that the germplasm is separate and distinct from the somatoplasm. Weismann argues that the germplasm is continuous from generation to generation and that only changes in the germplasm are transmitted to further generations. Weismann proposes a theory of chromosome behavior during cell division and fertilization and predicts the occurrence of a reduction division (meiosis) in all sexual organisms.

1883
Ernst Mach (1838–1916), Austrian physicist, publishes his Die Mechanik in ihrer Entwickelung historisch-kritisch dargestellt in which he offers a radical philosophy of science that calls into question the reality of such Newtonian ideas as space, time, and motion. His work influences Einstein and prepares the way for relativity.

1883
Frank Wigglesworth Clarke (1847–1931), American chemist and geophysicist, is appointed chief chemist to the U.S. Geological Survey. In this position, he begins an extensive program of rock analysis and is one of the founders of geochemistry.

1883
George Francis Fitzgerald (1851–1901), Irish physicist, first suggests a method of producing radio waves. From his studies of radiation, he concludes that an oscillating current would produce electromagnetic waves. This is later verified experimentally by Hertz in 1888 and used in the development of wireless telegraphy.

1883
Johann Gustav Kjeldahl (1849–1900), Danish chemist, devises a method for the analysis of the nitrogen content of organic material. His method uses concentrated sulfuric acid and is simple and fast.

1883
Thomas Alva Edison (1847–1931), American inventor, discovers the emission of electrons from hot bodies. He inserts a small metal plate near the filament of a light bulb and finds that the plate draws a current when he connects it to the positive terminal of the light bulb circuit, even though the plate is not touching the filament. Called the Edison effect, this is later a major factor in the invention of the vacuum tube.

1884
Svante August Arrhenius (1859–1927), Swedish chemist, first proposes the concept of ions being atoms bearing electrical charges.

1886
Paul-Louis-Toussint Héroult (1863–1914), French metallurgist, and Charles Martin Hall (1863–1914), American chemist, independently invent an electro-chemical process for extracting aluminum from its ore. This process makes aluminum cheaper and forms the basis of the huge aluminum industry. Hall makes the discovery in February of this year, and Héroult achieves his in April.

1886
Roland von Eötvös (1848–1919), Hungarian physicist, first introduces the concept of molecular surface tension. His study of Earth's gravitational field, which leads him to invent a precise torsion balance, results in his proof that inertial mass and gravitational mass are equivalent. This proves to be a major principle in Einstein's general theory of relativity.

1887
Albert Abraham Michelson (1852–1931), German-American physicist, collaborates with the American chemist, Edward Williams Morley (1838–1923), to test the age-old hypothesis that Earth moves through luminiferous ether (the supposed light-carrying element that exists outside or above Earth's atmosphere). They use Michelson's interferometer—that splits a beam of light in two, sends each on a different path, and then reunites them—to test out this idea. The failure of this extremely sensitive instrument to detect even the slightest change in the velocity of light proves that the ether does not exist (for it would have had to change slightly if it had gone through a substance). This result overturns all ether-based theories and makes physicists search for some explanation of the invariance of the speed of light.

1887
Ernst Mach (1838–1916), Austrian physicist, is the first to note the sudden change in the nature of the airflow over a moving object that occurs as it approaches the speed of sound. Because of this, the speed of sound in air (under certain temperature conditions) is called Mach 1. Mach 2 is twice that speed and so on.

1887
Herman Frasch (1851–1914), German-American chemist, patents a method for removing sulfur compounds from oil. Once the foul sulfur smell is removed through the use of metallic compounds, petroleum becomes a marketable product.

1887
Woldemar Voigt, anticipated Lorentz transform to derive Doppler shift.

1888
Heinrich Rudolf Hertz (1857–1894), German physicist, for the first time generates electromagnetic (radio) waves and devises a detector that can measure their wavelength. From this he is able to prove experimentally James Clerk Maxwell's (1831–1879) hypothesis that light is an electromagnetic phenomenon. Hertz's work not only discovers radio waves, but experimentally unites the three main fields of physics—electricity, magnetism, and light.

1890
Hendrik Antoon Lorentz (1853–1928), Dutch physicist, suggests that the atom consists of charged particles whose oscillations can be affected by a magnetic field. This is later confirmed by his pupil, the Dutch physicist, Pieter Zeeman (1865–1943), in 1896.

1891
Edward Goodrich Acheson (1856–1931), American inventor, discovers that carbon heated with clay yields an extremely hard substance. He names it carborundum, and eventually finds it to be a compound of silicon and carbon. For half a century it remains second only to a diamond in hardness, and becomes very useful as an abrasive.

1893
Augusto Righi (1850–1920), Italian physicist, demonstrates that Hertz (radio) waves differ from light only in wavelength and not because of any essential difference in their nature. This helps to establish the existence of the electromagnetic spectrum.

