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In 1866, Gregor Mendel, in "Versuche über Pflantenhybriden," interpreted heredity in terms of a pairing of dominant and/or recessive unit characters; that is, ones that could in practice be treated as indivisible and independent particles.  "What chiefly fascinated Mendel...was the nature of heredity which the vigour of grafts showed to be stronger than environment, that is, the stock on which they had been grafted.  He...began to produce hybrids, not to improve the yields, but to follow the behavior of characters from generation to generation" (Jacob 1970:202-203). [added 02/01/03]

In 1866, Edmund Vulpian noted that curare interrupts the communication between the nerve and the muscle fibers.
In 1866, Haeckel, in Generelle Morphologie der Organismen, challenged the plant/animal division of the living world, recognizing that single-celled forms, the protists, did not fit into either category, and must have arisen separately from plants and animals.  At the same time, he published his 'biogenetic law' wherein ontogeny is erroneously said to recapitulate phylogeny.  However, in attempting to rationalize it, he invoked the mechanism of changes in developmental timing, coining the word 'heterochrony.'  He also coined 'ecology,' 'ontogeny,' and 'phylogeny.'  The recapitulation theory of development was widely held at the time, and earlier by Goethe, Johann Gottfried von Herder, and biologists associated with naturphilosophie.  Haeckel's version of Darwinism persisted, e.g., in the ideas of the socialist Karl Kautsky, August Weismann, Freud, Carl Gustave Jung, and the Hitlerite Monist League (Gould 1977:115-116). Haeckel also published misleading illustrations in support of his theory.

In 1866, Max Schultze discovered two sorts of 'receptors' in the retina.

In 1866, Alfred Nobel patented kiselguhr, or dynamite, in Sweden. [added 02/01/03]

In 1866, Huggins made the first spectroscopic observations of a nova.

In 1866, Giovanni Virginio Schiaparelli postulated that meteors are debris from comets.
 

In 1867, Theodor Meynert showed that the laminated form of the cortex was due to the distribution in parallel layers of different categories of neurons (Meynert 1867-1868) .

In 1867, Helmholtz, in Handbuch de Physiologischen Optik, Volume III, said that "disparate images from corresponding retinal points enter the sensorium distinct and intact, and that their union into a single image is an unconscious act of judgement dependent on prior experience" (Turner 1976:248).  
In 1867, Fleeming Jenkin, in a review of Origin of Species, pointed out that variation would be eliminated with an inheritance which was a blend of the parents.  Blending inheritance is analogous to mixed paints.  This criticism caused Darwin, in subsequent editions, to resurrect Lamarck's theory of acquired characters, which was not finally put aside until the rediscovery of Mendel and unit characters in 1900.
In 1867, Darwin, working on his theory of sexual selection and failing to understand why caterpillars are often brightly adorned, wrote Wallace, who explained his theory of warning coloration, which today is proven.
In 1867, Aleksander Onufriyevich Kovalevsky extended the germ layer concept to invertibrates.

In 1867, Wilhelm Griesinger published the second edition of his psychiatric textbook in which he said that mental diseases are brain diseases and that the onset of psychosis was experienced as an intrusion of a 'thou' on the 'I,' or ego.
In 1867, Karl Marx, in Das Kapital, maintained the value, or exchange relation, of commodities is characterized by its alienation from its use-value, and thus its value as the product of human labor, which the capitalist treats as a variable and against which he accounts his surplus.
   

In 1868, Josef Breuer and Ewald Hering, by occluding the trachea at the end of inhaling or exhaling, demonstrated that the lungs 'self-regulate' breathing, i.e., they contain receptors that detect the degree to which they are stretched. These receptors transmit signals to the brain via the vagus nerve which initiates the opposite signal back to the lungs. This was one of the first 'feedback' mechanisms demonstrated in mammals.

In 1868, Ångström, in an atlas of the solar spectrum, measured the wavelengths of over a thousand spectral lines in units which came to be called an 'angstrom' in his honor.
In 1868, Boltzmann, in "Studien über das Gleichgewicht der lebendingen Kraft zwischen bewegten materiellen Punkten" on thermal equilibrium, extended Maxwell's theory of the distribution of energy among colliding molecules in equilibrium in a conservative force field.  By assuming a fixed amount of energy divided among a finite number of molecules, i.e., all combinations of energies are equally probable, the problem could treated by combinatorial analysis.  "The result was a new exponential formula, now known as the 'Boltzman factor' and basic to all modern calculations in statistical mechanics" (Brush 1976:261).  

In 1868, Maxwell, in "On Governors," published a mathematical analysis of governors, the first significant paper on feedback mechanisms.
 

In 1869, Dmitri Ivanovich Mendeléev and, independently, Julius Lother Meyer formulated the 'Periodic law.'  Meyer showed that Newlands' 'Law of octaves' only holds for the first two periods.  He also evolved the atomic volume curve which represented graphically the relation between the atomic weights and the volumes of the elements, expressed by dividing atomic weights by specific gravities.  Mendeléev placed the chemical elements in seven rows in an order where those elements having similar chemical properties were aligned vertically.  He also left gaps in his table where he predicted elements would be found, which, in due course, they were, removing any doubt as to the validity of the periodic table.

In 1869, Eduard von Hartmann published Philosophie des Unbewussten, in which the 'unbewussten',' or 'unconscious,' included both Georg Frederick Hegel's 'idée' and the 'will' of Schopenhauer and others.  Modern discussions of the unconscious are generally dated from this time.

In 1869, George M. Beard distinguished 'neurasthenia,' a nervous disease of men, from hysteria, a women's disease, as, in an earlier time, men's 'hypochondriasis' had been distinguished from women's ' vapeurs.'  Subforms of neurasthenia came to be called phobias.

In 1869, Karoly Maria Benkert invented 'homosexuality' as a behavioral category. 

In 1869, Francis Galton, in Hereditary Genius, suggested a genetic basis for intelligence.  He established that the science of heredity could be concerned with deviations measured in statistical units.  His discovery of the standard deviation gave him the mathematical machinery to handle variability and to treat population as a unit of explanation.
In 1869, Ludwig Valentin Lorenz, as a result of his optical research and his wave equation, developed an equation relating the density of a body and its index of refraction and verified it in the case of water.  In 1878, Hendrik Antoon Lorentz, independently, developed the same constant, now known as the Lorentz-Lorenz formula.  

In 1869, Elwin Bruno Christoffel, in "Ueber die Transformation der homogen Differentalausdrücke zweiten Grades," introduced an operation which transformed one quadratic differential form into another, i.e., two types of curvature components.  This was a basic question arising from Riemann's geometry and was later called 'covariant differentiation' by C. Gregorio Ricci-Curbastro (Ehlers 1981:527-542). 
In 1869, Georg Cantor published his proof of the apparent paradox which stated says that an infinite class has the unique property that the whole is no greater than some of its parts.  The proof involves acknowledgement that the class of integers is infinite and countable and, then, establishing a one-to-one correspondence between the class of integers and its subset, the class of even numbers.  This was the beginning of set theory. The first transfinite number was created to describe the cardinality of countable infinite classes. 

In 1869, Charles Joseph Minard, in a graph showing Napoleon's march to Moscow and back, set a new standard for such representations plotting multivariate data: The size of the army, its location on a two-dimensional surface, its direction, and the temperature on various dates during the retreat.

In 1869, John Hyatt produced 'celluloid,' the first synthetic plastic to be put into wide use.
   

In 1870, Gustave Fritsch and Edward Hitzig demonstrated an inseparable link between electricity and cerebral function, but did not show where the electricity was produced.
In 1870, Camillo Golgi established that neurons in the brain sent information to the motor nerves and received it from the sensory nerves.  He developed a silver impregnation method that allowed microscopic visualization of the anatomy of the whole neuron.
In 1870, Friedrich Goltz suggested that the semicircular canals of the inner ear are the sense organs that detect the position of the head relative to to the gravitational field. [added 02/01/03]

[The demonstrations of the 1870s and 1880s that the internal processes of cell division were fundamentally the same in plants and animals magnified the cell as a universal unit of structure.]

In 1870, William Kingdon Clifford, introducing the details of non-Euclidean geometry to the English, raised the question of "variation in the curvature of space," describing it as "analogous to little hills on the surface [of the Earth] which is on average flat," that "the ordinary laws of geometry are not valid in them[, and] that this property of being curved or distorted is continually being passed on from one portion of space to another after the manner of a wave" (Clifford 1876:21-22).
   

In 1871, St. George Mivart, in On the Genesis of Species, claimed that, contrary to Darwin, species arise suddenly with large-scale changes already intact: Inheritance by blending, as Darwin proposed, meant that variation would have to be sustained by an extremely high mutation rate.
In 1871, Darwin, in The Descent of Man, and Selection in Relation to Sex, suggested that there was no sharp discontinuity between the evolution of humans and animals, that "the difference was one of degree and not of kind" (Darwin 1871:127), and that, therefore, not only was the behavior of animals guided in part by primitive reasoning processes, but human behavior must also be guided in part by instincts, e.g., the "instinctive tendency to speak" (ibid.:101).
In 1871, Johann Friedrich Miescher isolated a substance from the nuclei of white blood cells which is soluble in alkalis but not in acids. This substance came to be called 'nucleic acid.' (Miescher 1871).

In 1871, Maxwell, in Theory of Heat, proposed the idea that an intelligent being, named by W. Thomson 'Maxwell's Demon,' could by simple inspection of molecules (i.e., without doing work) violate the second law.  "The demon points to...the problem of reconciling the irreversible increase in entropy of the universe demanded by thermodynamics with the dynamical laws governing the motion of molecules, which reversible with respect to time" (Everitt 1976:227).  Maxwell also introduced the terms 'vector' and 'scalar potential.'  
In 1871, Crookes, in the course of trying to weigh thallium, created a vacuum "on the order of one millionth of an atmosphere [which] made possible the discovery of X-rays and the electron" (Brock 1976:475).  

In 1871, Strutt, also known as Baron Rayleigh, propounded a general law relating the intensity of light scattered from small particles to its wavelength when the dimensions of the particles are much less than the wavelength. He expressed this scattering as a function of the inverse fourth power of the wavelength of the incident light.  
   

In 1872, John Thomas Gulick pointed out the inevitability of divergence among isolated groups even without environmental difference.

In 1872, Ludwig and Edward Pfünger showed that oxidation occurs in tissues, not in the blood.

In 1872, Boltzmann, in "Weitere Studien über das Wärmegleichgewicht unter Gasmolekülen," argued that the second law of thermodynamics, and the spontaneous increase in entropy which it predicts, can only be understood in terms of large populations of particles, not individual trajectories, the primitive object of classical physics.  Influenced by Darwin, he replaced the study of individuals "with the study of populations, and showed that slight variations taking place over a long period of time can generate evolution at a collective level" (Prigogine 1996:20; Boltzmann 1905:193-197).  Assuming that all microscopic states of a system have the same probability, he established that entropy was statistical; however, by the same token he could not establish that long-term deviation from equilibrium was not impossible, even though very improbable.  He proposed an equation which gives a mathematical description of a state and how it is changing; i.e., if the Maxwellian E-function (Boltzmann's H-function) is identical to entropy, then the definition of entropy can be extended to nonequilibrium states.  
In 1872, Christian Felix Klein outlined his synthesis of geometric group transformations, in which he showed that there were three types of geometry: the Bolyai-Lobachevsky type where straight lines have two infinitely distant points, the Riemann type where the points are imaginary, and Euclid's type.  The so-called 'Klein bottle,' with no inside, came out of these studies.  The best known of his transformations is the so-called 'Klein four-group,' which was exploited by the Structuralists after the second world war.

In 1872, Julius Wilhelm Richard Dedekind, in Stetigkeit und die Irrationalzahlen, maintained that the essence of the continuity of a line consists in the possibility of dividing that by a single point, i.e., an irrational number, e.g., a fraction.  This division is known as a Schnitt, or 'Dedekind Cut.'  By putting the points into a one-to-one correspondence with the rational numbers, a continuum can consist of rational numbers and the fundamental theorems on limits can be proved rigorously.  Dedekind regarded arithmetic as a "natural consequence of the simplest arithmetic act, that of counting" (Dedekind 1872:4).  The redefinition of number and limit as ordinal concepts make "calculus...not a branch of the science of quantity, but of the logic of relations" (Boyer 1949:294).
In 1872, Claude Monet painted "Impression: Sunrise," which has been used to mark the beginning of Modern Art because it lent its name to 'Impressionism.'  This is a style concerned with portraying variations in light and color brought on by hour and season as deduced both from observation and optical principles.  On the other hand, his contemporary, Paul Cézanne simplified forms to their basic geometric equivalents and was honored as their master by early abstract painters Henri Matisse and Pablo Picasso. 
   

In 1873, Anton Schneider described chromosomes during the process of mitosis during cell division.

