Jean Baptiste Perrin

French physicist Jean Baptiste Perrin (1870-1942) helped to prove that atoms and molecules exist, an achievement that earned him the 1926 Nobel Prize in physics.

Jean Baptiste Perrin was born in Lille, France, on September 30, 1870, and raised, along with two sisters, by his widowed mother. His father, an army officer, died of wounds he received during the Franco-Prussian War. The young Perrin attended local schools and graduated from the Lycée Janson-de-Sailly in Paris. After serving a year of compulsory military service, he entered the Ecole Normale Supérieure in 1891, where his interest in physics flowered and he made his first major discovery.

Between 1894 and 1897 Perrin was an assistant in physics at the Ecole Normale, during which time he studied cathode rays and X rays, the basis of his doctoral dissertation. At this time, scientists disagreed over the nature of cathode rays emitted by the negative electrode (cathode) in a vacuum tube during an electric discharge. Physicists disagreed among themselves over whether cathode rays were particles—a logical assumption, since they carried a charge—or whether they took the form of waves.

In 1895 Perrin settled the debate simply and decisively using a cathode-ray discharge tube attached to a larger, empty vessel. When the discharge tube generated cathode rays, the rays passed through a narrow opening into the vessel, and produced fluorescence on the opposite wall. Nearby, an electrometer, which measures voltage, detected a small negative charge. But when Perrin deflected the cathode rays with a magnetic field so they fell on the nearby electrometer, the electrometer recorded a much larger negative charge. This demonstration was enough to prove conclusively that cathode rays carried negative charges and were particles, rather than waves. This work laid the basis of later work by physicist J. J. Thomson, who used Perrin's apparatus to characterize the negatively charged particles, called electrons, which were later theorized to be parts of atoms.

In 1897 Perrin married Henriette Duportal, with whom he had a son and a daughter. He received his doctorate the same year, and began teaching a new course in physical chemistry at the University of Paris (the Sorbonne). He was given a chair in physical chemistry in 1910 and remained at the school until 1940. During his early years at the University of Paris, Perrin continued his study of the atomic theory, which held that elements are made up of particles called atoms, and that chemical compounds are made up of molecules, larger particles consisting of two or more atoms. Although the atomic theory was widely accepted by scientists by the end of the nineteenth century, some physicists insisted that atoms and molecules did not actually exist as physical entities, but rather represented mathematical concepts useful for calculating the results of chemical reactions. To them, matter was continuous, not made up of discrete particles. Thus, at the dawn of the twentieth century, proving that matter was discontinuous (atomic in nature) was one of the great challenges left in physics. Perrin stood on the side of the "atomists, " who believed that these tiny entities existed. In 1901 he even ventured (with no proof) that atoms resembled miniature solar systems. His interest in atomic theory led him to study a variety of related topics, such as osmosis, ion transport, and crystallization. However, it was colloids that led him to study Brownian motion, the basis of his Nobel Prize-winning discovery of the atomic nature of matter.

In 1827 the English botanist Robert Brown reported that pollen grains suspended in water were in violent and irregular motion, a phenomenon at first ascribed to differences in temperature within the fluid. Before the end of the century, however, scientists generally accepted the notion that the motion might be caused by bombardment of the pollen grains by molecules of the liquid—an apparent triumph for atomic theory. Yet some scientists remained skeptical.

In 1905 Albert Einstein calculated the mathematical basis of Brownian motion, basing his work on the assumption that the motion was due to the action of water molecules bombarding the grains. But Einstein's work, though elegant, lacked laboratory experiments needed to demonstrate the reality of his conclusions. It fell to Perrin to bolster Einstein's calculations with observations. From 1908 to 1913, Perrin, at first unaware of Einstein's published paper on the subject, devoted himself to the extremely tedious but necessary experiments—experiments now considered classics of their kind. He hypothesized that if Brownian movement did result from molecular collisions, the average movements of particles in suspension were related to their size, density, and the conditions of the fluid (e.g., pressure and density), in accordance with the gas laws. Perrin began by assuming that both pollen grains and the molecules of the liquid in which they were suspended behave like gas molecules, despite the much greater size of the grains.

