The 100 Most Influential Scientists of All Time (26 page)

In addition to his electromagnetic theory, Maxwell made major contributions to other areas of physics. While still in his 20s, he demonstrated his mastery of classical physics by writing a prizewinning essay on Saturn's rings, in which he concluded that the rings must consist of masses of matter not mutually coherent—a conclusion that was corroborated more than 100 years later by the first Voyager space probe to reach Saturn.

The Maxwell relations of equality between different partial derivatives of thermodynamic functions are included in every standard textbook on thermodynamics. Though Maxwell did not originate the modern kinetic theory of gases, he was the first to apply the methods of probability and statistics in describing the properties of an assembly of molecules. Thus he was able to demonstrate that the velocities of molecules in a gas, previously assumed to be equal, must follow a statistical distribution (known subsequently as the Maxwell-Boltzmann distribution law). In later papers Maxwell investigated the
transport properties of gases—i.e., the effect of changes in temperature and pressure on viscosity, thermal conductivity, and diffusion.

Maxwell was far from being an abstruse theoretician. He was skillful in the design of experimental apparatus, as was shown early in his career during his investigations of colour vision. He devised a colour top with adjustable sectors of tinted paper to test the three-colour hypothesis of Thomas Young and later invented a colour box that made it possible to conduct experiments with spectral colours rather than pigments. His investigations of the colour theory led him to conclude that a colour photograph could be produced by photographing through filters of the three primary colours and then recombining the images. He demonstrated his supposition in a lecture to the Royal Institution of Great Britain in 1861 by projecting through filters a colour photograph of a tartan ribbon that had been taken by this method.

(b. Jan. 27 [Feb. 8, New Style], 1834, Tobolsk, Siberia, Russian Empire—d. Jan. 20 [Feb. 2], 1907, St. Petersburg, Russia)

R
ussian chemist Dmitry Ivanovich Mendeleyev developed the periodic classification of the elements. Mendeleyev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. In his version of the periodic table of 1871, he left gaps in places where he believed unknown elements would find their place. He even predicted the likely properties of three of the potential elements. The subsequent proof of many of his predictions within his lifetime brought fame to Mendeleyev as the founder of the periodic law.

Mendeleyev published a textbook on organic chemistry in 1861 that had been awarded the prestigious Demidov Prize. He then set out to write another one. The result was
Osnovy Khimii
(1868–71;
The Principles of Chemistry
), which became a classic, running through many editions and many translations. When Mendeleyev began to compose the chapter on the halogen elements (chlorine and its analogs) at the end of the first volume, he compared the properties of this group of elements to those of the group of alkali metals such as sodium. Within these two groups of dissimilar elements, he discovered similarities in the progression of atomic weights, and he wondered if other groups of elements exhibited similar properties.

After studying the alkaline earths, Mendeleyev established that the order of atomic weights could be used not only to arrange the elements within each group but also to arrange the groups themselves. Thus, in his effort to make sense of the extensive knowledge that already existed of the chemical and physical properties of the chemical elements and their compounds, Mendeleyev discovered the periodic law.

His newly formulated law was announced before the Russian Chemical Society in March 1869 with the statement “elements arranged according to the value of their atomic weights present a clear periodicity of properties.” Mendeleyev's law allowed him to build up a systematic table of all the 70 elements then known. He had such faith in the validity of the periodic law that he proposed changes to the generally accepted values for the atomic weight of a few elements and predicted the locations within the table of unknown elements together with their properties. At first the periodic system did not raise interest among chemists. However, with the discovery of the predicted elements,
notably gallium in 1875, scandium in 1879, and germanium in 1886, it began to win wide acceptance. Gradually the periodic law and table became the framework for a great part of chemical theory. By the time Mendeleyev died in 1907, he enjoyed international recognition and had received distinctions and awards from many countries.

Since Mendeleyev is best known today as the discoverer of the periodic law, his chemical career is often viewed as a long process of maturation of his main discovery. Indeed, in the three decades following his discovery, Mendeleyev himself offered many recollections suggesting that there had been a remarkable continuity in his career, from his early dissertations on isomorphism and specific volumes, which involved the study of the relations between various properties of chemical substances, to the periodic law itself. In this account, Mendeleyev mentioned the Karlsruhe congress as the major event that led him to the discovery of the relations between atomic weights and chemical properties.

In the field of chemical science, Mendeleyev made various contributions; for example, in the field of physical chemistry, he conducted a broad research program throughout his career that focused on gases and liquids. In 1860, while working in Heidelberg, he defined the “absolute point of ebullition” (the point at which a gas in a container will condense to a liquid solely by the application of pressure). In 1864 he formulated a theory (subsequently discredited) that solutions are chemical combinations in fixed proportions. In 1871, as he published the final volume of the first edition of his
Principles of Chemistry
, he was investigating the elasticity of gases and gave a formula for their deviation from Boyle's law (now also known as the Boyle-Mariotte law, the principle that
the volume of a gas varies inversely with its pressure). In the 1880s he studied the thermal expansion of liquids.

A second major feature of Mendeleyev's scientific work is his theoretical inclinations. From the beginning of his career, he continually sought to shape a broad theoretical scheme in the tradition of natural philosophy. This effort can be seen in his early adoption of the type theory of the French chemist Charles Gerhardt and in his rejection of electrochemical dualism as suggested by the great Swedish chemist Jöns Jacob Berzelius. Before and during Mendeleyev's time, many attempts at classifying the elements were based on the hypothesis of the English chemist William Prout that all elements derived from a unique primary matter. Mendeleyev insisted that elements were true individuals, and he fought against those who, like the British scientist William Crookes, used his periodic system in support of Prout's hypothesis.