In 1873, Moritz Wagner emphasized the effects of different environments on isolated groups of animals.
In 1873 and 1874, Ernst Mach, Breuer, and Alexander Crum Brown, each independently and each based on Goltz's 1870 suggestion, published the insight that the flow of endolymph in the canals of the inner ear during motion stimulates the receptors in the ampullae at the end of the canals. Crum Brown also pointed out that the two canals received their stimuli from motions in opposite directions. [added 02/01/03]
In 1873, Maxwell, A Treatise on Electricity and Magnetism, tried to finish off the notion of action-at-a-distance and wrote a summary of his equations in terms of symmetry and vector structure.  This relational Lagrangian method enabled him to forego any mention of mechanical aether, supposed by many physicists of the time to be the fundamental electromagnetic substance.  Maxwell perceived that these equations had wave solutions and that electromagnetic waves of all frequencies were generated by accelerating electric charges and travelled at the same speed.  Moreover, based on his electromagnetic theory, he established that light exerts a radiation pressure.  This conclusion had many implications (Everitt 1976:212).  He also proposed that these waves could be generated in the laboratory by creating an quickly oscillating current.  
In 1873, Josiah Willard Gibbs, in "Graphical Methods in the Thermodynamics of Fluids," gave the fundamental equation for entropy, dU = TdS - PdV, where U is the internal energy, T is the absolute temperature, S is entropy, P is the pressure on the system, and V is its volume.  
In 1873, Joseph Antoine Ferdinand Plateau, in Statique expérimentale et théorique des Liquides soumis aux seules Forces moléculaires, showed experimentally that liquid surfaces always assume a curvature, i.e., the smallest possible area, when there is no appreciable external force, like gravity, exerted (Adam 1930:1).  
   

In 1874, W. Betz extrapolated to the telencephalon the posterior-anterior sensorimotor dichotomy that prevails along the nerve axis, from the spinal cord to the brain.
In 1874, S. Bodkin published his observation that, in patients with leukemia, transcutaneous electrical stimulation of the enlarged spleen led to reduction in size and an increase in leukocyte count.
In 1874, Franz Brentano, in Psychologie vom empirischen Standpunkte, maintained that mental processes should be treated as intentional acts rather than passive processes.  Among the auditors of classes which he taught were Edmund Husserl, Thomas Masaryk, Franz Kafka, Rudolf Steiner, and Freud.

In 1874, Marie Alfred Cornu described a graphical curve, known as the 'Cornu spiral,' for calculating light intensities in Fresnel diffraction. 
In 1874, Strutt, in "The Kinetic Theory of the Dissipation of Energy," pointed out the 'reversibility paradox' occasioned by Boltzmann's H -function, i.e., the "apparent contradiction between...the reversibility of individual collisions and the irreversibility predicted by the theorem itself for a system of many molecules" (Brush 1976:263).  
In 1874, Boltzmann, in "Zur Theorie der elastischen Nachwirkung," introduced 'memory effects' into the relation between stresses and strains of an elastic continuum, i.e., "the circumstance in which a strain that occurred previously reduces the force required to produce a strain of the same kind" (Boltzmann, quoted in Cercignani 1998:161).  This laid the foundations for 'hereditary mechanics,' a term introduced by E. Picard in 1907.  
In 1874, William Stanley Jevons, in Principles of Science, demonstrated a symbolic and logical method, intended to supplant Boole and John Venn, that involved permutations of ABC corresponding to the eight compartments of Venn's three-circle diagram.  Jevons also designed labor-saving logic machines for exploiting his method, among them an 'abacus' similar to a primitive IBM punchcard machine.
   

In 1875, Richard Caton demonstrated that the brain's electricity originated in the cerebral cortex.
In 1875, Eduard Seuss coined the term 'biosphere' for where life can exist, i.e., on the Earth's surface and adjacent atmosphere.  

In 1875, Galton demonstrated "the usefulness of twin studies for elucidating the relative influence of nature (heredity) and nurture (environment) upon behavioral traits" (King and Stansfield 1997:382).

In 1875, Crookes developed a 'light-mill,' or radiometer.  This is a sealed and evacuated (as far as possible) glass chamber containing a paddle wheel with vanes blackened on one side and silvered on the other.  This spins rapidly when it is impinged upon by radiant heat; i.e., "a rise in pressure occurred on the hotter side of the vanes, which consequently moved away from the incident radiation" (Brock 1976:480n10).  

In 1875, George Henry Lewes, in the second volume of Problems of Life and Mind, used 'emergent' to describe a 'resultant' which "arises out of...combined agencies, but in a form which does not display the agents in action....  The emergent is unlike its components in so far as...it cannot be reduced either to their sum or their difference" (Lewes 1875:368-369). 
   

In the 1870s, Mach stated the principle that the inertia of a piece of matter is attributable to the interaction between that piece of matter and the rest of the Universe, i.e., "a body in an empty universe has no inertia" (Hiebert 1978:599). This idea has roots in the writings of Leibniz and was widely accepted among so-called Energists, e.g., Mayer, who held the energy was a substance, i.e., matter, and that atoms were only a convenience (Cercignani 1998:203). [revised 02/01/03]
   

Beginning in 1876 with anthrax, Robert Koch devised the method of employing aniline dyes to stain microorganisms. By this means he was able to isolate pure cultures of bacteria and showed the bacterial origin of many infectious diseases, including tuberculosis, cholera, bubonic plague, and sleeping sickness. This confirmed the germ theory of disease. [revised 02/01/03]

In 1876, Wallace published his special contribution to the study of evolution, The Geography of Animal Distribution.
In 1876, Carl Wernicke published a paper in which he described a new type of aphasia, involving an impairment of comprehension rather than execution, and located at a different locus from the aphasia described by Broca.  According to Wernicke, interconnections between functional sites make more complex intellectual functions possible.
In 1876, Alexander Graham Bell invented the telephone.

In 1876, Nikolas August Otto designed the first four-stroke piston engine.
   

In 1877, Ernst Abbé published the first in a series of contributions to the theory of microscopic optics.
In 1877, Maxwell, in "On Boltzmann's Theorem on the Average Distribution of Energy in a System of Material Points," proved that "the densities of the constituent components in a rotating mixture of gases would be the same as if each gas were present by itself.  Hence gaseous mixtures could be separated by means of a centrifuge" (Everitt 1976:224).  

In 1877, Schiaparelli reported detailed observations of Martian 'canali,' or channels.
   

In 1878, F. Heinke published a study on herring, which climaxed the focus on animal studies.

In 1878, Emil Hermann Fischer figured out the chemical formula for phenylhydrazine, a compound he had discovered. This led to his research on sugars, of which he synthesized glucose and about thirty others, purines, of which he synthesized about one hundred thirty, and to the development of synthetic drugs like novacaine. [added 02/01/03]
In 1878, Wilhelm Wundt founded the first laboratory devoted to physiological psychology.  He intuited that dreaming is the product of the simultaneous enhancement and impairment of different parts of the brain.
In 1878, J. W. Gibbs, in the abstract of "On the Equilibrium of Heterogenous Substances," asserted that "when the entropy of a system has reached a maximum, the system will be in a state of equilibrium" (Gibbs 1878:354).  In the paper itself, published in two parts, in 1876 and 1878, he proceeded to generalize thermodynamic equilibrium theory, removing one restriction after another, and deriving, for example, the chemical phase rule: "In a heterogenous system composed of several homogenous phases, the fundamental equilibrium condition leads to the requirement that temperature, pressure, and the chemical potential of each independent chemical component must have the same values throughout the system" (Klein 1976:390).  Other experimental facts, "such as the theories of catalysis, of solid solutions, and of the actions of semi-permeable diaphragms and osmotic pressure, [he] showed...are in fact simple, direct and necessary consequences of the fundamental laws of thermodynamics" (Bumstead 1903:xviii).  He also defined what has came to be known as 'Gibb's function,' or 'free energy,' a measure of a system's ability to do work; i.e., that portion of the total energy "which can be freely converted to other forms of energy....  In any spontaneous reaction occurring at a constant temperature and volume the free energy must decrease.  Hence the free energy, not the total energy change measured by the evolution of heat, determines the direction of any reaction" (Turner 1976:244).  
In 1878, Maxwell, in "On Stresses in Rarefied Gases Arising From Inequalities of Temperature," explaining the action of a radiometer, noted that "when a viscous fluid moves past a solid body, it generates tangential stresses by sliding [i.e., 'slip' effects] over the surface with a finite velocity" (Everitt 1976:224).  Indepenently, Osbourne Reynolds came to a similar conclusion about the same time.  
   

In 1879, Walther Flemming named 'chromatin' and 'mitosis,' made the first accurate counts of chromosome numbers,and discerned the longitudinal splitting of chromosomes.
In 1879, Crookes, in "On the Illumination of Lines of Molecular Pressure, and the Trajectory of Molecules," attempted to determine the paths of the 'lines of molecular pressure,' or cathode rays, in an evacuated glass tube through which two electrodes are passed.  When high voltage is applied, electrons are emitted from the radiometer vanes, which act as a cathode, and, under reduced pressure, the vane turns and the electrons are accelerated toward the anode.  Many of these electrons, or cathode rays, miss the anode and, striking the tube wall, exhibit fluorescence.  
In 1879, Jules-Henri Poincaré showed how automorphic functions can be used to express coordinates of any point in an algebraic curve as uniform functions of a single parameter.

In 1879, Stefan, in "åber die Beziehung zwischen der Wärmestrahlung," conjectured that that the radiant energy emitted by an enclosure equivalent to a black body is proportional to the fourth power of the body's temperature.  
In 1879, Planck, in Vorlesungen über Thermodynamik, opposed the idea that the validity of the second law depends upon the existence of an observer or his lack of information.  The implication is that irreversibility is natural.
In 1879, Albert Abraham Michelson determined the speed of light to be 186,350 miles per second + or - 30 miles per second.
In 1879, Edwin Herbert Hall discovered a component of an electric field which when crossed with a magnetic field becomes perpendicular to the electric field.  Known as 'Hall current,' or the 'Hall effect,' it was not explained until the advent of quantum theory.

In 1879, Gottlob Frege, in Begriffsschrift, proffered the first system of propositional calculus, also known as the calculus of sentential conjunctions.
   

In 1880, Sydney Ringer studied the use of body temperature in diagnosis and inorganic ions in heart contractions, making possible the analysis of heart metabolism.
   

In 1881, Wallace proposed to date the beginning of the Cambrian period about 28 million years ago.
In 1881, Lucian Galard and John D. Gibbs obtained patents for systems of alternating electrical current.
In 1881, Venn, in Symbolic Logic, represented logical propositions diagrammatically.
   
In 1882, Eduard Strasburger coined the terms 'cytoplasm' and 'nucleoplasm.'
In 1882, Dmitri Iosefovich Ivanovsky demonstrated that tobacco mosaic disease is caused by "a self-replicating agent (or virus) that will pass through bacterial filters and can neither be seen with light microscope nor grown upon bacteriological media" (King and Stansfield 1997:382).
In 1882, Helmholtz, independently of Gibbs, distinguished between 'bound' and 'free energy' in chemical reactions, the formula for which, free energy equals internal energy minus the temperature of the system times its entropy, is known as the Gibbs-Helmholtz equation.  

In 1882, Michelson described an 'interferometer,' an interference meter, which had a half-silvered mirror in order to split incident beams of light into two parts at right angles to each other. 
   

In 1883, Ilia Il'ich, also known as Élie, Metchnikoff identified the phagocyte as a purveyor of cellular defense, thereby raising questions of organismic identity, i.e., how do organisms protect themselves from their environment?  He recognized that phagocytes, cells capable of engulfing particles, such as bacteria, define the 'self' constituents; that is, they devour tadpole tails as frogs metamorphosize into adults.  Viewing the immune system as "self-referential, not antigen-driven," he saw inflammation as "self-directed 'immune' surveillance" (Tauber 1990:566).  This biological line of investigation developed into 'humoral theory,' after the classic term for body fluids, and was driven by the need to understand what identified non-host elements. It may be noted that bloodletting did not go out of fashion until about this time.
In 1883, Edouard van Beneden, studying nuclear division in the germ cells of a round worm, explained the "longstanding paradox that the maternal and paternal contributions to the character of the progeny seem often to be equal, despite the enormous difference in size between the egg and the sperm" (Alberts et al. 1994:1014). This explanation was made possible by his discovery that, while gamete nuclei, i.e., the sperm and egg nuclei, each have two chromosomes, the fertilized egg has four chromosomes. This implies that chromosomes carry genetic information and that germ cells, in contrast to somatic cells, must undergo a special sort of nuclear division in which the chromosome complement is halved. This process came to be known as 'meiosis,' a word which means that something appears to be of less size or significance than it really is. [revised 02/01/03]
In 1883, Weismann stated that his 'germ-line theory,' namely, that the separation of the germ-line from the phenotype of the body, or soma, is final from the point in the egg's development when it is determined which cells will become the ovary or the testes--and potentially immortal.  In human beings, for example, this point occurs at the 59th day of gestation.  This doctrine refuted Lamarck's theory that acquired characters can be inherited.  It also made it possible to understand the genetics of animals (though not plants), and, hence, evolution without understanding development.

In 1883, Max Rubner said that a body's metabolic rate was proportional to its surface area.

In 1883, Oscar Hertwig described 'mesenchyme,' a term he coined for the protoplasmic network filled with a intercellular fluid which gives rise to connective and other tissue.

In 1883, Wilhelm Roux suggested that the filaments within the cell's nucleus carry the hereditary factors.

In 1883, Karl Georg Friedrich Rudolf Leuckart and A. P. Thomas, independently, working on the life cycle of sheep liver flukes, determined the snails were intermediate hosts.

In 1883, Galton advocated selective breeding of human beings, or 'eugenics,' which he coined from a Greek word meaning "hereditarily endowed with noble qualities" (Galton 1883:24).  Eugenics was discredited through the uses to which it was put, especially during the 1930s and 1940s.

In 1883, George John Romanes published Mental Evolution in Animals, the first modern text comparing the psychology of humans and animals in objective terms.

In I883, Jean-Martin Charcot was able to obtain recognition of the neurological reality of hypnotism from the French Academy of Sciences.  He thought only hysterics were susceptible to hypnosis, i.e., that hypnosis was itself a pathological condition.
In 1883, Pierre Curie discovered piezoelectricity, a form of electric polarity, in crystals.