1893
Ferdinand-Frédéric-Henri Moissan (1852–1907), French chemist, produces artificial diamonds in his electric furnace.

1894
Guglielmo Marconi (1874–1937), Italian electrical engineer, uses Hertz's method of producing radio waves and builds a receiver to detect them. He succeeds in sending his first radio waves 30 feet to ring a bell. The next year, his improved system can send a signal 1.5 miles.

1894
John William Strutt Rayleigh (1842–1919), English physicist, and William Ramsay (1852–1916), Scottish chemist, succeed in isolating a new gas in the atmosphere that is denser than nitrogen and combines with no other element. They name it "argon," which is Greek for inert. It is the first of a series of rare gases with unusual properties whose existence had not been predicted.

1895
Pierre Curie (1859–1906), French chemist, studies the effect of heat on magnetism and shows that there is a critical temperature point above which magnetic properties will disappear. This comes to be called the Curie point.

1895
Wilhem Conrad Röntgen (1845–1923), German physicist, submits his first paper documenting his discovery of x rays. He tells how this unknown ray, or radiation, can affect photographic plates, and that wood, paper, and aluminum are transparent to it. It also can ionize gases and does not respond to electric or magnetic fields nor exhibit any properties of light. This discovery leads to such a stream of ground-breaking discoveries in physics that it has been called the beginning of the second scientific revolution.

1895
William Ramsay (1852–1916), Scottish chemist, discovers helium in a mineral named cleveite. It had been speculated earlier that helium existed only in the Sun, but Ramsay proves it also exists on Earth. It is discovered independently this year by Swedish chemist and geologist Per Theodore Cleve (1840–1905). Helium is an odorless, colorless, tasteless gas that is also insoluble and incombustible.

1896
Antoine Henri Becquerel (1852–1908), French physicist, studies fluorescent materials to see if they emit the newly-discovered x rays and discovers instead that uranium produces natural radiation that is eventually called radioactivity in 1898 by the Polish-French chemist, Marie Sklodowska Curie (1867–1934).

1897
Joseph John Thomson (1856–1940), English physicist, discovers the electron. He conducts cathode ray experiments and concludes that the rays consist of negatively charged "electrons" that are smaller in mass than atoms.

1898
Marie Sklodowska Curie (1867–1934), Polish-French chemist, discovers thorium, which she proves is radioactive.

1899
Ernest Rutherford (1871–1937), British physicist, discovers that radioactive substances give off different kinds of rays. He names the positively charged ones alpha rays and the negative ones beta rays.

1900
Carl Correns, Hugo de Vries, and Erich von Tschermak independently rediscover Mendel's laws of inheritance. Their publications mark the beginning of modern genetics. Using several plant species, de Vries and Correns perform breeding experiments that paralleled Mendel's earlier studies and independently arrive at similar interpretations of their results. Therefore, upon reading Mendel's publication, they immediately recognized its significance. William Bateson describes the importance of Mendel's contribution in an address to the Royal Society of London.

1900
Friedrich Ernst Dorn (1848–1916), German physicist, analyzes the gas given off by (radioactive) radium and discovers the inert gas he names radon. This is the first clear demonstration that the process of giving off radiation transmutes one element into another during the radioactive decay process.

1900
Hugo Marie de Vries describes the concept of genetic mutations in his book Mutation Theory. He uses the term mutation to describe sudden, spontaneous, and drastic alterations in the hereditary material.

1900
Max Karl Ernst Ludwig Planck (1858–1947), German physicist, publishes his classic and revolutionary paper on quantum physics. He tells of his discovery that light or energy is not found in nature as a continuous wave or flow, but is emitted and absorbed discontinuously in little packets, or quanta. Further, each quantum, or packet of energy, is indivisible. Planck's new notion of the quantum seemed to contradict the mechanics of Newton and the electromagnetics of Maxwell, and replace them with new rules. In fact, his quantum physics were new rules in a new type of physics—physics of the very fast and the very small. His theory is soon applied (by Einstein) and incorporated (by Bohr), and becomes the watershed between all physics that comes before it (classical physics) and all that is after (modern physics).

1900
Paul Karl Ludwig Drude (1863–1906), German physicist, proposes the first model for the structure of metals. His model explains the constant relationship between electrical conductivity and the heat conductivity in all metals.

1900
Paul Ulrich Villard (1860–1934), French physicist, discovers what are later called gamma rays. While studying the recently discovered radiation from uranium, he finds that in addition to the alpha rays and beta rays, there are other rays, unaffected by magnets, that are similar to x rays, but shorter and more penetrating.

1901
Antoine Henri Becquerel (1852–1908), French physicist, studies the rays emitted by the natural substance uranium and concludes that the only place they could be coming from is within the atoms of uranium. This marks the first clear understanding of the atom as something more than a featureless sphere. It implies a dynamic reality that might also contain electrons. Becquerel's discovery of radioactivity and his focus on the uranium atom make him the father of modern atomic and nuclear physics.