With the discovery of electrons and radioactivity in the 1890s, Mendeleyev perceived a threat to his theory of the individuality of elements. In
Popytka khimicheskogo ponimania mirovogo efira
(1902;
An Attempt Towards a Chemical Conception of the Ether
), he explained these phenomena as movements of ether around heavy atoms, and he tried to classify ether as a chemical element above the group of inert gases (or noble gases). This bold (and ultimately discredited) hypothesis was part of Mendeleyev's project of extending Newton's mechanics to chemistry in an attempt to unify the natural sciences.

(b. Sept. 14 [Sept. 26, New Style], 1849, Ryazan, Russia—d. Feb. 27, 1936, Leningrad [now St. Petersburg])

R
ussian physiologist Ivan Petrovich Pavlov was known chiefly for his development of the concept of the
conditioned reflex. In a now-classic experiment, he trained a hungry dog to salivate at the sound of a bell, which was previously associated with the sight of food. He developed a similar conceptual approach, emphasizing the importance of conditioning, in his pioneering studies relating human behaviour to the nervous system. He was awarded the Nobel Prize for Physiology or Medicine in 1904 for his work on digestive secretions.

During the years 1890–1900 especially, and to a lesser extent until about 1930, Pavlov studied the secretory activity of digestion. He devised an operation to prepare a miniature stomach, or pouch; he isolated the stomach from ingested foods, while preserving its vagal nerve supply. The surgical procedure enabled him to study the gastrointestinal secretions in a normal animal over its life span. This work culminated in his book
Lectures on the Work of the Digestive Glands
in 1897.

By observing irregularities of secretions in normal unanesthetized animals, Pavlov was led to formulate the laws of the conditioned reflex, a subject that occupied his attention from about 1898 until 1930. He used the salivary secretion as a quantitative measure of the psychical, or subjective, activity of the animal, in order to emphasize the advantage of objective, physiological measures of mental phenomena and higher nervous activity. He sought analogies between the conditional (commonly though incorrectly translated as “conditioned”) reflex and the spinal reflex.

According to the physiologist Sir Charles Sherrington, the spinal reflex is composed of integrated actions of the nervous system involving such complex components as the excitation and inhibition of many nerves, induction
(
i.e
., the increase or decrease of inhibition brought on by previous excitation), and the irradiation of nerve impulses to many nerve centres. To these components, Pavlov added cortical and subcortical influences, the mosaic action of the brain, the effect of sleep on the spread of inhibition, and the origin of neurotic disturbances principally through a collision, or conflict, between cortical excitation and inhibition.

Beginning about 1930, Pavlov tried to apply his laws to the explanation of human psychoses. He assumed that the excessive inhibition characteristic of a psychotic person was a protective mechanism—shutting out the external world—in that it excluded injurious stimuli that had previously caused extreme excitation. In Russia this idea became the basis for treating psychiatric patients in quiet and nonstimulating external surroundings. During this period Pavlov announced the important principle of the language function in the human as based on long chains of conditioned reflexes involving words. The function of language involves not only words, he held, but an elaboration of generalizations not possible in animals lower than the human.

Pavlov was able to formulate the idea of the conditioned reflex because of his ability to reduce a complex situation to the simple terms of an experiment. Recognizing that in so doing he omitted the subjective component, he insisted that it was not possible to deal with mental phenomena scientifically except by reducing them to measurable physiological quantities.

Although Pavlov's work laid the basis for the scientific analysis of behaviour, and notwithstanding his stature as a scientist and physiologist, his work was subject to certain
limitations. Philosophically, while recognizing the preeminence of the subjective and its independence of scientific methods, he did not, in his enthusiasm for science, clarify or define this separation. Clinically, he accepted uncritically psychiatric views concerning schizophrenia and paranoia, and he adopted such neural concepts as induction and irradiation as valid for higher mental activity. Many psychiatrists now consider his explanations too limited.

(b. Dec. 19, 1852, Strelno, Prussia [now Strzelno, Pol.]—d. May 9, 1931, Pasadena, Calif., U.S.)

G
erman-born American physicist Albert Abraham Michelson established the speed of light as a fundamental constant and pursued other spectroscopic and metrological investigations. He received the 1907 Nobel Prize for Physics.

Michelson came to the United States with his parents when he was two years old. From New York City, the family made its way to Virginia City, Nev., and San Francisco, where the elder Michelson prospered as a merchant. At 17, Michelson entered the United States Naval Academy at Annapolis, Md., where he did well in science but was rather below average in seamanship. He graduated in 1873, then served as science instructor at the academy from 1875 until 1879.

In 1878 Michelson began work on what was to be the passion of his life, the accurate measurement of the speed of light. He was able to obtain useful values with homemade apparatuses. Feeling the need to study optics before he could be qualified to make real progress, he traveled to Europe in 1880 and spent two years in Berlin, Heidelberg, and Paris, resigning from the U.S. Navy in 1881. Upon his return to the United States, he determined the velocity of
light to be 299,853 kilometres (186,329 miles) per second, a value that remained the best for a generation, until Michelson bettered it.

While in Europe, Michelson began constructing an interferometer, a device designed to split a beam of light in two, send the parts along perpendicular paths, then bring them back together. If the light waves had, in the interim, fallen out of step, interference fringes of alternating light and dark bands would be obtained. From the width and number of those fringes, unprecedentedly delicate measurements could be made, comparing the velocity of light rays traveling at right angles to each other.