In 1883, Mach, in Die Mechanik in ihrer Entwicklung, translated as The Science of Mechanics: A Critical and Historical Account of Its Development, attempted to eliminate metaphysics by reducing science to the sum of what appears to the senses, and, in particular, attacked Newton's assumption that absolute rotation is observable. "The object of science [is] to replace, or save experiences, by the reproduction and anticipation of facts in thought...; [but] we never reproduce the facts in full..., only that side of them which is important to us, moved directly or indirectly by practical interest" (Mach 1883:481-482).  Concepts both compete for adherents and adapt to facts and to one another in order to survive.  Mach also did work in the field of ballistics, where the 'Mach number' borrows his name.
In 1883, Boltzmann, in "Ableitung des Stefan'schen Gesetzes," based on the fact that electromagnetic waves exert pressure on the walls of a radiation-filled enclosure, worked out theoretically a relation between thermodynamics and Maxwell's electromagnetic equations, i.e., the fourth power law previously found experimentally by Stefan.

In 1883, Reynolds introduced the 'Reynolds' number,' a dimensionless quantity associated with the smoothness of the flow of a fluid, which characterizes laminar and turbulent flow by relating kinetic to viscous forces.

In 1883, Gottlieb Daimler patented the gasoline combustion engine.
   

In 1884, A. Kossel isolated a protein from the nuclei of goose erythrocytes and called them 'histones.'

In 1884, Julius Kollman described the phenomena of 'neoteny' in his study of the axolotl form of Ambystoma tigrinum.

In 1884, J. Hughlings Jackson published his speculation that the neuropathological dissolution of function tends to roughly reverse the order of the acquisition of that function.

In 1884, Freud published a paper in which he found cocaine, an alkaloid in coca, effective against fatigue and neurasthenia.
In 1884, Jacobus Hendricus van't Hoff explained the principle of equilibrium in chemical dynamics and osmotic electrical conductivity.
In 1884, Edwin A. Abbott, in Flatland: A Romance of Many Dimensions, recounted the adventures of 'A. Square,' a character who inhabits a two-dimensional world populated by other geometrical figures--triangles, squares, pentagons, hexagons, etc.  Toward the end of the story, on the first day of 2000, a spherical creature from 'Spaceland' carries A. Square off to show him the three-dimensional nature of the larger world.  There A. Square speculates that Spaceland may itself exist as a subspace of a larger four-dimensional universe, an "infallible confirmation of the series [of end-points of a line, a square, a cube, etc.], 2, 4, 8, 16" (Abbott 1884:on line).

In 1884, Hilaire de Chardonnet invented the first artificial textile, which was made from cellulose.  It was later named rayon.
   

By 1885, Hertwig and Strasburger developed the conception that the nucleus is the basis of heredity.  Subsequently, Hertwig asserted that from the biological point of view sex is merely the union of two cells.
In 1885, Roux, testing Weismann's idea of heredity and germ plasm, did one of the first experiments in what became experimental embryology when he showed that embryonic chick cells could be maintained alive in a saline solution.

In 1885, Ernst Hartwig noticed a nova in the Andromeda nebulae.  Before it faded, he noted its peak intensity which was as great as the rest of the galaxy combined.

In 1885, Friedrich Wilhelm Nietzsche completed Also sprach Zarathustra in which he created the term 'id,' meaning the resevoir of human instinctual drives.  Other psychological terms employed frequently in his writings include sublimation and inhibition.
   

In 1886, Hippolyte Bernheim published his argument that hypnotism was a special case of general human suggestibility; i.e., anyone could be hypnotized.

[In the course of the nineteenth century, the practice of hypnotism brought with it greater popular, as well as medical, awareness of the split between conscious and unconscious behavior, as may be seen in the stories of E. T. A. Hoffman, E. A. Poe, Honoré de Balzac, Alexander Dumas, Victor Hugo, R. L. Stevenson, Fyodor Dostoevsky, and others (Ellenberger 1970:158-170).  Greater experience with hypnotism also brought disillusion with its limitations, e.g., sensitized patients confirming their doctor's unspoken expectations, etc.].

In 1886, Pierre Janet, in L'Automatisme Psychologique, introduced the term 'subconscious' in the context of patients' fixed ideas.

In 1886, Richard von Krafft-Ebing, in Psychopathia Sexualis, included among his classifications the terms 'sadism' (after Donatien de Sade) and 'masochism' (after Leopold Sacher-Masoch).
   

In 1887, Wilhelm His , in "Zur Geschichte des menschlichen Rückenmarkes und der Nervenwurzeln," published his discovery that in the early stages of development the nervous system is made up of independent, closely packed cells without axons.
In 1887, Auguste Forel showed that certain degenerative effects remained limited to the cell body and its dendrites.
In 1887, Svante August Arrhenius announced the theory of electrolytic dissociation which says that in aqueous solution the molecules of all acids, bases, and salts are split into ions.  This theory depends on van't Hoff's equilibrium principle.

In 1887, Michelson and Edward W. Morley, using an interferometer to investigate whether the speed of light depends on the direction the light beam moves, failed to detect the motion of the Earth with respect to the aether, thereby refuting the hypothesis that the aether exists. 

In 1887, Heinrich Rudolf Hertz produced Maxwellian electromagnetic waves, the first radio waves.  He demonstrated that they travel at the velocity of light and can be reflected, refracted, and polarized like light.  They also led him to drop Helmholtz's action-at-a-distance point of view.  The unit of frequency was named in his honor.
   

In 1888, Roux removed from a frog's egg one of the two cells existing after the first cleavage and obtained a half embryo.
In 1888, Santiago Ramón y Cajal, employing Golgi's staining technique, launched attacks on the 'nerve-net hypothesis' by demonstrating the anatomical independence of the axon from its target cell, i.e., that neurons are juxtaposed, not continuous.  At the time it was not realized that nerve cells do not interact through cell bodies, but through their axons and dendrites.  This permitted two hypotheses: that of the nerve-net or continuous network and that of the neuron or contiguous, but independent cells.  This question was not settled until the increased resolving power of electron microscopy after 1950.
In 1888, George Henry Falkiner Nuttall showed the blood serum contained bactericidal substances, from which he concluded that phagocytes were merely accessory to the protection offered by serum.
In 1888, Heinrich Wilhelm Gottfried Waldeyer named Roux's filaments 'chromosomes.'
In 1888, Theodore Boveri described the 'centriole' which in animals and most plants lies just outside a cell's nucleus.

Between 1888 and 1893, Marius Sophus Lie and Friedrich Engel published the six volumes of Theorie der Transformationsgruppen in which they showed that different sorts of symmetry form mathematical groups.  Lie divided these groups into "deux grandes classes: les groups intégrables et les groupes non intégrables" (Cartan 1894:103). 

In 1888, Nicola Tesla patented his invention of alternating electric current.
   

In the late 1880s, Louis Lewin and Arthur Heffter isolated the peyote alkaloid, mezcal.
   

In 1889, Wallace published his book on natural selection, which he called Darwinism.

In 1889, George Francis FitzGerald, in "The Ether and the Earth's Atmosphere," suggested that the null results of the Michelson-Morley experiment could be explained by the shrinkage of a body due to motion at speeds close to that of light, and that the only assumption necessary is that intermolecular forces obey the same laws as electromagnetic forces (FitzGerald 1889:390).  In 1892, Lorentz, independently, reached the same conclusions about the contraction of a moving body.  
In 1889, Giuseppe Peano postulated five properties of natural numbers in attempting to be as rigorous with numbers as Euclid had been with geometry.
   

In 1890, Boveri and Jean Louis Guignand established the numerical equality of paternal and maternal chromosomes at fertilization.

In 1890, Hans Driesch separated two cells of a fertized sea urchin egg by shaking with very different results than Roux: From a single cell arose an entire sea urchin.  Roux's frog experiment was repeated and by merely turning the cell over it developed into a whole frog.  Thus, a cell's competence to develop fully was established.  Roux was not deterred by this result and continued to maintain, against Weismann's germ-line theory, that the mechanics of development were distributed throughout the cytoplasm and triggered by each prior stage.
In 1890, Richard Altmann reported the presence within cells of organisms which live as intracellular symbionts, and were later named mitochondria.
In 1890, Emil Adoph von Behring and Shibasaburo Kitasato showed that antitoxins, that is, antibodies, could be produced--without any knowledge of their chemical nature--which neutralized the soluble toxins, classically, diptheria, tetanus, and botulism.
In 1890, Theobold Smith demonstrated the transmission of disease by an 'arthropod vector,' a tick.
In 1890, William James, in Principles of Psychology, accepted the idea that all behavior derives from the nervous system.  He also suggested that learning is accompanied by an increase in neuronal efficiency, and that primary memory, or short-term memory, is memory of the immediate past still in consciousness where it is the object of selective attention.

In 1890, Eugen Bleuler declared that 'dementia praecox' was curable.

In 1890, Michaelson, in "On the Application of Interference Methods to Astronomical Measurements," described the utility of interferometers "in measuring the angular size and the one-dimensional brightness distribution of sources that are too small to be resolved by a single telescope" (Lang and Gingerich 1979:2).

Before 1890, Herman Hollerith invented a punch card tabulator which was used in the United States Census of 1890. Hollerith's company eventually became IBM.  
   

In the 1890s, William Coley observed that some of his cancer patients were able to eliminate their cancer after contracting severe bacterial infections and so causing him to use bacterial extracts to activate the immune system with some good results.
   

In 1891, Marie Eugene Dubois discovered 'Javaman,' now known as Homo erectus.

In 1891, Waldeyer named nerve cells 'neurons.'.

In 1891, George Redmayne Murray successfully treated hypothyroid patients with a preparation of sheep thyroid.
   

In 1892, Poincaré published the first of three volumes on celestial mechanics in which he made fundamental mathematical discoveries, such as his demonstration that dynamical systems are non-integrable, i.e., they are neither static nor deterministic.  He also identified the reason for this, which is the existence of resonances between the degrees of freedom of, e.g., harmonic oscillators.  His memoirs on 'analysis situs,' as it was then called, marked the beginning of modern topology.
In 1892, Lorentz, in "La théorie électromagnétique de Maxwell et son application aux corps mouvants," proposed a theory of 'charged particles,' in which a body carries a charge if it has an excess of positive or negative particles, an electric current in a conductor is a flow of particulate particles, and the particles create the electromagnetic field.  "Because Lorentz completely separated ether [i.e., the field] and matter, he needed only one pair of directed magnitudes-one electric and one magnetic-to define the field at a point" (McCormmach 1976:494).  He also derived Fresnel's 'drag coefficient,' the measure of the motion that a moving transparent body communicates to light passing through it, and demonstrated that the drag was the result of the interference of light.  
   

In 1893, Andreas Franz Wilhelm Schimper proposed the idea that the photosynthetic parts of plant cells came from cyanobacteria.

In 1893, Freud and Breuer published Über den Psychischen Mechanismus Hysterischer, marking the beginning of psychoanalysis.  They claimed that "hysterics suffer mainly from reminescences" (Breuer and Freud 1893:7).

In 1893, Michaelson completed the measurement of the standard meter in terms of the wave length of the red line of the cadmium spectrum, providing an absolute and reproducible standard.
In 1893, Emile Durkheim published the first of a series of sociological books in which he explained how the ostensible explanations of theistic world-views, e.g., curiosity about the seasons or the rest of the natural environment, are actually "phrased so as to satisfy a dominant social concern, the problem of how to organize together in society" (Douglas 1966:91).
   

In 1894, William Bateson, in Materials for the Study of Variation, emphasized the importance of discontinuous variations and described and named homeotic mutations.
In 1894, H. J. H. Fenton discovered a reaction, to which he lent his name, now considered to be one of the most important mechanisms of oxidative damage in living cells.
In 1894, Ramón y Cajal, combining and extending the ideas of E. Tanzi and E. Lugaro, proposed the plastic, or functional, change hypothesis for neuronal growth, according to which synapses on 'associative pathways' are able to strengthen through use and to make new associations through learning.

In 1894, George Oliver and Eduard Albert Sharpey-Schaeffer demonstrated the effect of an extract of the adrenal gland, that is to say, a hormone, which contracted blood vessels and muscles and raised blood pressure.

In 1894, Emil Fischer suggested that a cell contains a chemically active substance whose geometrical configuration is complementary to that of another substance, fitting it like a key in a lock (Fischer 1894; de Duve 1991:22n8).  These studies still form the basis of our notions of enzyme specificity.
[["From the time of [N.] Carnot on, when it was discovered that the new science of thermodynamics did not require any sort of picture or model to explain the nature of heat, there had been a widespread hostile attitude among Continental physicists [Mach is one example] to any form of hypothetical model in science.  By [the time of Boltzmann, Planck, and Hertz], however, new developments in physics were requiring fresh patterns of explanation, and these warranted complex mechanical hypotheses" (Janik and Toulmin 1973:143)]].
In 1894, Hertz, in Principien der Mechanik, having earlier held that "Maxwell's theory is in Maxwell's equations" (Hertz, quoted in McCormmach 1976:346), noted that mathematical formulas could confer a logical structure on physical reality, creating 'possible sequences' of observed events.  Hertz opened Principien with the statement that "all physicists agree that the problem of physics consists in tracing the phenomena of nature back to the simple laws of mechanics" (Ibid.:348).  He went on to trace the classic mechanical formulations of Newton, Lagrange, and Hamilton, but it was one of the last times anyone would make that statement.  
In 1894, Strutt and William Ramsay discovered and isolated argon in the process of explaining the discrepancy between the weight of nitrogen obtained from the air and from ammonia.   