1901
Guglielmo Marconi (1874–1937), Italian electrical engineer, successfully sends radio signal from England to Newfoundland.

1902
Discovery of Tyrannosaurus Rex fossil.

1902
Lapworth names the Ordovician Period after a Welsh Iron Age tribe.

1902
Oliver Heaviside (1850–1925), English physicist and electrical engineer, and Arthur Edwin Kennelly (1861–1939), British-American electrical engineer, independently and almost simultaneously make the first prediction of the existence of the ionosphere, an electrically conductive layer in the upper atmosphere that reflects radio waves. They theorize correctly that wireless telegraphy works over long distances because a conducting layer of atmosphere exists that allows radio waves to follow the Earth's curvature instead of traveling off into space.

1903
Antoine Henri Becquerel (1852–1908), French physicist, shares the Nobel Prize in physics with the husband-and-wife team of Marie Sklodowska Curie (1867–1934), Polish-French chemist, and Pierre Curie (1859–1906), French chemist. Becquerel wins for his discovery of natural or spontaneous radioactivity, and the Curies win for their later research on this new phenomenon.

1903
Archibald Edward Garrod provides evidence that errors in genes cause several hereditary disorders in human beings. His book The Inborn Errors of Metabolism (1909) is the first treatise in biochemical genetics.

1903
Ernest Rutherford (1871–1937), British physicist, and Frederick Soddy (1877–1956), English chemist, explain radioactivity by their theory of atomic disintegration. They discover that uranium breaks down and forms a new series of substances as it gives off radiation.

1903
Walter S. Sutton publishes a paper in which he presents the chromosome theory of inheritance. The theory, which states that the hereditary factors are located in the chromosomes, is independently proposed by Theodor Boveri and is generally referred to as the Sutton–Boveri hypothesis.

1903
William Ramsay (1852–1916), Scottish chemist, and Frederick Soddy(1877–1956), English chemist, discover that helium is continually produced by naturally radioactive substances.

1904
Ernest Rutherford, age of Earth by radioactvity dating.

1904
Hendrik Antoon Lorentz (1853–1928), Dutch physicist, extends his idea of local time (different time rates in different locations) and arrives at what are called the Lorentz transformations. These are mathematical formulas describing the increase of mass, shortening of length, and dilation of time that are characteristic of a moving body. These eventually form the basis of Einstein's special theory of relativity.

1904
William Ramsay (1852–1916), Scottish chemist, receives the Nobel Prize in Chemistry for the discovery of the inert gaseous elements in air, and for his determination of their place in the periodic system.

1905
Albert Einstein (1879–1955), German-Swiss physicist, uses Planck's theory to develop a quantum theory of light which explains the photoelectric effect. He suggests that light has a dual, wave-particle quality.

1905
Albert Einstein (1879–1955), German-Swiss physicist, publishes special theory of relativity.

1905
Albert Einstein (1879–1955), German-Swiss (later German-born American) physicist, publishes an elegantly brief but seminal paper in which he asserts and proves his most famous formula, E=mc² (Energy = mass times the square of the speed of light squared) relating mass and energy.

1905
Percival Lowell postulates a ninth planet beyond Neptune.

1905
Walther Hermann Nernst (1864–1941), German physical chemist, announces his discovery of the third law of thermodynamics. He finds that entropy change approaches zero at a temperature of absolute zero, and deduces from this the impossibility of attaining absolute zero.

1907
Albert Einstein, equivalence principle and gravitational red shift.

1907
Georges Urbain (1872–1938), French chemist, discovers the last of the stable rare earth elements, and names it lutetium after the Latin name of Paris.

1907
Pierre Weiss (1865–1940), French physicist, offers his theory explaining the phenomenon of ferromagnetism. He states that iron and other ferromagnetic materials form small domains of a certain polarity pointing in various directions. When some external magnetic field forces them to be aligned, they become a single, strong magnetic force. This explanation is still accurate.

1908
Tunguska event occurs when a comet or asteroid enters the atmosphere, causing major damage to a forested region in Siberia.

1908
Alfred Wegener proposes the theory of continental drift.

1908
Ernest Rutherford (1871–1937), English physicist, is awarded the Nobel Prize in Chemistry for his investigations into disintegration of the elements and the chemistry of radioactive substances.

1908
Ernest Rutherford (1871–1937), British physicist, and Hans Wilhelm Geiger (1882–1945), German physicist, develop an electrical alpha-particle counter. Over the next few years, Geiger continues to improve this device which becomes known as the Geiger counter.

1908
Percy Williams Bridgman (1882–1961), American physicist, begins a lifetime of pioneering work in the field of high pressures. Working eventually in the unexplored field of attaining 400,000 atmospheres, he is forced to invent much of his equipment himself. As he extends the range of pressures, he becomes the founder of the laws of high pressure physics.