According to Einstein's equations governing Brownian motion, the way the particles maintained their position in suspension against the force of gravity depended partly on the size of the water molecules. In 1908 Perrin began his painstaking observations of suspensions to determine the approximate size of the water molecules by observing suspensions of particles. He spent several months isolating nearly uniform, 0.1-gram pieces of gamboge—tiny, dense extracts of gum resin, which he suspended in liquid. According to Einstein's molecular theory, not all particles will sink to the bottom of a suspension. The upward momentum that some particles achieve by being bombarded by molecules of the fluid will oppose the downward force of gravity. At equilibrium, the point at which the reactions balance each other out, the concentrations of particles at different heights will remain unchanged.

Perrin devised an ingenious system to make thousands of observations of just such a system. He counted gamboge particles at various depths in a single drop of liquid only one twelve-hundredth of a millimeter deep. The particle concentration decreased exponentially with height in such close agreement with the mathematical predictions of Einstein's theory that his observations helped to prove that molecules existed.

In essence, his system behaved like the Earth's atmosphere, which becomes increasingly rarified with height, until, at the top of a very tall mountain, people may find it difficult to breathe. Furthermore, it was already known that a change in altitude of five kilometers is required to halve the concentration of oxygen molecules in the atmosphere, and that the oxygen atom has a mass of sixteen. Based on his knowledge of the gas laws, Perrin realized that if, in his tiny system, the height required to halve the concentration of particles was a billion times less than the height it took to halve the concentration of oxygen in the atmosphere, he could, by simple proportion, calculate the mass of a gamboge particle relative to the oxygen molecule.

Einstein had linked to Brownian motion the concept of Avogadro's number, the number of molecules in any gas at normal temperature and pressure, now known to be 6.023 x 1023. According to Avogadro's hypothesis, equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. Furthermore, the total mass of a specific volume of gas is equal to the mass of all the individual molecules multiplied by the total number of these individual molecules. So a gram-molecule of all gases at the same temperature and pressure should contain the same number of molecules. (A gram-molecule, or mole, is a quantity whose mass in grams equals the molecular weight of the substance; for example, one gram-molecule of oxygen equals sixteen grams of oxygen.) Only if this were true would the concept that each individual molecule contributes a minute bit of pressure to the overall pressure hold true, and individual entities called molecules could be said to exist.

Perrin calculated the gram-molecular weight of the 0.1-gram particles in the equilibrium system and therefore knew the number of grams in a gram-molecule of the particles. Then he divided the gram-molecular weight by the mass in grams of a single particle. The result, 6.8 x 10 23, was extremely close to Avogadro's number. Thus, Perrin had demonstrated that uniform particles in suspension behave like gas molecules, and calculations based on their mass can even be used to calculate Avogadro's number. This demonstrated that Brownian motion is indeed due to bombardment of particles by molecules, and came as close as was possible at the time to detecting atoms without actually seeing them. "In brief, " Perrin said during his Nobel Prize acceptance speech, "if molecules and atoms do exist, their relative weights are known to us, and their absolute weights would be known as soon as Avogadro's number is known."

Perrin's work ranged farther afield than equilibrium distribution of particles and Avogadro's number, however. As an officer in the engineering corps of the French army during World War I, he contributed his expertise to the development of acoustic detection of submarines. His commitment to science, however, did not inhibit his social graces. He was a popular figure who took a genuine interest in young people, and held weekly parties for discussion groups in his laboratory. Following the war, Perrin's reputation continued to grow. In 1925, he became one of the first scientists to use an electric generator capable of producing a continuous current of 500, 000 volts. At the time, he predicted that someday much larger machines of this type would let physicists bombard atoms, and thus make important discoveries about the structure of these particles.