In 1894, Élie Joseph Cartan, in "Sur la Structure des Groupe de Transformations Finis et Continus," laid out all possible variations of Lie groups and gave them names, e.g., U(1), or unitary group with one matrix row, or SU(2), or special unitary group with two matrix rows (Cartan 1894:133-287). 
In 1894, Oliver Joseph Lodge invented the 'coherer,' a detector used in early radio receivers.
   

In 1895, Richard F. J. Pfeiffer presented the theory that bactericidal substances in the blood, or "Antikörper," were "highly active" and "were formed 'under the influence' of the body's cells and were consumed in the process of destroying bacteria" (Keating and Ousman 1991:248).

In 1895, Smith produced a blood deficiency disease in guinea pigs by depriving them of leafy vegetables.
In 1895, Johannes Eugenius Bülow Warming, in Plantesamfund (which was translated as The Ecology of Plants in 1909), plotted the distribution of plants against temperature and moisture.
In 1895, Wilhelm Conrad Röntgen, using a Crookes' tube, observed a new form of penetrating radiation, which he named X-rays.
In 1895, Lorentz, in Versuch einer Theoris der electrischen und optischen Erscheinungen in bewegten Körpen, spoke of 'ions,' instead of charged particles, and produced an equation connecting the continuous field with discrete electricity.  

In 1895, Guglielmo Marconi sent longwave wireless telegraphic, or radio, signals over a distance of more than a mile.
   

In 1896, Romanes promulgated the notion that behavior is species-dependent and phyletic, or inherited.  He also coined the term 'Neo-Darwinism' in order to differentiate pre- and post-Weismannian concepts of evolution.
In 1896, Conwy Lloyd Morgan, James Mark Baldwin, and Henry Fairfield Osborn, each independently, proposed a theory of how acquired characters could be inherited.  Lloyd Morgan concluded that evolutionary changes in anatomy can give rise to new behavior patterns.  Baldwin named his version 'Organic Selection' "since it required the direct cooperation of the organism itself."  The idea behind this so-called 'Baldwin effect' is that learning creates habituation which, in turn, provides the adaptive occasion for selection, or "overproduction with survival of the fittest" (Baldwin 1896:546,548-549).
In 1896, Ferdinand Isidore Widal introduced serological diagnosis through his discovery that typhoid patients agglutinate typhoid bacilli.
In 1896, Eduard Buchner discovered a chemical in yeast, which he called zymase.  He noted that the crushed yeast, that is, cell-free yeast, fermented sugar.  This observation opened the era of modern biochemistry.

In 1896, Freud suggested analyzing childhood conflicts in the study of neuroses.  He also devised a psychoanalytic technique called 'free association' which allows emotionally-charged, repressed material to be consciously recognized.  Over the next few years, Freud expanded his interpretive repertoire to include "dreams..., slips of the tongue, bungled actions, the forgetting of names, and what he called 'screen memories' (vivid but essentially counterfeit recollections from childhood)" (Kerr 1993:76).

In 1896, Pieter Zeeman observed that a single spectral line splits into a group of closely spaced lines when the substance producing the line is subjected to a uniform magnetic field.  This is known as the 'Zeeman effect,' and was explained by Lorentz on the basis of his electron theory.  
In 1896, Antoine Henri Becquerel discovered radioactivity in uranium.
In 1896, Boltzmann, in Vorlesunfer über Gastheorie, provided what he felt was a reasonable basis for statistical mechanics: "We...obtain the correct average values if we consider...an infinite number of equivalent systems, which started from arbitrary initial conditions" (Boltzmann 1896:310).  These mean values he called the 'Ergoden,' or "the so-called 'quasi-ergotic hypothesis, [i.e.,] the trajectory of a [kinetic] system may pass arbitrarily close to every point on an energy surface" (Brush 1964:11).  
   

In 1897, Paul Ehrlich , in Die Wertbemessung des Diphtherieheilserums und deren theoretische Grundlagen,determined that a toxin was toxic because it had a chemical, i.e., molecular receptor, structure complementary to the molecular structure of the susceptible cell.  If there were no matching receptors, this meant there was no disease.  If there were some matches, but the cell did not have enough receptors to deal with all the toxins, the cell would produce more and release them into the blood (Ehrlich 1897).  This was the first selective theory of antibody formation. By 1900, he had revised this theory to mintain that antibodies were continuously formed under normal conditions.
In 1897, Charles Sherrington named the junction between the neurons, a 'synapse.'
In 1897, Christiaan Eijkman proved that in a rice diet only rice with hulls intact would prevent 'beriberi.'
In 1897, Gabriel Bertrand designated certain inorganic substances co-enzymes because they were necessary to activate certain enzymes.
In 1897, Felix Hoffman synthesized a form of acetysalicylic acid that enabled the mass production of aspirin two years later.
In 1897, Jean Henri Fabre observed a series of stereotypic sequences in insect behavior, later named 'fixed-action patterns' by Konrad Lorenz.

In 1897, Wilhelm Fliess suggested that all organisms were fundamentally bisexual, with the implication that adolescence is as much a time of sexual repression as of sexual flowering.
In 1897, Joseph John Thomson, using a Crookes' tube, demonstrated that cathode rays consisted of units of electrical current made up of negatively charged particles of subatomic size.  Believing them to be integral to all matter, in "Cathode Rays," he hypothesized a model of atomic structure in which negatively charged particles, or electrons, were embedded in a sphere of positive electricity. (Thomson 1897)
In 1897, Boltzmann, responding to Ernst Zermelo on time irreversibility, preferred a "universe, which is in thermal equilibrium as a whole and therefore dead, [but has] here and there relatively small regions the size of our galaxy..., which during the relatively short time of aeons deviate significantly from thermal equilibrium," i.e., corresponding to the existence of life, to an entire universe of "unidirectional change...from a definite initial state to final state" (Boltzmann, quoted in Cercignani 1998:102).  Earlier, to the same end, he had maintained that "the probability that such a small part of [the universe] as our world should be in its present state is no longer small" (Boltzmann 1895:415).  

In 1897, Jacobus Cornelius Kapteyn, in a program of measuring the proper motions of stars, found two preferred directions of motion, roughly toward and away from the center of our Galaxy.  This is known as 'star streaming.'
In 1897, Peirce attempted to publish a topographical system of symbolic logic which he belived could give geometric expression to any conceivable assertion or logical argument.  "Once a formal structure had been adequately graphed, it could then be experimented upon in a manner similar to the way a scientist experiments with a structure in nature" (Gardner 1982:56).
   

In 1898, Henry Fairfield Osborne enunciated the evolutionary concept of 'adaptive radiation,' the descent from an ancestral form of related species occupying and exploiting different types of available habitats.

In 1898, Edward L. Thorndike devised the first reliable techniques for measuring learning in animals.
In 1898, Golgi described the 'Golgi apparatus.' [added 02/01/03]

In 1898, Stokes suggested that X-rays were pulses of radiation emitted when electrons, ejected from a cathode, hit a target.
In 1898, Marie Sklodowska Curie and P. Curie discovered and isolated radium and polonium. and clarified that radiation was an atomic property.  M. Curie coined the term 'radioactive.'
In 1898, J. Thomson, in "On the Charge of Electricity Carried by the Ions Produced by Röntgen-Rays," showed that neon gas consisted of two types of charged electrons, or ions, each with a different charge, or mass, or both.  This raised the possibility that varieties of a single element might exist with the same atomic number but differ in mass.  
In 1898, Planck, in "Über irreversible Strahlungsvorgänge.  Vierte Mitteilung" and probably responding to Boltzmann's criticism, adopted the kinetic theory and changed his program completely, saying that "all the radiation processes which do not exhibit the feature of irreversibility" must be excluded (Planck, quoted in Cercignani 1976:218).  
In 1898, Wien, while studying streams of ionized gas, identified a positive particle equal in mass to the hydrogen atom, which later was named the 'proton.'

In 1898, Poincaré, in "De la mesure du temps," postulated the limiting and constant speed of light and formulated the principle of relativity non-mathematically. 

In 1898, George Johnstone Stoney showed that the stability of the atmosphere of a given planet depends on its temperature and its mass.  If the velocity of individual molecules, as determined by their temperature, exceed the planet's 'escape velocity,' as determined by its gravitational pull, the lighter molecules are more likely to escape. 
In 1898, Ramsey and Morris Travers discovered neon, krypton, and xenon.
In 1898, James Dewar liquified hydrogen.
   

In 1899, Charles O. Whitman , working with pigeons, and Oskar Heinroth, working with ducks, independently discovered that the stereotypic responses of birds could be used as a taxonomic criteria for phylogenetic classification.

In 1899, the sixth edition of Emil Kraepelin's textbook, Psychiatrie, codified a diagnostic distinction, based on outcome statistics, by adding dementia praecox, i.e., 'schizophrenia,' to 'manic-depression' and 'paranoia.'
In 1899, Ernest Rutherford characterized the radiation from radium as being quite complex, easily absorbed, and stopped by a few centimeters of air.  These he named 'alpha rays.'  He also characterized uranium radiation as far more penetrating.  These he named 'beta rays.'
In 1899, Becquerel showed that radioactivity in uranium consists in charged particles that are deflected by a magnetic field.

In 1899, Strutt, in "On the Transmission of Light Through an Atmosphere Containing Small Particles in Suspension, and On the Origin of the Blue of the Sky," explained 'elastic scattering of electromagnetic radiation,' called 'Rayleigh scattering,' as reflected photons, i.e., photons which bounce off atoms and molecules without any change of energy.  Therefore, for example, the 'elastic scattering' of photons when they bounce off molecules and atoms in the Earth's atmosphere accounts for the color of the sky and red sunsets. 
In 1899, J. Thomson, in "On the Masses of Ions in Gases at Low Pressure," pointed out that ions have "a very much smaller mass than ordinary atoms; so that in the convection of negative electricity...we have...something which involves the splitting up of the atom" (Thomson 1899:548).  
In 1899, Lorentz, in "Théorie simplifiée des phénomènes électriques et optique dans des corps en mouvement," treated his contraction hypothesis mathematically in terms of electrons and, except for not determining the coefficient and not generalizing the subject to any mass, the resulting transformations for space and time coordinates are the same as those in his 1904 article.  

In 1899, Thomas Chrowder Chamberlain raised the question of whether the "present knowledge relative to the behavior of matter under such extraordinary conditions as obtain in the interior of the sun [is] sufficiently exhaustive to warrent the assertion that no unrecognized sources of heat reside there?  What the internal constitution of atoms may be is yet open to question.  It is not improbable that they are complex organizations and seats of enormous energies" (Chamberlain 1899:12).  Chamberlain was a geologist and "up to the end of the nineteenth century, the minimum estimates of geologists were far in excess of the maximum which physicists would allow for the age of the solar system on the basis of known sources of energy radiated by the sun.  When the enormously greater energy from the conversion of mass became known, there was no difficulty in reconciling estimates" (Wright 1948:920).
   
   

In 1900, Julius Bernstein hypothesized that nerve cells have both a resting and a stimulated potential. During the resting potential, the cell is impermeable to the negatively charged ions. When the cell is stimulated, the ions can pass in both directions (Bernstein 1902). [revised 02/01/03]
In 1900, the significance of Mendel's work was realized when there were three independent accounts of it by Hugo Marie de Vries, Carl Erich Correns, and Erich von Tschermak von Seysegegg.
In 1900, Karl Pearson developed the chi-square, or c2, test, a statistical procedure that enables the determination of how closely an experimental set of values conforms to theoretical expectation.
In 1900, Mikhail Semenovich Tsvet, or sometimes Tswett, established that in leaves there are two green pigments, chlorophyll a and b, differing in color, fluorescence, and spectral absorption. Subsequently he discerned another green pigment, chlorophyll c.[added 02/01/03]
In 1900, Planck, in "Zur Theorie des Gesetzes der Enieverteilung im Normalspektrum," introduced the 'quantum theory' to explain a formula, E=hf, where E is energy, f is frequency, and h is a new constant, 6.63 x 10-34 J-sec., which accounts for experimental data in black-body radiation. Quantum is a Latin word, widely used in German to mean 'portion.'.  This theory holds that oscillating atoms absorb and emit energy, or light, only in discrete bundles, or 'quanta,' rather than continuously, as classical physics would have it. Each quanta has a value proportional to the frequency of the oscillation. 

In 1900, Joseph Larmor, in Aether and Matter, showed that Maxwell's theory of the electromagnetic field can be derived from a Lagrangian, which equals the square of the magnetic field strength H2 minus the square of the electric field E2

In 1900, Rutherford identified a third type of radiation, which he called 'gamma radiation.' Rather than consisting of particles, like alpha and beta radiation, gamma rays are electromagnetic photons.  
   

In 1901, de Vries devoted his book, in Die Mutationstheorie, promulgated the principle characters, or genes, and the speculation that that made possible, namely, evolution of species by discontinuities, or 'saltations,' rather than by imperceptible gradations.  This led to Darwin's eclipse for some years, at least, for those biologists who believed with de Vries that mutation superceded selection as the cause of evolution.
In 1901, Richard Bucke proposed the possibility of man's evolution from self-consciousness to 'cosmic consciousness.'
In 1901, Josiah Royce contended that "the distinction between Self and the not-Self had a predominently social origin" (Royce 1901:245).
In 1901, Planck discovered the first indications of the granular structure of electromagnetic radiation while working on the spectrum of blackbody radiation.
In 1901, Oliver Heaviside and, independently, Arthur E. Kennelly predicted the existence of an atmospheric layer, later named the ionosphere, that would reflect radio waves.  This encouraged Marconi, the following year, to make a successful attempt to send radio signals across the Atlantic ocean.

In 1901, Ricci-Curbastro and Tullio Levi-Civita, in Méthodes de calcul différentiel absolu et leurs applications, developed a coordinate-free tensor calculus using Christoffel's symbols. 