1909
Thomas Hunt Morgan selects the fruit fly Drosophila as a model system for the study of genetics. Morgan and his coworkers confirm the chromosome theory of heredity and realize the significance of the fact that certain genes tend to be transmitted together. Morgan postulates the mechanism of "crossing over." His associate, Alfred Henry Sturtevant demonstrates the relationship between crossing over and the rearrangement of genes in 1913.

1911
Arthur Holmes publishes the first geological time scale with dates based on radioactive measurements.

1911
Ernest Rutherford (1871–1937), British physicist, discovers that atoms are made up of a positive nucleus surrounded by electrons. This modern concept of the atom replaces the notion of featureless, indivisible spheres that dominated atomistic thinking for 23 centuries—since Democritus (c. 470–c.380).

1911
Victor Hess identifies high altitude radiation from space.

1912
Friedrich Karl Rudolf Bergius (1884–1949), German chemist, discovers how to treat coal and oil with hydrogen to produce gasoline.

1913
Charles Fabry (1867–1945), French physicist, first demonstrates the presence of ozone in the upper atmosphere. It is found later that ozone functions as a screen, preventing much of the Sun's ultraviolet radiation from reaching Earth's surface. Seventy-five years after the discovery of ozone, in 1985, a hole in the ozone layer over Antarctica is discovered via satellite.

1913
Niels Henrik David Bohr (1885–1962), Danish physicist, proposes the first dynamic model of the atom. It is seen as a very dense nucleus surrounded by electrons rotating in orbitals (defined energy levels).

1913
Between 1911-13, Danish astronomer Ejnar Hertzsprung (1873-1967) and American astronomer Henry Norris Russell (1877-1957) independently develop what is now known as the Hertzsprung-Russell diagram describing stellar evolution.

1913
Using the x-ray spectroscope which they invented, William Lawrence Bragg (1890–1971), Australian-English physicist, and his father William Henry Bragg (1862–1942), make the first determinations of the structures of simple crystals and demonstrate the tetrahedral distribution of carbon atoms in diamonds. Their perfection of x-ray crystallography leads to the later examination of the molecule structure of thousands of crystalline substances.

1914
Ejnar Hertzsprung measures the distance of the Large Magellanic Cloud using Cepheid variable stars.

1914
Ernest Rutherford (1871–1937), British physicist, discovers a positively charged particle he calls a proton.

1915
Albert Einstein (1879–1955), German-Swiss physicist, completes four years of work on his theory of gravitation, or what becomes known as the general theory of relativity.

1915
Richard Martin Willstätter (1872–1942), German chemist, is awarded the Nobel Prize in Chemistry for his research on plant pigments, especially chlorophyll.

1916
Karl Schwarzschild theorizes the existence of black holes.

1917
Albert Einstein, introduction cosmological constant and steady state model of the universe.

1917
Vesto Melvin Slipher observes that most galaxies have red shifts.

1917
Willem de Sitter describes a model of a static universe with no matter.

1918
Harlow Shapley determines the size and shape of our galaxy.

1918
Harlow Shapley measures distance to globular clusters using Cepheid variable stars.

1919
Arthur Eddington records data on the Sun's gravitational deflection of starlight during a solar eclipse, confirming Einstein's general theory of relativity.

1920
Ernest Rutherford (1871–1937), English physicist, names the positively charged part of the atom's nucleus a "proton."

1920
Shapley and Curtis, The Great Debate over the scale and structure of the universe.

1922
Alexsandr Friedmann develops a model of an expanding/oscillating universe with matter included.

1923
Arthur Holly Compton (1892–1962), American physicist, discovers what is later called the Compton effect. While accurately measuring the wavelengths of scattered x rays, he discovers that some of the rays had lengthened their wavelength in scattering. He accounts for this by stating that a photon of light strikes an electron, which recoils and subtracts some energy from the photon, thereby increasing its length. It is Compton who coins the term photon to describe light as a particle.

1923
Louis Victor Pierre Raymond de Broglie (1892–1987), French physicist, introduces the particle-wave hypothesis, stating that an electron or any other subatomic particle can behave either as a particle or as a wave. This idea is confirmed in 1927.

1924
Edwin Hubble measures the distance of other galaxies using Cepheid variables in galaxies outside ours.

1925
Robert Andrews Millikan (1868–1953), American physicist, names the radiation coming from outer space cosmic rays.

1925
Werner Karl Heisenberg (1901–1976), German physicist, develops a new system of mathematics called matrix mechanics which employs columns of numbers that describe all possible transitions within an atom. The solutions to the matrix correspond to the wavelengths of the hydrogen spectral lines.

1925
Wolfgang Pauli (1900–1958), Austrian physicist, announces his discovery of the exclusion principle which develops into one of the most powerful basic descriptive tools in physics. It states that two electrons with the same quantum numbers cannot occupy the same atom. This principle serves to explain the chemical properties of the elements.