In 1929 after being appointed director of the newly founded Rothschild Institute for Research in Biophysics, he was invited to the United States as a distinguished guest at the opening of Princeton University's new chemical laboratory. In 1936 Perrin replaced Nobel laureate Irene Joliot-Curie as French undersecretary of state for scientific research in the government of Premier Léon Blum. The following year, as president of the French Academy of Science, he assumed the chair of the scientific section of an exhibit in the Grand Palais at the 1937 Paris exposition. The project enabled him to help the average person, including children, to appreciate the wonders of science, from astronomy to zoology.

His flourishing reputation was further enhanced in 1938 when he informed the French Academy of Science, of which he had been a member since 1923, and was then president, that his collaborators had discovered the ninety-third chemical element, neptunium, a substance heavier than uranium. Four years earlier, Enrico Fermi (who was awarded the 1938 Nobel Prize in physics and directed the first controlled nuclear chain reaction) had artificially created Neptunium, a so-called transuranium element, by bombarding uranium (element 92) with neutrons. Perrin's announcement that Neptunium existed in nature excited speculation among physicists that there also might exist even more undiscovered elements, which turned out to be the case.

His blossoming career did not shield the French physicist from concerns over what he considered to be a steady encroachment by totalitarian governments around the world on the freedom of science to express itself. A socialist and outspoken opponent of fascism, Perrin expressed his concerns during a speech delivered at the Royal Opera House in London before the International Peace Conference, reported in the New York Times. He asserted that world science stands or falls with democracy, and decried the fact that scientists seemed unable to understand "how financiers and capitalists as a whole cannot see that it is to their interest not to support those powers which, if they are successful, will ruin them." Perrin also criticized what he called "an irrational world that made it difficult to extend higher education or grant more aid to science but relatively easy to raise money for costly armaments." He voiced concern over what he believed was the coming war—World War II—which he feared would cost millions of lives, as well as threaten "the democracy that is the spirit of science." Perrin also warned that the victory of totalitarianism would mean "perhaps a thousand years of ruthless subjugation and standardization of thought, which will destroy the freedom of scientific research and theorizing."

Perrin's fears were realized in September of 1939, when France joined Great Britain in entering World War II against Germany following that country's invasion of Poland. By the end of September, the French government appointed Perrin president of a committee for scientific research to help the war effort. The situation became particularly grim in the summer of 1940, when German troops swept into Paris. Perrin fled the city and took up residence in Lyon as a refugee. In December 1941 he moved to the United States, where he lived with his son, Francis Perrin, a visiting professor of physics and mathematics at Columbia University. While in the United States, Perrin sought American support for the French war effort and helped to establish the French University of New York.

Perrin spoke out against the German occupation and French collaboration with the enemy. He was particularly disturbed when the Germans began operating an armaments industry in the suburbs of Paris using forced labor. Following Allied aerial bombardment of the factories, the New York Times reported that Perrin defended the action as "one of the sad necessities" of the war. Speaking before five hundred guests at the first dinner of the French American Club in New York City, in March 1942, Perrin asked, "Who does not understand that it was imperative to put an end to this?" A few weeks later, Perrin took ill, and ten days later he died at the age of seventy-one at Mount Sinai Hospital in New York.

Three years after the defeat of Germany and the end of the war, diplomats and scientists in New York paid homage to Jean Perrin at ceremonies held at the Universal Funeral Chapel. Afterwards, Perrin's ashes were placed aboard the training cruiser Jean d'Arc at Montreal, on which they were transported to France for burial at the Pantheon, a magnificent former eighteenth-century church converted to civic use. Among his many honors in addition to the Nobel Prize, Perrin received the Joule Prize of the Royal Society of London in 1896 and the La Caze Prize of the French Academy of Sciences in 1914. In addition, he held honorary degrees from the universities of Brussels, Liège, Ghent, Calcutta, and Manchester and from New York, Princeton, and Oxford universities. Perrin was also a member of the Royal Society of London and scientific academies in Italy, Czechoslovakia, Belgium, Sweden, Romania, and China.

Further Reading on Jean Baptiste Perrin

New York Times, March 27, 1938, p. 7; August 3, 1938, p. 21; March 10, 1942, p. 7; April 18, 1942, p. 15.

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