In 1901, Willis H. Carrier invented the industrial air conditioner. 
   

In 1902, W. Bateson, in Mendel's Principles of Heredity: A Defense, in which he demonstrated that Mendel's principles apply also to animals.  In the same year, he coined 'allomorph,' and, before 1909, also 'genetics,' 'homozygote,' 'heterozygote,' and 'epistatic.'
By 1902, Karl Landsteiner found that human blood was one of four types, A, B, A-B, and O, thus making transfusions safe.
In 1902, E. Overton supported Bernstein's idea with evidence that exogamous sodium ions were responsible for the impulse (Overton 1902).
In 1902, Fischer proposed that proteins consist of chains of amino acids. [added 02/01/03]
In 1902, Ivan Pavlov combined associative learning with reflex acts, postulating the existence of associated stimuli, or 'conditioned responses.'  Later, he also described two non-associated behavioral modifications, 'habituation' and 'sensitization.'
In 1902, Lucien Claude Cuénot proposed that a gene plus two enzymes controlled hair color in mice; if both enzymes were present, it was grey, or if only one, it was black.
In 1902, J. W. Gibbs, in Elementary Principles in Statistical Mechanics Developed With Special Reference to the Rational Foundation of Thermodynamics, offered a more general approach to statistical mechanics than Boltzmann or Maxwell.  What Boltzmann had called the Ergoden, Gibbs called the 'grand canonical ensemble,' and the process of achieving it is known as 'ensemble averaging.'  The principle theme is the "analogy...between the average behavior of a canonical ensemble of systems and the behavior of a physical system obeying the laws of thermodynamics" (Klein 1976:392).  This ensemble, or function, has a simple physical interpretation: the probability of finding at a certain time t a point in the small region of phase space around the point q at momentum p.  That Gibbs' and Boltzmann's books did not become obsolete is due to the adequacy of classical theory in dealing with the relations between molecules, whereas quantum theory is necessary to deal with a molecule's internal structure.  

In 1902, Poincaré, in La science et l'hypotheése, noted that it doesn't matter whether or not aether exists, that 'what is essential for us is that everything happens as if it existed....  [It] is only a convenient hypothsis, [and] some day, no doubt, the aether will be thrown aside as useless" (Poincaré 1902:211-212). 

In 1902, Bertrand Arthur William Russell found the 'ultimate paradox:' If the set of all sets which do not contain themselves nonetheless contains itself, then it cannot belong to the set of all sets which do not contain themselves.  If it does not contain itself, then it must belong to the set of all sets which do not contain themselves.
   

In 1903, the beginning of cytogenetics occurred when, in independent accounts, Bovari and Walter Stanborough Sutton pointed out that chromosomes permutated themselves in cell division, halved their complement in germ cell formation, and paired again in fertilization, in a "physical dance that kept perfect step with Mendel's abstract algebra" (Judson 1979:206).
In 1903, Willem Einthoven invented a string galvanometer which enabled him to produce the first electrocardiogram, or graphic record of the action of the heart.
In 1903, Richard Kraus distinquished natural, or normal, antibodies from acquired immune antibodies according to their 'avidity,' or strength.  Both Ehrlich and Landsteiner, in contrast, were intent on reducing the difference to one of quantity.
In 1903, Metchnikoff, in Études sur la nature humaine, translated as The Rhythm of Life, argued that death from old age was only to be feared because of the accompanying pain and that, as science advanced and old age became less onerous, a natural wish to die would manifest itself.
In 1903, Tsvet made the principle of adsorption the basis of a new method which would permit the extraction from a solution of pigments in unchanged forms (Tsvet 1903), and subsequently developed and named it 'chromatography' (Tsvet 1906). [added 02/01/03]

In 1903, Orville Wright and Wilbur Wright achieved flight in a manned, gasoline power-driven, heavier-than-air flying machine.
   

["The emergence of genetics coincided with the redefinition of the term heredity to refer exclusively to transmission: what had previously been seen as two aspects of a single subject (transmission and development) came to be regarded as distinct concerns.  By the early decades of the twentieth century, the study of transmission had become the province of genetics, whereas that of development--now split off from genetics--continued as the province of embryology" (Keller 1995:4-5).  At the same time as thinking about evolution turned from Haeckelian comparative anatomy and Weismannian speculation to the laboratory, embryologists shifted Haeckelian phylogenetic recapitulation and Weismannian concentration on heredity to a concern for experiment and developmental mechanics, specifically to His's immediate causes of morphologies and Roux's Entwicklungsmechnik, or developmental mechanics.  Unlike many cytologists, and later geneticists, who centered their investigations on the chromosomes, embryologists centered theirs on the cytoplasm of the egg.  The geneticists were essentially reductionist; the embryologists integrative or holistic.  Genetic methods looked for differences through interbreeding; embryological methods, for commonality.]
   

In 1904, Nuttall, using precipitin tests of blood serum proteins, inferred the close phylogenetic relationship between humans and apes.
In 1904, T. R. Elliott recognized that chemical agents, and specifically adrenaline, acted as neurotransmitters in peripheral nerves, helping the nerve signal across the synapse (Elliott 1904).

In 1904 and 1905, Arthur Harden discovered that the presence of phosphate was essential to the enzymes which ferment sugar.

In 1904, Lorentz, in "Electromagnetic Phenomena in a System Moving with Any Velocity Less than that of Light," formulated the so-called 'Lorentz transformation,' which describes the increase in mass, the shortening of length, and the time dilation of a body moving at speeds close to that of light, by which the space-time coordinates of a moving system can be correlated with those of any other system. "The quality of not changing under this or some other transformation is called invarience [which] is the mathematical expression of symmetry....  Both Maxwell's electrodynamics and [Albert] Einstein's special relativity are descriptions which are invariant under Lorentz transformation" (Park 1990:355-356). 

In 1904, Hantaro Nagaoka proposed a 'Saturn model' of the atom with a nucleus and many electrons in a ring around it.

In 1904 or earlier, Poincaré gave the name the 'principle of relativity' to the proposition that, since the Universe contained no standard of absolute rest, anything is moving only in respect to something else. 

In 1904, Ramsey discovered radon.

In 1904, L. P. Teisserenc de Bort published the results of 581 free balloon ascents in which instruments measured the temperatures and pressures in the atmosphere to a height of about 14 km.
   

In 1905, Metchnikoff introduced the theory that white blood cells are able to engulf and kill bacteria (Metchnikoff 1905).
In 1905, Nuttall demonstrated the importance of bacteria for digestion.
In 1905, John Newport Langley discovered acetycholine, but it was not recognized in the brain until F. McIntosh did so in 1941 (Langley 1905).
In 1905, F. Knoop deduced the beta-oxidation of fatty acids.
In 1905, Edmund Beecher Wilson, author of Cell Biology in Development and Heredity (published in 1896 and numerous later editions and others), discovered that the X chromosome is linked to the sex of the bearer.

In 1905, Freud, in Drei Abhandlungen zur Sexualtheorie, redrew the line between normality and mental illness: "Not some mysterious hereditary degeneration read back into infancy, but an otherwise normal childhood experience that would bear a resemblance to the adult behavior (or in the case of neurosis, to adult repressed fantasy)" (Kerr 1993:93).  In the same year, in a postscript to another paper, Freud argued that patients sought to reexperience old erotic situations by transferring them to their physician.

In 1905, Arrhenius expressed concern about global warming as a result of burning fossil fuels.

In 1905, in the first of three articles in a single issue of Annalen der Physik, "Übereien die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt," Einstein sought an explanation of the photoelectric effect, the anomaly that electrons are emitted from the surface of a metal only if the incident light is sufficiently short wave length.  Einstein determined that a massless quanta of light, which he called a 'photon,' in order to break the attractive forces holding the electrons in the metal, would have to impart the required energy according to Planck's radiation law.  "This elegantly quantified reversion to Newton's corpuscular theory of light by Einstein was one of the milestones in the the development of quatum mechanics" (Dictionary of Physics 2000:387-3880.

In 1905, in the second article, "Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in rubenden Flüssigkeiten suspendierten Teilchen," Einstein studied some consequences of assuming that liguids and gases are composed of atoms.  Even though too small to see, he conjectured that the presence of atoms could be confirmed if objects large enough to see were influenced by their fluctuations.  This he demonstrated by showing that "a particle suspended in a liquid and observed with a microscope would be seen to dance around under the influence of the random fluctuations in pressure that are to be expected if the liquid consists of atoms in rapid motion" (Park 1990:309).  This phenomena was well known to microscope users as Brownian movement.  By inverting Boltzmann's formula, Einstein described its mathematics, deriving the probability of a macroscopic state for the distribution of gas molecules, in terms of the entropy associated with that state. 

In 1905, in the third article, "Zur Electrodynamik bewegter Körper" ("On the Electrodynamics of Moving Bodies"), Einstein evolved the Special Theory of relativity by working out the consequences of two postulates: One, the laws of nature are the same for all frames of reference in uniform, i.e., not accelerating, relative motion, and, two, light is propagated at a constant velocity which, unlike things in ordinary experience, is independent of the movement of the emitting body and the observer.  In other words, observers in motion with respect to one another will disagree about length and time in the other's system.  This theory "led to the discovery that time is associated as a fourth coordinate on an equal footing with the other three coordinates of space, and that the scene of material events, the world, is therefore a four-dimensional, metrical continuum" (Weyl 1918a:201).  The Special Theory was invented as "a way--the only way--to assure the complete validity and self-consistency of Maxwell's equations" (Wheeler and Ford 1998:166).  It also resulted in mathematical equations which confirmed the 'Lorentz transformation' and contained the velocity of a moving body at the velocity of light relative to an observer, V=(v1+v2)/(1+v1v2/c2)(Einstein 1905a:37-65). "The real (and great) merit of the Special Theory...was pedagogical.  It arranged the old confusing material in a clear deductive pattern" (Everitt 1976:215).  

Later in 1905, in a second paper of the Special Theory of relativity, "Ist die Trägheit einer Körpers von seinem Energiehalt abhängig?" ("Does the Inertia of a Body Depend upon Its Energy Content?"), Einstein wrote that "if a body gives off the energy L in the form of radiation, its mass diminishes by L/c2....  The mass of a body is a measure of its energy content" (Einstein, quoted in Kantha 1996:46). This was published in Annalen der Physik in 1906 and put an end to speculation that the Sun's energy came from radiation (Einstein 1905b:69-71).

In 1905, Poincaré, in "Sur la dynamique de l'electron," obtained, independently of Einstein, many of the results of the Special Theory of relativity.  However, he postulated nonelectric forces, or 'stresses,' to give stability to an electron; these were rendered irrelevant by quantum theory.

In 1905 and 1907, Ejnar Hertzsprung published papers relating colors and brightnesses of stars in a systematic way, and recognizing dwarf and giant stars.
   

In 1906, W. Bateson and Reginald Crundall Punnett reported less-than-independent assortment, or 'linkage,' in gene alleles on the same chromosome in sweet peas.

In 1906, Sherrington showed, in his book The Integrative Action of the Nervous System, that those cells which send their fibers and impulses directly to the limb muscles can be influenced to fire by excitation or not to fire by inhibition.
In 1906, Frederick Gowland Hopkins noticed that 'accessory food factors,' later called vitamins, were essential to the growth of rats.
In 1906, Andrei Andreyevich Markov described sequences of randomly linked probability variables in which the future variable is determined by the present variable, but is independent of the way in which the present variable arose from its predecessors. These 'Markov chains' launched the theory of stochastic processes.
In 1906, Walther Hermann Nernst stated a new tenet, often called the Third Law of Thermodynamics, according to which if a chemical change takes place between substances that are at absolute zero there is no change in entropy. [added 02/01/03]

Beginning in 1906, Ferdinand de Saussure lectured on the structural principles of general linguistics, including the reciprocity of phonemes and the opposition of diachrony and synchrony (Saussure 1915).
   

In 1907, Ross Granville Harrison cultivated amphibian spinal cord, demonstrating that axons are extensions of single nerve cells.  In so doing, he invented tissue culture (Harrison 1907).
In 1907, Alois Alzheimer characterized the senile degeneration , to which he loaned his name, by the 'senile plaques' and 'neurofibrillary tangles' which he found in an autopsied brain.
In 1907, Arrhenius published Immunochemistry: The Application of the Principles of Physical Chemistry to the Study of Biological Antibodies, thereby coining the term 'immunochemistry.'

In 1907, Arrhenius hypothesized that life on earth is descended from interstellar microorganisms, sucked in by gravity and pushed out by radiation.  This hypothesis is usually called 'panspermia,' meaning 'life everywhere.'  Since, at that time, it was assumed that, even though stars were born and died, the Universe was in essence eternal and unchanging, the question of its origin did not have to be addressed (Gribbin and Gribbin 2000:3-4).

In 1907, Einstein, in "Über die vom Relativitätsprinzip geforderte Trägheit der Energie," deduced the expression for the equivalence of mass and energy, Ko=mV2, where Ko is energy, m is mass, and V2 is the speed of light squared.  ["E=mc2" was the title of a Science Illustrated article which Einstein wrote in 1954 (Kantha 1996:46).]  This relation says that "a sufficiently energetic packet of radiation (a photon) can convert into matter with the appropriate mass, and vice-versa" (Gribbin 1998a:172).

In 1907, Luitzen Egbertus Jan Brouwer completed his doctoral dissertation on the logical foundations of mathematics which marked the beginning of the Intuitionist School.
   