1925
Vesto Slipher argues that red-shifts of light can be interpreted in terms of distance and velocity.

1925
Walter Elsasser, explanation of electron diffraction as wave property of matter.

1926
American Robert Goddard launches the first liquid-fueled rocket capable of stable flight.

1926
Astronomers assert that the Pauli exclusion principle offers an explanation of white dwarf stars.

1926
Enrico Fermi (1901–1954), Italian-American physicist, introduces Fermi-Dirac statistical mechanics. This theory explains the behavior of clouds of electrons in a substance and also shows that gas particles obey Pauli's exclusion principle.

1926
Erwin Schrödinger (1887–1961), Austrian physicist, publishes a paper in which he mathematically develops wave mechanics and offers what becomes known as the Schrödinger wave equation. This replaces the electron in the Bohr model of the atom with wave trains. This explanation becomes so satisfactory that it finally places Planck's quantum theory on a firm mathematical basis.

1926
Eugene Paul Wigner (1902–1995), Hungarian-American physicist, develops the principles involved in applying group theory to quantum mechanics and evolves the concept of the symmetry in space and time that marks the behavior of subatomic particles.

1926
Gilbert Newton Lewis (1875–1946), American chemist, first introduces the term photon to describe the minute, discrete energy packet of electromagnetic radiation that is essential to quantum theory.

1926
John Desmond Bernal (1901–1971), English physicist, advances x-ray crystallography by developing the Bernal chart with which he can deduce crystal structure by analyzing photographs of x-ray diffraction patterns.

1926
Max Born (1882–1970), German-British physicist, publishes his first paper on the probability interpretation of quantum mechanics. In working out the mathematical basis of quantum mechanics, he gives electron waves a probabilistic interpretation as to their behavior.

1927
Astronomer Jan Oort argues that observations indicate the solar system orbits a galactic center and that the Milky way is a spiral shaped galaxy.

1927
Big bang theory is formulated to provide a coherent cosmology consistent with new developments in quantum and relativity physics.

1927
Charles Lindbergh makes first solo transatlantic flight.

1927
Georges Lemaitre offers a model of an expanding universe.

1927
Hermann Joseph Muller induces artificial mutations in fruit flies by exposing them to x rays. His work proves that mutations result from some type of physical-chemical change. Muller wrote extensively about the danger of excessive x rays and the burden of deleterious mutations in human populations.

1927
Niels Henrik David Bohr (1885–1962), Danish physicist, states complementarity principle, that a phenomenon can be considered in each of two mutually exclusive ways, and each way is valid in its own terms. He applies it specifically to the simultaneous wave and particle behavior of an electron, but it later is used by disciplines besides atomic physics.

1927
Paul Adrien Maurice Dirac (1902–1984), English physicist, develops equations that unite quantum mechanics and relativity theory.

1927
Werner Karl Heisenberg (1901–1976), German physicist, postulates his uncertainty principle, also known as the principle of indeterminacy. This states that it is impossible to determine accurately and simultaneously two variables of an electron, such as position and momentum. More generally, it states that when working with atom-sized or subatomic particles, the very act of measuring such particles significantly affects the results obtained. Philosophically, this is a troubling notion, for it calls into question much of traditional beliefs in straightforward cause and effect.

1928
George Gamow (1904–1968), Russian-American physicist, develops the quantum theory of radioactivity which is the first theory to successfully explain the behavior of radioactive elements, some of which decay in seconds and others after thousands of years.

1929
Astronomer Edwin Hubble argues evidence that galaxies are moving away from each other and that the universe is expanding.

1929
Walther Wilhelm Georg Franz Bothe (1891–1957), German physicist, invents coincidence counting by using two Geiger counters to detect the vertical direction of cosmic rays. This allows the measurement of extremely short time intervals, and he uses this technique to demonstrate that the laws of conservation and momentum are also valid for subatomic particles.

1929
William Francis Giauque (1895–1982), American chemist, discovers that oxygen is a mixture of three isotopes. This leads to a debate between chemists and physicists concerning an atomic weight standard, which is not resolved until 1961.

1930
Clyde Tombaugh (1906–1997) discovers Pluto.

1931
Paul Adrien Maurice Dirac (1902–1984), English physicist, proposes the antielectron, or positron, a positively charged electron.

1932
James Chadwick (1891–1974), English physicist, proves the existence of the neutral particle of the atom's nucleus, called the neutron. It proves to be by far the most useful particle for initiating nuclear reactions.

1932
John Douglas Cockcroft (1897–1967), English physicist, and Irish physicist Ernest Thomas Sinton Walton (1903–1995), use their new particle accelerator to bombard lithium and produce two alpha particles (having combined lithium and hydrogen to produce helium).This is the first nuclear reaction that has been brought about through the use of artificially accelerated particles and without the use of any form of natural radioactivity. This ultimately proves highly significant to the creation of an atomic bomb.

1932
Karl Jansky makes first attempts at radio astronomy.

1932
Lev Davidovich Landau proposes the existence of neutron stars.

1932
Ruska builds first electron microscope.

1932
Thomas H. Morgan receives the Nobel Prize in Medicine or Physiology for his development of the theory of the gene. He is the first geneticist to receive a Nobel Prize.

1932
Werner Karl Heisenberg (1901–1976), German physicist, wins the Nobel Prize in physics for the creation of quantum mechanics, whose application has led to the discovery of the allotropic forms of hydrogen.

1933
Baade and Zwicky argue that a collapse of a white dwarf may set off a supernova and then produce a neutron star.

1933
Enrico Fermi (1901–1954), Italian-American physicist, develops a theory of beta decay which uses Pauli's neutrino as part of its explanation.

1934
Arnold O. Beckman (1900–), American chemist and inventor, invents the pH meter, which uses electricity to accurately measure a solution's acidity or alkalinity.

1934
Enrico Fermi (1901–1954), Italian-American physicist, bombards uranium with neutrons and obtains not only a new element, number 93 (neptunium), but also a number of other products that he is unable to identify. What he eventually discovers is that he has not only created the first synthetic element, but he has also produced the first nuclear fission reaction.

1934
Frédéric Joliot-Curie (1900–1958) and Irène Joliot-Curie (1897–1956), husband-and-wife team of French physicists, discover what they call artificial radioactivity. They bombard aluminum to produce a radioactive form of phosphorus. They soon learn that radioactivity is not confined only to heavy elements like uranium, but that any element can become radioactive if the proper isotope is prepared. For producing the first artificial radioactive element they win the Nobel Prize in chemistry the next year.

1934
Leo Szilard (1898–1964), Hungarian-American physicist, first conceives of the idea of a nuclear chain reaction (in which a neutron induces an atomic breakdown, which releases two neutrons which break down two more atoms, and so on). Although his method uses beryllium rather than uranium and would be impractical, it is correct in principle. He keeps it a secret, foreseeing its importance in making nuclear bombs, but it is soon discovered by other scientists.

1935
Robert Watson-Watt develops RADAR.

1935
Subrahmanyan Chandrasekhar, calculation of mass limit for stellar collapse of a white dwarf.

1936
Carl David Anderson (1905–1991), American physicist, discovers the muon. While studying cosmic radiation, he observes the track of a particle that is more massive than an electron but only a quarter as massive as a proton. He initially calls this new particle, which has a lifetime of only a few millionths of a second, a mesotron, but it later becomes known as a muon to distinguish it from Yukawa's meson.

1936
Theodosius Dobzhansky publishes Genetics and the Origin of Species, a text eventually considered a classic in evolutionary genetics.

1937
Emilio Segre (1905–1989), Italian-American physicist, and Carlo Perrier bombard molybdenum with deuterons and neutrons to produce element 43, technetium. This is the first element to be prepared artificially that does not exist in nature.

1938
Bethe, Critchfield, von Weizsacker, argue that stars are powered by the CNO-cycle of nuclear fusion.

1939
Leo Szilard (1898–1964), Hungarian-American physicist, and Canadian-American physicist, Walter Henry Zinn (b. 1906), confirm that fission reactions (nuclear chain reactions) can be self-sustaining using uranium.

1939
Linus Carl Pauling (1901–1994), American chemist, publishes The Nature of the Chemical Bond, a classic work that becomes one of the most influential chemical texts of the twentieth century.

1939
Lise Meitner (1878–1968), Austrian-Swedish physicist, and Otto Robert Frisch 1904–1979), Austrian-British physicist, suggest the theory that uranium breaks into smaller atoms when bombarded. Meitner offers the term fission for this process.

1939
Niels Hendrik David Bohr (1885–1962), Danish physicist, proposes a liquid-drop model of the atomic nucleus and offers his theory of the mechanism of fission. His prediction that it is the uranium-235 isotope that undergoes fission is proved correct when work on an atomic bomb begins in the United States.

1939
Oppenheimer and Snyder argue that a collapsing neutron star forms what would later be termed a black hole.

1941
Arnold O. Beckman (1900–), American physicist and inventor, invents the spectrophotometer. This instrument measures light at the electron level and can be used for many kinds of chemical analysis.

1941
R. Sherr, Kenneth Thompson Bainbridge, American physicist, and H. H. Anderson produce artificial gold from mercury.

1942
Astronomer Grote Reber constructs radio map of the sky.

1942
Enrico Fermi (1901–1954), Italian-American physicist, heads a Manhattan Project team at the University of Chicago that produces the first controlled chain reaction in an atomic pile of uranium and graphite. With this first self-sustaining chain reaction, the atomic age begins.

1943
First operational nuclear reactor is activated at the Oak Ridge National Laboratory in Oak Ridge, Tennessee.

1943
J. Robert Oppenheimer (1904–1967), American physicist, is placed in charge of United States atomic bomb production at Los Alamos, New Mexico. He supervises the work of 4,500 scientists and oversees the successful design construction and explosion of the bomb.

1944
Otto Hahn (1879–1968), German physical chemist, receives the Nobel Prize in Chemistry for his discovery of nuclear fission.