In 1908, Archibald Edward Garrod, in Inborn Errors of Metabolism, recognized that gene products are proteins and showed that certain rare, inherited disorders were caused by the absence of specific enzymes.  W. Bateson, in 1902, had suggested to him the probability that an inherited disorder was due to a recessive gene.
In 1908, Godfrey Harold Hardy worked out the equilibrium formula for a population heterogenous for a single pair of alleles: Assuming the truth of Mendel's laws (and generalizing them), the resulting combinations will expand into the binomial distribution, or p2(AA) + 2pq(Aa) + q2(aa), where p is the initial frequency of the dominant A in a population and q the initial frequency of the recessive a.  This formula was derived independently by Wilhelm Weinberg, and is thus known as the Hardy-Weinberg equilibrium formula.  It provided the first baseline for assessing the effects of mutation.
In 1908, William McDougall, in An Introduction to Social Psychology, postulated that human beings have as many as a dozen different basic instincts, e.g., curiosity, pugnacity, self-abasement, etc.
In 1908, Robert Andrews Millikan determined the probable minimum unit of an electrical charge, that is, of an electron. Later, he named 'cosmic rays.' 
In 1908, Planck, attacking Mach's position that physical theories were based solely on sense data, held that "the physicist creates the system of the physical world by imposing form upon it..., creat[ing] the mathematical structures which organize empirical facts" (Janik and Toulmin 1973:138).

In 1908, Hermann Minkowski took Einstein's algebraic expression of the Special Theory of relativity and geometrized it, coupling space and time into a four-dimensional continuum, and providing a framework for all later mathematical work in relativity.  "Henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality" (Minkowski 1908:75).

In 1908, Frank W. Very suggested that the "atmospheres of the major planets would allow optically visible sunlight to pass through to the ground, which would heat up and reradiate at infrared wavelengths.  Because the atmospheres are opaque to the infrared spectrum, this radiation would be trapped beneath the atmosphere where it could heat up the planetary surface" (Lang and Gingerich 1979:153).

In 1908, Henrietta Swan Leavitt, after years of analyzing the two Magellanic Clouds, reported finding 1,777 variable stars, and, having derived the periods of a few 'Delta Cepheid-type' variables, also reported that the brighter among them tended to have longer pulsation cycles.

In 1908, George Ellery Hale completed building the 60-inch reflecting telescope on Mount Wilson in California.
In 1908, Zermelo founded axiomatic set theory.
   

In 1909, Charles D. Walcott discovered the Cambrian Burgess Shale fossils.

In 1909, Andrija Mohorovicic observed a discontinuity within the Earth that marks the junction between the crust and the mantle.
In 1909, reports by Correns and Erwin Baur described the non-Mendelian inheritance of a factor influencing chloroplast development, thus beginning the recognition of extra-nuclear or cytoplasmic genetics.
In 1909, F. Meves proposed that mitochondria originate from preexisting structures of the same kind and carry their own heredity.
In 1909, Wilhelm Johannsen published Elemente der exakten Erblichkeitslehre which was concerned with how to grow pure lines of beans in view of the fact that natural selection can influence change only if there is genetic variability.  To this end he distinquished between 'genotype' and 'phenotype,' the one being variant due to heredity and the other being due to environment.  Naming Mendel's algebraic units 'genes,' Johannsen understood that to mean that each gene underlies a single trait.
In 1909, W. Bateson, in a much expanded new edition of Mendel's Principles of Heredity, echoed Mivart's idea that what was selected was born fit.  Bateson believed that the variation giving rise to new species was saltational, but present from the beginning of life and waiting for disinhibition and expression.  He coined the term 'genetics,' but abjured theorizing: Heredity, the Mendelian variations which he encountered in experiments, failed to explain big changes.  By contrast, Pearson assumed that selection brought about stable varieties or species based on the small, incremental differences or gene frequencies in individuals belonging to groups of vast size.
In 1909, Kørbinian Brodmann , in Vergleichende Lokaisationslehre der Groshirnrinde, published a map of the cortex with 52 areas, each with a function.  This map is still in use.
In 1909, Edward Tyson Reichert conceived the ambition to plot the evolutionary relationships among species by the divergences between their hemoglobin molecules.  To this end he published six hundred micrographs of hemoglobin crystals.
In 1909, Charles Jules Henri Nicolle showed that typhoid fever is transmitted by body lice.
In 1909, Fritz Haber, in order to synthesize ammonia from its elements, developed the first commercially important high-pressure chemical process. [added 02/01/03]
In 1909, Hans Geiger and E. Marsden, under Rutherford's direction, scattered alpha particles with thin films of heavy metals, providing evidence that atoms possessed a discrete nucleus.
In 1909, Vito Volterra, in "Sulle equazioni integro-differenziali della teoria dell'elasticità," writing on the hereditary phenomena, said that the delayed effects tend to zero when time tends to infinity.  

In 1909, Karl Bohlin suggested that the center of the Milky Way lies within the large collection of globular clusters in the direction of Sagittarius.

In 1909, Vesto M. Slipher showed "photographic emulsions could record the infrared spectrum and...found that the major planets exhibited infrared absorption lines not present in sunlight.  Subsequently, these bands were identified with amonia and methane" (Lang and Gingerich 1979:67).
In about 1909, David Hilbert's work on integral equations established the basis for his subsequent work on infinite-dimensional space, which came to be called 'Hilbert space.'
   

In 1910, Francis Peyton Rous induced a tumor using a filtered extract of chicken tumor cells.
In 1910, Konstantin S. Mereschovsky published an essentially modern view of the bacterial origin of what later came to be called eukaryotic cells.
In 1910, Thomas Hunt Morgan discovered the white-eye sex linkage in Drosophila, relating it to Mendel's recessive traits, and thus initiating fruit fly genetics.  His insistence that genes were not just logical constructs from Mendelian ratios developed into the general theory of linkage within a chromosome, according to which the strength of the linkage is inversely proportional to the likelihood that a 'crossover' will occur during meiosis.  Later, he maintained that cytoplasm could be ignored in studying genetics [Meiosis consists in two divisions of the diploid nucleus of the fertilized cell accompanied by one division of its chromosomes. Initially, each chromosome replicates to produce two sister chromatids as in ordinary division, or mitosis. At this point the special features of meiosis become evident. Each chromosome must then somehow pair with its homologue. The pairing allows genetic recombination, or crossingover, to occur, whereby a random fragment of a maternal chromatid may be exchanged for a corresponding fragment of a homologous paternal chromatid (Alberts et al. 1994:1016). The duplicated homologues separate and "the chromosomes of each pair pass to opposite poles without separation of their chromatids [or half-chromosomes]. These chromatids then separate at the second division. Each of the four nuclei therefore has one of the four chromatids of each pair of chromosomes" (Darlington 1939:11). This is a haploid gamete and contains half the number of chromosomes of the egg. [Meiosis consists in two divisions of the diploid nucleus of the fertilized cell accompanied by one division of its chromosomes. Initially, each chromosome replicates to produce two sister chromatids as in ordinary division, or mitosis. At this point the special features of meiosis become evident. Each chromosome must then somehow pair with its homologue. The pairing allows genetic recombination, or crossingover, to occur, whereby a random fragment of a maternal chromatid may be exchanged for a corresponding fragment of a homologous paternal chromatid (Alberts et al. 1994:1016). The duplicated homologues separate and "the chromosomes of each pair pass to opposite poles without separation of their chromatids [or half-chromosomes]. These chromatids then separate at the second division. Each of the four nuclei therefore has one of the four chromatids of each pair of chromosomes" (Darlington 1939:11). This is a haploid gamete and contains half the number of chromosomes of the egg. [revised 02/01/03]
In 1910, P. Boysen-Jensen proved the existence of 'auxins' which are chemicals instrumental in the the growth of higher plants.

In 1910, Georges Claude discovered that electricity conducted through a tube of the rare inert gas, neon, gives a bright red glow and that other gases gave off other colors, e.g., argon gives blue, helium gives yellow and white, etc.  Fluorescent light, introduced in 1935, is a variant containing argon and krypton.
In 1910, Alfred North Whitehead and Russell, in Principia Mathematica, put forth the theory that there is a discontinuity between a class and its members and attempted to overcome certain logical paradoxes by the formal device of branding them meaningless.
   

In 1911, Alfred Henry Sturtevant, an undergraduate student of Morgan's, constructed the first rudimentary map of the fruit fly chromosome, establishing that genes are real.  By 1917, the map was sufficiently continuous to be published.
In 1911, Casimir Funk isolated a crystal, which came to be known as B-complex, and coined the name 'vitamine.'
In 1911, Tsvet, having discovered many forms of xanthophyll and their chemical relation to carotene, proposed to call the general group 'carotenoids' (Tsvet 1911). [added 02/01/03]

In 1911, Bleuler renamed dementia praecox 'schizophrenia.'

In 1911, Arnold Johannes Wilhelm Sommerfeld hypothesized that "the interaction between electrons and atoms was definitely and uniquely controlled by Planck's quantum of action" (Cao 1997:126-127). 
In 1911, Rutherford, in "The scattering of a and b particles by matter and the structure of the atom," thinking about the nature of the nuclei which could produce radiation, described the atom as a small, heavy nucleus, surrounded by electrons.
In 1911,  Charles Thomson Rees Wilson developed the 'cloud chamber,' a device in which the paths of particles of ionizing radiation are made visible.  The excess moisture in supersaturated vapor is deposited on the tracks of the ions.

In 1911, Heike Kamerlingh Onmes discovered 'superconductivity,' the ability of certain materials at low temperatures to carry electric current without resistance.

In 1911, Einstein, in "Einfluss der Schwerkraft auf die Ausbreitung des Lichtes" ("The Influence of Gravity upon the Propagation of Light"), said that if a "light beam is bent in an accelerating frame of reference, then if the theory is correct it must also be bent by gravity, and by the exactly equivalent amount" (Gribben 1998a:90; Einstein 1911:99-108).

In 1911, Hertzsprung published graphs plotting color or spectral class against the absolute magnitude of stars.  In 1913, Henry Norris Russell, independently, presented similar graphs.  These are now called Hertzsprung-Russell, or HR, diagrams and are the basis of the theory of stellar evolution.  Russell also suggested that nuclear energy is generated inside stars when they reach a critical temperature.

In 1911, Jacob Halm argued that the masses of stars are correlated with spectral type and therefore with their luminosities.
   

In 1912, Alfred Lothar Wegener proposed a unified theory of continental drift, which opposed to the sinking of continents, based on fossil and glacial evidence.

In 1912, J. F. Gudernatsch, working with frogs, found that removing the thyroid gland prevents metamorphosis and that feeding thyroid extracts induces precocious metamorphosis.

In 1912, Ernest Everett Just, in "The Relation of the First Cleavage Plane to the Entrance Point of the Sperm," said that the former "passes either directly through the entrance-point of the sperm or a degree or so from it" (Just 1912). [added 02/01/03]

In 1912, John Broadus Watson launched his polemic favoring the objective study of psychology as physicochemically-based behavior and reputiating introspection as unscientific.  He denied the value of studying either consciousness or instinct, suggesting one could never be certain that a given behavior is free of learning.
In 1912, Jung conceptualized and named 'introvert' and 'extrovert,' and suggested the study of current conflicts for insights into the triggering of repressed, infantile contents.
In 1912, Max Theodor Felix von Laue obtained the first diffraction effects by letting X-rays fall on a crystal.  Almost immediately, William Lawrence Bragg proposed a simple relationship between an X-ray diffraction pattern, or characteristic interference pattern, and the arrangement of atoms in a crystal that produced the pattern, thereby inventing X-ray crystallography.
In 1912, Louis Carl Heinrich Paschen and Ernest E. A. Back discovered that atomic line spectra have a splitting pattern in a very strong magnetic field.

In 1912, Victor Hess, in the course of a balloon flight, noted increasing radiation above 5000 feet and proposed an extra-terrestial source.

In 1912, Slipher obtained spectrograms of the Andromeda Nebulae, M31, which all showed clear evidence of a Doppler blueshift.  By 1914, he had measured a dozen more Doppler shifts, all but one toward red.

In 1912, Leavitt concluded that those Cepheid-types in the smaller of the two Magellanic Clouds are so far away that they may be regarded as being roughly at the same distance and was thus able to work out the relationship between their luminosity, or energy output, and orbital period.
   

In 1913, Lawrence Joseph Henderson proposed that the concept of fitness, which in animals is the relative ability to transmit its genes to the next generation, be extended to the environment.  This has ramifications for the origin of life.

In 1913, Shiro Tashiro discerned slight increases in carbon dioxide production by stimulated nerves.

In 1913, C. Fabry and M. Buisson reported the existence of ozone, a gas created by a photochemical reaction between sunlight and oxygen.
In 1913, Henry Gwyn-Jeffreys Moseley bombarded the atoms of various elements with X-rays and found that the wavelength decreased in proportion to the increase in the atomic weight of the element emitting the rays.  From observing the wavelength, he discovered that the inner stucture responded in a characteristic group of lines, enabling the assignment of 'atomic numbers.'  The periodic table turned out to coincide with these numbers rather than, as had been supposed, the atomic weight.
In 1913, Niels Bohr,in "On the constitution of atoms and molecules," strongly influenced by Sommerfeld, applying the Planck quantum hypothesis to Rutherford's atomic model and postulating stable states and single frequencies, calculated closely the frequencies of the spectrum of atomic hydrogen (which has a single electron).  "Only certain photon energies are ever seen, identified by their corresponding frequencies or wavelengths, and this explains the appearance of the spectrum" (Park 1990:312).  This supported his proposal that electrons moved around the nucleus in restricted orbits and his explanation of the manner in which the atom absorbs and emits energy by leaping from one orbital to another without traversing the space in between, and was the first theory of quantum mechanics.
In 1913, Frederick Soddy discovered that different forms of the same element were, in fact, groups of elements with the same chemical character, but varying in their masses, and that radioactive decay is accompanied by the transmutation of one element to another.  To express this new found complexity of matter, the term isotopic element, or isotope, was used.

In 1913, Einstein and Marcel Grossman, in "Entwurf einer verallgemeinerten Relativitätstheorie und eine Theorie der Gravitation," investigated curved space and curved time as it related to a theory of gravity.  Einstein contributed the physics and Grossman the mathematics.

In 1913, Hertzsprung, using statistical parallax, got an indication of the distance to a couple of Cepheids and was able to extrapolate a rough measure in numbers of the distance to the Small Magellanic Cloud, which was much farther away than had been imagined.

In 1913, Cartan, in "Les groupes projectifs qui ne kaissent invariante aucune multiplicité plane," announced his discovery of linear two-dimensional representations of the three-dimensional orthogonal matrix known as "a spinor..., a sort of 'directed' or 'polarized' isotropic vector; a rotation about an axis through which an angle 2p changes the polarization of this isotropic vector" (Cartan 1937:41).  'Spin' is quanticized rotation and always comes in multiples of a basic unit which is equal to one-half times Planck's contant h divided by 2p.  Cartan's concept was used by Einstein in the mathematics of the theory of General Relativity. 

In 1913, Leo Baekeland invented a plastic laminate, known as Bakelite, and later as formica.
   
In 1914, Nicholas Vaschide published his hypothesis that sleep is not just the absence of being awake, but is a vital instinctual, i.e., biological, process.
In 1914, James Franck and Gustav Hertz confirmed experimentally Bohr's theory of the stationary states of the energy levels in atoms by producing "jumps between them, supplying the excitation energy by collisions with accelerated electrons" (Segrè 1976:137).
In 1914, Arthur Stanley Eddington hypothesized that spiral nebula were actually distant galaxies.

In 1914, Harlow Shapley established that the Cepheid variables are pulsating stars, not binaries.
   

In 1915, Jacques Loeb, in The Organism as a Whole, maintained that a complicated organism was unimaginable without a prestructure in the egg which he characterized in colloidal chemistry terms.  He also maintained that behavior consisted in stereotypic movements, directly elicited and controlled by sensory stimuli.

In 1915, Morgan, Sturtevant, Calvin Blackman Bridges, and Hermann Joseph Muller published The Mechanism of Mendelian Inheritance.

In 1915, Bridges discovered the first homeotic mutation in Drosophila, 'bithorax.'
In 1915, Haber, directing Germany's chemical warfare activities, initiated the use of poison gas releasing 150 tons of compressed chlorine on Allied positions around Ypres. Nernst also was a leader in the Germany's chemical warfare effort. In 1917, James Bryant Conant was put in charge of the United States' Chemical Warfare Service which was attempting to develop mustard gas (L. F. Haber 1986). [added 02/01/03]

In 1915, in three lectures delivered to the Prussian Academy, and published the following year as Grundlage der allgemeinen Relativitätatheorie (Foundation of General Relativity), Einstein completed the mathematical generalization of the theory of relativity: Whereas spacetime in the Special Theory is geometrically flat, in the General Theory spacetime is curved and includes gravity as a determinant, i.e., "a ray will experience a curvature of its path when passing through a gravitational field, this curvature being similar to that experienced by the path of a body which is projected through a gravitational field" (Einstein 1916:127).  Or, in other words, under the force of gravity, objects follow "a path of least resistance, the equivalent of a straight line, through a curved portion of space, or spacetime" (Gribbin 1998a:90-91).  This is the first example of concepts from differential geometry being used to represent physical structures; thus, in order to get from a flat to a curved spacetime manifold, "one replaces in all tensorial (or spinorial) relations the flat metric h by the curved one, g, and one substitutes covariant derivatives with respect to the latter for those with respect to the former" (Ehlers 1981:536).  In fact, in one these lectures, Einstein attributes "dem Zauber dieser Theorie," i.e., the magic of the theory, to the differential calculus methods of "Gauss, Riemann, Christoffels, Ricci und Levi-Civita" (Einstein, quoted in Ibid.:536).

Einstein's 1915 theory replaced the Kepler-Newton theory of planetary motion, which was based on the assumption of absolute space, with one which is able to account for the slow rotation, in the direction of motion, which the orbital ellipse of a planet undergoes.  Einstein's value for the bending of light waves by the Sun is "almost precisely the same value" as von Soldner's for light particles, which is to say that it is not space-bending but time-bending which differs from Newtonian calculations (Gribbin 1998a:53).  Employing Riemann's non-Euclidean geometry and equations which are highly non-linear, Einstein was able to predict radically new phenomena: The bending of light around the Sun and the precession of the perihelion of Mercury.
   

In 1916, Gilbert Newton Lewis said that the chemical bond consists of two electrons held jointly by two atoms.
In 1916, Karl Schwarzschild , in a paper which Einstein delivered to the Prussian Academy and which was based on the General Theory of Relativity, calculated that a star collapsing under its own gravitational force would cease to radiate energy beyond a certain parameter.  This parameter is known as the 'Schwarzschild radius' and shrinking beyond it creates a 'black-hole.'  Since inside a black-hole, according to Schwarzschild's solution, the curvature becomes infinite, and since this is a 'singularity,' i.e., not generally believable to physicists, it was not taken seriously for many years. In fact, when it was taken seriously, it was realized that there were two opposite solutions and that the singularity was an artefact of the coordinate system chosen by Schwarzschild to measure spacetime around a black-hole.  The second solution is the origin of the notion of the existence of 'white-holes' and describes the expansion out of an initial singularity.  Black-holes were not actually called that until, in 1968, John Archibald Wheeler used that name for what had been previously a 'Schwarzschild singularity,' a 'collapsed star,' or a 'frozen star' (Wheeler 1968:9). 

In 1916, Eddington, in "On the Radiative Equilibrium of Stars," said that the pressure of radiation takes its place beside gas pressure and gravity as an equilibrating force.
In 1916, Sommerfeld modified Bohr's atomic model by specifying elliptical orbits for the electrons.
In 1916, Irving Langmuir concluded that adsorption, the condensation of gas on a surface, is a single molecular layer thick and chemically bonded to the surface.  It is not, as most thought, analogous to the physical attraction which holds the earth's atmosphere. He also noted that the length of hydrocarbon chains makes no difference to the shape of the surface energy curve provided that there are more than 14 carbons in the molecule (Langmuir 1917).  
   

In 1917, J. Schmidt demonstrated that the differences between individuals in a population were genetic.  Richard Benedict Goldschmidt, in 1920, and Francis Bertody Sumner, in 1924, demonstrated it in other populations.  Their findings caused the downfall of the mutationalist and Lamarckian theories of de Vries and Bateson and "permitted a selectionist interpretation of slight differences among local populations that were obviously caused by differences in the environment" (Mayr 1959:4).
In 1917, D'Arcy Thompson, in Growth and Form, took basic body plans and changed the size and position of the parts relative to one another in geometric ways, showing how evolution might have proceeded.  His thesis was that form "is determined by its rate of growth in various directions, hence rate of growth deserves to be studied as a necessary preliminary to the theoretical study of form" (Thompson 1917:51).
In 1917, Landsteiner and H. Lampl found that antibodies could be produced which reacted with synthetic haptens, that is to say, with incomplete antigens which are unable to induce antibody formation.  This finding seemed for a long time to support a template model of antibody formation (Landsteiner and Lampl 1917).

In 1917, William D. Harkins noticed that terrestial elements "of low atomic weight are more abundant than those of high atomic weight and that, on average, the elements with even atomic numbers are about 10 times more abundant than those with odd atomic numbers of similar value, [and conjectured] that the relative abundances of the elements depend on nuclear rather than chemical properties and that heavy elements must have been synthesized from light ones" (Lang and Gingerich 1979:374).

In 1917, Einstein, in "Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie" ("Cosmological Considerations on the General Theory of Relativity"), by adding a 'cosmical constant,' was able to describe a universe that conformed to what he and everyone else supposed: A closed and static sphere coextensive with the Milky Way (Einstein 1917:177-188).  The same year, Willem de Sitter offered a different solution to the General Relativity equations, one which contained "negligibly small values for the mass density and pressure in ordinary matter" and thus permitted "exponential expansion" (Peebles 1993:77,393).  In fact, these equations, without the addition of a cosmical contant and including matter, were solvable provided that the universe was not static but rather expanding or contracting.  Moreover, mathematicians soon predicted a redshift as test-particles moved away from each other.

In 1917, Heber D. Curtis pointed out that the nova he observed in spiral nebulae were 100 times farther away than nova in the the Milky Way.  This supported the island universe theory of spiral nebulae.

In 1917, Slipher, using spectral analysis of spiral nebulae, recognized that they were generally receding from us at a high velocity.
In 1917, James Hopwood Jeans submitted an essay in which he described a general theory of the configuration of equilibrium of compressible and non-homogeneous masses of astronomical matter, enabling him to explain the behavior of certain nebulae and describe the evolution of gaseous stars.

In 1917, Levi-Civita recognized "the geometrical meaning of the Christoffel symbols as determining a natural parallel transport of vectors and tensors on Riemannian manifolds [which was important] in the subsequent development of differential geometry and field physics" (Ehlers 1981:527). 
   

In 1918, Ronald Aylmer Fisher wrote the initial paper, "The correlation of relatives under the supposition of Mendelian inheritance," of what came to be known as population genetics, the joining of Mendelian experiments with a statistical approach to large populations.

As early as 1918, Paul Portier became convinced that mitochondria were direct descendents of bacteria.

In 1918, Bridges, working with Drosophila, suggested that gene duplications promote the evolution of organisms toward greater complexity.

In 1918, J. S. Szymanski demonstrated that animals are capable of maintaining approximately 24-hour activity patterns without external or temperature clues. 

In 1918, Amelie Emmy Noether, in "Invarianten beliebiger Differentialausdrücke" and "Invariante Variationsprobleme," demonstrated the theorem that "wherever there is symmetry in nature, there is also a conservation law, and vice-versa.  In other words, the symmetries of space and time are not only linked with conservation of energy, momentum, and angular momentum, but each implies the other" (Crease and Mann 1986:189). 
In 1918, Hermann Klaus Hugo Weyl, in "Gravitation und Elektricitat" and two other papers, produced the first unified field theory in which the electromagnetic and gravitational fields appeared as a property of space-time (Weyl 1918a:201-216).  To do so, he found it necessary to describe random changes in another aspect of a system as being precisely compensated by changes in another aspect.  This he called 'measuring rod symmetry,' and later 'gauge invariance' (Weyl 1918b:176; 1928:100).  A gauge transformation is simply a relabelling exercise, e.g., when the Earth rotates, the distance between New York and London remains the same.  According to Weyl, "a state of equilibrium is likely to be symmetric....  The feature that needs explanation is, therefore, not the symmetry of [a] shape but deviations from this symmetry" (Weyl 1952:25-26). 

In 1918, Shapley, using the Mount Wilson observatory, Leavitt's period-luminosity law, and employing a statistical method of his own devising--i.e., he assumed the brightest stars in globular clusters had all the same intrinsic brightness, was able to show the dimensions of the Milky Way galaxy and the Earth's peripheral place in it.
In 1918, Ludwig Wittgenstein, in Tractatus Logico-Philosophicus, pointed out that "what can be said at all can be said clearly; and whereof one cannot speak thereof one must be silent" and "the sense of the world must lie outside the world" (Wittgenstein 1918:27,183).
   

In 1919, Harry Steenbock demonstrated the relationship between the plant pigment 'carotene' and vitamin A.

In 1919, Ernst Spath produced a synthetic version of peyote's psychoactive alkaloid, which he called 'mescaline.'

In 1919, Pierre Janet pointed out that the hypnotic condition must be learned by the subject: If the subject has never heard of hypnotism, it is unlikely that he can be hypnotized.
In 1919, E. Rutherford discovered the proton, which contains the positive charge within the nucleus of an atom, and published the first evidence of artificially-produced splitting of atomic nuclei; that is, he produced hydrogen through the bombardment of nitrogen with alpha radiation. His discovery made possible the description of the electrostatic force; namely, if each of two bodies have an excess of electrons or an excess of protons, repulsion occurs, but if the two bodies differ in their excesses, then attaction occurs.  
In 1919, Francis William Aston designed the mass spectograph and discovered neon isotopes with it, enabling him to explain nonintegral atomic weights. This revealed that the helium atom was less massive than four hydrogen atoms, pointing to the transmutation of the first two elements. "Of the nearly 300 isotopes of elements, Aston isolated and measured the masses of more than 200" (Hoyle 1994:149).
In 1919, Eddington and Frank W. Dyson measured the bending of starlight by the gravitational pull of the sun, thus confirming Einstein's general theory of relativity.
   

In 1920, Hermann Staudinger began to work on macromolecules, such as proteins, which had hitherto been thought by many to be aggragates.  Others questioned the strength of the atomic forces.

In 1920, Otto Loewi showed that the terminal branches of nerve fibers release stimulating and inhibiting chemicals.
In 1920, Friedrich A. von Hayak, in The Sensory Order, postulated that all perception is a product of memory and an act of classification of the qualities of objects and events performed by maps of cortical cells.  These interconnections are reinforced by the experience of prior contact.  This essay was not published until 1952.

In 1920, Jung, in Psychologische Typen, first used the term 'anima,' a word borrowed from Plato, who used it to represent the soul of the individual.  Jung used it to represent the archtype of the mediator between consciousness and the collective unconscious (for men; for women, he used 'animus').  Ignoring these mediators meant the failure to acknowledge all parts of a cognitive whole with the consequence that the hidden part would be dominant.