1944
Sin-Itiro Tomonaga (1906–1979), Japanese physicist, works out the theoretical basis for quantum electrodynamics independently of American physicists Richard Philips Feynman (1918–1988) and Julian Seymour Schwinger (1918–1994) (who also work independently of one another). This method allows the behavior of electrons to be determined with far greater precision than before. It also leads to the formulation of quantum electrodynamics (QED).

1945
First atomic bomb is detonated by the United States near Almagordo, New Mexico. The experimental bomb generates an explosive power equivalent to 15–20 thousand tons of TNT.

1945
Joshua Lederberg and Edward L. Tatum demonstrate genetic recombination in bacteria.

1945
United States destroys the Japanese city of Hiroshima with a nuclear fission bomb based on uranium-235 on August 6. Three days later, a plutonium-based bomb destroys the city of Nagasaki. Japan surrenders on August 14 and World War II ends. This is the first use of nuclear power as a weapon.

1946
George Gamow proposes the Big Bang hypothesis.

1947
A U.S. aircraft travels faster than the speed of sound.

1947
First carbon-14 dating.

1948
Gamow and others assert theory of nucleosynthesis is consistent with hot big bang.

1950
Astronomer Jan Oort offers explanation of origin of comets.

1951
Rosalind Franklin obtains sharp x-ray diffraction photographs of DNA.

1952
Alfred Hershey and Martha Chase publish their landmark paper "Independent Functions of Viral Protein and Nucleic Acid in Growth of Bacteriophage." The famous "blender experiment" suggests that DNA is the genetic material. When bacteria are infected by a virus, at least 80% of the viral DNA enters the cell and at least 80% of the viral protein remains outside.

1952
First thermo-nuclear device is exploded successfully by the United States at Eniwetok Atoll in the South Pacific. This hydrogen-fusion bomb (H bomb) is the first such bomb to work by nuclear fusion and is considerably more powerful than the atomic bomb exploded over Hiroshima on August 6, 1945.

1952
First use of isotopes in medicine.

1953
James D. Watson and Francis H. C. Crick publish two landmark papers in the journal Nature: "Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid" and "Genetical implications of the structure of deoxyribonucleic acid." Watson and Crick propose a double helical model for DNA and call attention to the genetic implications of their model. Their model is based, in part, on the x ray crystallographic work of Rosalind Franklin and the biochemical work of Erwin Chargaff. Their model explains how the genetic material is transmitted.

1953
Murray Gell-Mann (b. 1929), American physicist, suggests that basic particles contain an intrinsic property known as strangeness that can explain a number of new observations being made about them. A similar concept is developed independently by the Japanese physicist Kazuhiko Nishijima (b. 1926).

1953
Stanley Miller produces amino acids from inorganic compounds similar to those in primitive atmosphere with electrical sparks that simulate lightning.

1953
USGS decides to split the Carboniferous into Mississippian and Pennsylvanian.

1954
Linus Carl Pauling (1901–1994), American chemist, receives the Nobel Prize in Chemistry for his research into the nature of the chemical bond and its applications to the elucidation of the structure of complex substances.

1955
First synthetic diamonds are produced in the General Electric Laboratories.

1956
Joe Hin Tijo and Albert Levan prove that the number of chromosomes in a human cell is 46, and not 48, as argued since the early 1920s.

1956
Neutrino discovered at Los Alamos.

1957
Francis Crick proposes that during protein formation each amino acid is carried to the template by an adapter molecule containing nucleotides and that the adapter is the part that actually fits on the RNA template. Later research demonstrates the existence of transfer RNA.

1957
Soviet Union launches Earth's first artificial satellite, Sputnik, into earth orbit.

1958
Astronomer Martin Ryle argues evidence for evolution of distant cosmological radio sources.

1958
George W. Beadle, Edward L. Tatum, and Joshua Lederberg are awarded the Nobel Prize in Medicine or Physiology. Beadle and Tatum are honored for the work in Neurospora that led to the one gene-one enzyme theory. Lederberg is honored for discoveries concerning genetic recombination and the organization of the genetic material of bacteria.

1958
National Aeronautics and Space Administration (NASA) established .

1959
Soviet Space program sends space probe to impact Moon.

1961
Edward Lorenz advances chaos theory and offers possible implications on atmospheric dynamics and weather.

1961
Murray Gell-Mann (b. 1929), American physicist, and Israeli physicist Yuval Ne'Eman (b. 1925), independently introduce a new way to classify heavy subatomic particles. Gell-Mann names it the eightfold way, and this system accomplishes for elementary particles what the periodic table did for the elements.

1961
Soviet Union launches first cosmonaut, Yuri Gagarin, into Earth orbit.

1962
James D. Watson, Francis Crick, and Maurice Wilkins are awarded the Nobel Prize in Medicine or Physiology for their work in elucidating the structure of DNA.