In 1920, E. Rutherford postulated the existence of the neutron, required in order to keep the positively-charged protons in the nucleus from repelling each other.  Their existence explains why some atoms have identical chemical properties to one another but slightly different mass.

In 1920, Eddington, in "The Internal Constitution of the Stars," spelled out the implications of Aston's discovery, namely: "Mass cannot be annihilated, and the deficit can only represent the mass of the electrical energy set free in the transmutation.  We can therefore at once calculate the quantity of energy liberated when helium is made out of hydrogen.  If 5 percent of a star's mass consists initially of hydrogen atoms, which are gradually being combined to form more complex elements, the total heat liberated will more than suffice for our demands, and we need look no further for the source of a star's energy" (Eddington 1920:19).

In 1920, Meghnad Saha, in "Ionization of the Solar Chromosphere," obtained an equation relating the degree of ionization of an atom, analogous to the dissociation of a molecule, to temperature and pressure, and thus accounting for the relative intensities of different spectral lines.

In 1920, Michelson and Francis G. Pease, using an optical interferometer, measured the first stellar diameters, Betelgeuse and five other supergiant stars.
   

In the early 1920s, it was ascertained that there were two sorts of nucleic acid, deoxyribonucleic acid and ribonucleic acid.

In the early 1920s, Victor Jollos hypothesized that the disappearance of environmentally-induced acquired traits even after hundreds of generations indicated that their acquisition should be assigned to the cytoplasm rather than the nucleus.
   

In 1921, Frederick Grant Banting and Charles Herbert Best isolated insulin while working on pancreatic secretions. Banting injected it into an apparently terminally ill patient who survived. [revised 02/01/03]   
In 1921, Felix d'Hérelle discovered bacterial viruses which he named 'bacteriophage' (d'Herelle 1926).
In 1921, Muller raised the question of the relationship of genes to viruses, or 'naked genes' (Muller 1922).[revised 02/01/03]   
In 1921, Langley described the autonomic nervous system and its functions.
In 1921, Hopkins isolated gluthione.
In 1921, Theodor Kaluza , in "Zum Unitätsproblem der Physik," wrote down Einstein's field equations in five dimensions.  This reproduced the usual four-dimensional gravitational equations plus Maxwell's equations for the electromagnetic field.  In other words, according to this hypothesis, electromagnetism is not a separate force, but an aspect of gravity in a higher dimension.
In 1921, Otto Stern and Walter Gerlach demonstrated 'space quantization' "by sending a molecular beam, a tenuous stream of molecules or atoms, through a suitable magnetic field and observing their deflection....  The atoms orient themselves only in discontinuous positions" (Segrè 1976:138).  
   

In 1922, Herbert Spencer Gasser and Joseph Erlanger, working together, and Edgar Douglas Adrian found that the electric pulses within neurons caused chemicals to be released and that their rate of conduction was proportional to the thickness of their sheaths.

In 1922, Walter Garstang , in "The Theory of Recapitulation: A Critical Re-statement of the Biogenetic Law," showed that phylogeny is not the cause but the product of different ontogenies (Garstang 1922).

In 1922, Elmer Verner McCollum led a team which showed that rickets is caused by a lack of a new food factor, vitamin D.
In 1922, Arthur Holly Compton demonstrated an increase in the wavelengths of X-rays and gamma rays when they collide with loosely bound electrons.  This verified the quantum theory since the effect requires the rays be treated as particles, not waves (Compton 1923:483-502).

In 1922, Arthur Holly Compton demonstrated an increase in the wavelengths of X-rays and gamma rays when they collide with loosely bound electrons.  This verified the quantum theory since the effect requires the rays be treated as particles, not waves (Compton 1923:483-502). 
In 1922, and 1924, Aleksandr Friedman, in "Über die Krümmung des Raumes," proposed several nonstatic, realistic models of an expanding (or contracting) decelerating universe which were consistent with Einstein's General Theory of Relativity.  These models assumed that the universe was of uniform curvature and uniform matter (idealized as dust exerting neglible pressure), that the expansion was not caused by galaxies moving apart, but by space itself stretching, and predicted that the beginning and the end of time would occur for dynamical reasons.

In 1922, Edwin Powell Hubble demonstrated the nebulae which failed to generate their own light were gaseous and located within the Milky Way galaxy.
 

In 1923, Oswald Avery demonstrated that different types of pneumococci had different and specific exterior capsules and that bacteria are of distinct, heritable varieties.
In 1923, Bridges discovered chromosomal translocation in Drosophila.
In 1923, Otto Heinrich Warburg reached the conclusion that "cancer cells differ from non-cancer cells, including growing embryonic cells, by their failure to suppress glycolysis in the presence of oxygen" (Krebs 1981:20).  Today, it is realized that this only one of many  ways cancer cells differ; it is a symptom and not the primary cause.
In 1923, Robert Feulgen discovered a selective staining technique for DNA localization, which is still in use.
In 1923, Lloyd Morgan, in his book Emergent Evolution, used the word 'emergent' to show that higher orders are not mere resultants of what went before, but were qualitatively new.
In 1923, Thorsten Ludvig Thunberg characterized photosynthesis as an oxidation-reduction reaction in which carbon dioxide is reduced and water is oxidized.
In 1923, Johannes Nicolaus Brönsted published his theory of the acid-base phenomena according to which any group of atoms that gives up a proton is called an acid, etc.  In the same year, G. N. Lewis published his theory that neutralization accounts for the coordinate covalent bond between the acid and the base.  His theory incorporated Brönsted's proton theory as a special case.

In 1923, Herbert M. Evans and K. Scott Bishop discovered vitamin E (Evans and Bishop 1923). 
In 1923, Jean Piaget, in Le Langage et la pensée chez l'enfant, maintained that child development proceeds in the same sequence of genetically determined stages.
In 1923, Eddington published The Mathematical Theory of Relativity, considered by Einstein the finest presentation of the subject in any language.

In 1923, Werner Heisenberg and Max Born, using Bohr's quantum mechanics, were finally able to calculate the wavelength of the two electron helium atom. 

In 1923, Louis Victor de Broglie (rimes with feuille), in "Ondes et quanta," hypothesized that a moving electron particle has wave-like properties.  "His central contribution was formula giving the relation between the momentum p of a particle and the wavelength l of its associated wave, analogous to the earlier relation between energy and frequency, l= h/p = h/mv, where for p is substituted the usual expression for the momentum of a moving object....  Assume a circular path, and assume that the wave makes a pattern that is closed on itself....  The circumference of the circle is a whole number of wavelengths: 2pr = nl, where n = 1,2,3,....  By de Broglie's hypothesis, this 2pr = nh/mv, whence mvr = nh/2p, and this, as if by miracle, is the formula Bohr had had to guess in 1913 in order to derive his formula for the energy levels of hydrogen" (Park 1990:316). 

In 1923, Electrolux produced the first electric refrigerator

In 1923, Vladimir Zworykin invented the iconoscope television camera-tube.
   

In 1924, Aleksander Ivanovitch Oparin published his speculation that life, that is to say, metabolism + self-reproductivity (but not replication) + mutability, is preceded by the formation of mixed colloidal units, called coacervates, and is the inevitable result of chemical self-organization in a reducing environment (Oparin 1924).  A reducing atmosphere is rich in hydrogen and hydrogen-containing gases, such as methane and ammonia, all of which "donate electrons to other substances and thereby produce energized molecules.  These molecules are then able to take part in chemical reactions that can lead to the creation of more complex substances" (Darling 2001:17). 
In 1924, E. Gorter and F. Grendel demonstrated that blood cells are surrounded be a membrane exactly two molecules thick (Gorter and Grendel 1925:439). [added 02/01/03]
In 1924, John Burdon Sanderson Haldane began a series of papers in which gene frequency substitutions in a population were treated systematically.
In 1924, Alfred Lotka, in Elements of Physical Biology, compared the global eco-system to "a great world engine" in which "plants and animals act as coupled transformers of energy" in "the mill-wheel" that is driven by "solar energy" (Lotka 1924:331-335).  Lotka gave analytical substance to the  vision intuitively adopted by field biologists.  Population ecologists took the 'logistical equation' for population growth, which is in fact based on an analogy with autocatalytic chemical reactions, from his book.
In 1924, Satyendranath Bose derived Planck's black box radiation law from photon statistics, that is, independent of classical electrodynamics. Instead of photons being statistically independent, he assigned them to cells and wrote of the cell's statistical independence. 

In 1924, Bohr, Hendric Anton Kramers, and John C. Slater tried and failed to solve the apparent contradiction between waves and particles by the concept of the 'probability wave.'  This probability is different from chance: "It meant a tendency for something.  It was a version of the old concept of potentia in Aristotelian philosophy.  It introduced...a strange kind of physical reality just in the middle between possibility and reality" (Heisenberg 1958:41; Bohr et al. 1924:785-812). 

In 1924, Bose, in "Wärmegleichgewicht im Strahlungenfeld bei Anwesenheit von Materie," and Einstein, in an appendix to Bose's paper, predicted the existence of the statistical phenomena, now known as 'Bose-Einstein condensation,' whereby a significant fraction of particles at a sufficiently low temperature could occupy the same quantum state of lowest energy. In other words, in this state, the atoms--later recognized as particles of integer spin and still later called 'bosons,' in Bose's honor--would lose their individual properties and would act collectively as a single entity.  Integer spin is either zero or an even number of multiples of the basic unit of spin. 
In 1924, Eddington, in "On the Relation between the Masses and Luminosities of the Stars," correctly derived the various mass-luminosity relations of stars; e.g., for stars like the Sun, the absolute luminosity is proportional to the fourth power of the mass, but for more massive stars the absolute luminosity is proportional to the cube of the mass.

In 1924, Carl Wirtz suggested that the smaller a galaxy appeared the larger its change in color toward the red end of the spectrum.  This is called 'cosmological redshift.'  But Wirtz lacked the direct evidence to conclude that these smaller, redder galaxies were farther away.

In 1924, Hubble, using the 100-inch telescope on Mount Wilson and the same scale which Shapley had used to map the Milky Way, measured the distance to the nearer spiral galaxies, which was two million light years.  So vast was this distance that "the whole Galaxy in which we live was suddenly shrunken, in the astronomical imagination, into a tiny mote floating in a vast, dark sea of emptiness" (Gribbin 1998a:65).
   

In 1925, Raymond Arthur Dart published his discovery of a skull of a new species, Australopithecus africanus, a missing link in the human fossil record.  He speculated that "Australopithecus had been a bloodthirsty carnivore, [giving birth to] the killer ape myth..., including a connection between warfare and hunting and the concept that aggressiveness drives cultural progress" (de Waal 2001:45).
In 1925, Gilbert Adair published his determination of the correct size of the hemoglobin molecule and subsequently wrote the equation for hemoglobin's cooperativity.
In 1925, George Richard Minot and William Parry Murphy noticed that feeding raw liver aids in the treatment of 'pernicious anemia.'  Their discovery led to the isolation of vitamin B12.
In 1925, Theodor Svedberg designed the ultracentrifuge.
In 1925,  Samuel A. Goudsmit and George Eugene Uhlenbeck assigned angular momentum to electrons and established that they have the quantum mechanical property of spin.

In 1925, Heisenberg, in "Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen," in order to avoid giving definite numbers to changing positions of quanta, changed the relationship between physical concepts and mathematical symbols through his use of symbols as imaginary quantities with arrays of numbers, called matrices.  The rules for calculating these symbols often depends on the order they are written down.  This became known as 'matrix mechanics.' 
In 1925, Wolfgang Pauli perceived the principle that no two electrons in the atom can be in the same quantum state; in other words, two electrons must have opposite spin, thus cancelling each other, and there can be no more than two in the same orbital. This is known as the 'Pauli exclusion principle.'
In 1925, Enrico Fermi devised a statistical mechanics valid for particles subject to the exclusion principle, i.e., particles of half-integral spin, e.g., electrons, protons, neutrons, etc.  Such particles came to be called 'fermions' in his honor.  As Paul Adrien Maurice Dirac, independently, made the same calculation, these equations are now known as Fermi-Dirac statistics.  

In 1925, Ernst Ising published a one-dimensional model of ferromagnetism in an attempt to explore the problem of atoms in a solid, each with a magnetic moment and a spin.  This model demonstrated that the energy of the system is taken to be proportional to the amount of magnetism and that at any temperature above absolute zero there would be no spontaneous magnetism.  One- and two-dimensional models have applications for phase transitions and interfaces in semiconductor technology.

In 1925, Gustaf Ising published a proposal for a 'linear accelerator.'

In 1925, Pierre Auger discovered that the ejection of an electron without the emission of a X- or gamma-ray photon is the result of the de-excitation of an excited electron within the atom.
In 1925, Walter Noddack and Ida Eva Tacke, who married Noddack the following year, discovered the element rhenium.  

In 1925, Cecilia H. Payne, in Stellar Atmospheres, "assumed that the number of effective atoms required to make a spectral line barely visible in a stellar spectrum is the same for all lines of all elements and that the reciprocals of those fractional concentrations could be used to give the relative abundances of the elements.  The results showed that the relative abundances [except for hydrogen] are similar to those in the Earth's crust" (Land and Gingerich 1979:244).

In 1925, Bertil Lindblad said that star streaming is evidence that the entire Galaxy is differentially rotating.

In 1925, George Y. Rainich, while re-expressing the content of the Maxwell-Einstein equations in a purely geometric form, established that, "under certain assumptions, the electromagnetic field is entirely determined by the curvature of space-time" (Rainich 1925:107). 
   
 

 
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