1963
Fred Vine and Drummond Matthews offer important proof of plate tectonics by discovering that oceanic crust rock layers show equidistant bands of magnetic orientation centered on the a site of sea floor spreading.

1964
Astronomers discover quasars.

1964
John Bell asserts a quantum inequality that limits the possibilities for local hidden variables in quantum theories.

1964
Roger Penrose defines nature of what would later be termed a black hole as a singularity or a dimensionless point of extreme mass.

1965
Arno Allan Penzias and Robert Woodrow Wilson detect cosmic background radiation.

1966
Marshall Nirenberg and Har Gobind Khorana lead teams that decipher the genetic code. All of the 64 possible triplet combinations of the four bases (the codons) and their associated amino acids are determined and described.

1966
Robert Sanderson Mulliken (1896–1986), American chemist, receives the Nobel Prize in Chemistry for his fundamental work concerning chemical bonds and the electronic structure of molecules by the molecular orbital method.

1966
X-ray source Cygnus X-1 discovered.

1967
Bell and Hewish discover pulsars.

1967
John Wheeler introduces the term "black hole".

1968
Electroweak theory—unification of electromagnetism with the weak force—achieved by American physicist Sheldon Lee Glashow (1932-), Pakistani physicist Abdus Salam (1926-1996), and American physicist Steven Weinberg (1933-).

1968
Joseph Weber, first attempt at a gravitational wave detector.

1969
Apollo 11 mission to the Moon. U.S. astronauts Neil Armstrong and Buzz Aldrin become first humans to walk of another world.

1969
Max Delbrück, Alfred D. Hershey, and Salvador E. Luria were awarded the Nobel Prize in Medicine or Physiology for their discoveries concerning the replication mechanism and the genetic structure of viruses.

1971
Cygnus X-1 identified as black hole candidate.

1972
Discovery of 2 million year old humanlike fossil, Homo habilis, in Africa.

1972
Gell-Mann theorizes QCD (Quantum Chromo Dynamics) and unification theory that includes strong force).

1972
Paul Berg and Herbert Boyer produce the first recombinant DNA molecules. Recombinant technology emerges as one of the most powerful techniques of molecular biology. Scientists are able to splice together pieces of DNA to form recombinant genes. As the potential uses, therapeutic and industrial, become increasingly clear, scientists and venture capitalists establish biotechnology companies.

1975
Scientists at an international meeting in Asilomar, California, call for the adoption of guidelines regulating recombinant DNA experimentation.

1976
U.S. Viking spacecraft lands and conducts experiments on Mars.

1977
Robotic submarine "Alvin" explores mid-oceanic ridge and discovers chemosynthetic life.

1977
Voyager spacecraft launched; contains golden record recording of Earth sounds.

1984
Ozone hole over Antarctica discovered.

1988
The Human Genome Organization (HUGO) is established by scientists in order to coordinate international efforts to sequence the human genome. The Human Genome Project officially adopts the goal of determining the entire sequence of DNA comprising the human chromosomes.

1990
Hubble Space Telescope launched.

1991
K-T event impact crater identified near the Yucatan Peninsula.

1991
Andrew A. Griffith, American chemist, uses an atomic force microscope to obtain extraordinarily detailed images of the electrochemical reactions involved in corrosion.

1993
George Washington University researchers clone human embryos and nurture them in a Petri dish for several days. The project provokes protests from ethicists, politicians and critics of genetic engineering.

1994
Astronomers observe comet Shoemaker-Levy 9 (S-L 9) colliding with Jupiter.

1994
Hubble Space Telescope confirms existence of black holes.

1994
Researchers at Fermilab discover the top quark. Scientists believe that this discovery may provide clues about the genesis of matter.

1995
Mayor and Queloz identify first extra-solar planet, a Jupiter-like planet orbiting an ordinary star.

1995
Paul Crutzen (1933–), Dutch meteorologist, Mario Molina (1943–), Mexican American chemist, and R. Sherwood Rowland (1927–), American atmospheric chemist, receive the Nobel Prize in Chemistry for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone.

1997
Microscopic analysis of Murchison meteorite lead some scientists to argue evidence of ancient life on Mars.

1997
Mars Pathfinder vehicle studies and photographs Martian surface.

1998
Ian Wilmut announces the birth of Polly, a transgenic lamb containing human genes.

1998
Two research teams succeed in growing embryonic stem cells.

1999
Scientists announce the complete sequencing of the DNA making up human chromosome 22. The first complete human chromosome sequence is published in December 1999.

2000
Astronomers discover a galaxy that is 18.6 billion light-years away from Earth. This is the most remote object ever observed. Scientists speculate that this galaxy was formed when the universe was one-sixteenth its present age.

2001
In February 2001, the complete draft sequence of the human genome is published. The public sequence data is published in the British journal Nature and the sequence obtained by Celera is published in the American journal Science.

2002
Satellites capture images of icebergs more than ten times the size of Manhattan Island breaking off Antarctic ice shelf.