Jean Perrin

Jean Baptiste Perrin (September 30, 1870 to April 17, 1942) was a French physicist and physical chemist. In 1895 he showed that cathode rays carry negative charge. From 1908 he measured how a cloud of microscopic resin grains thins out with height in a column of water, and how single grains wander, and used both measurements to count the molecules in a mole. His figures agreed with each other and with values from unrelated phenomena, and they ended a dispute over whether atoms and molecules are real that had run for most of the nineteenth century. He received the 1926 Nobel Prize in Physics "for his work on the discontinuous structure of matter, and especially for his discovery of sedimentation equilibrium".^[1]

He wrote the work up for general readers as Les Atomes (1913). His memoir of 1909 has a section called "Molecular reality", and the phrase is in the title of its English translation; the argument is described in Molecular reality (history of science). Perrin was also an organizer of science. Through the 1930s he lobbied for public money for research, in 1936 he became France's under-secretary of state for scientific research, and he is counted among the founders of the Centre national de la recherche scientifique (CNRS) and of the Palais de la Découverte, a museum where the experiments were performed in front of the visitors.^[2][3]

Perrin entered the École Normale Supérieure in 1891.^[4] In December 1895 he told the Académie des sciences that cathode rays directed into a hollow metal cylinder charged it with negative electricity, and that when a magnet bent the beam away from the opening the charging stopped, and resumed when the magnet was switched off.^[5] Whether the rays were particles or waves was still disputed. J. J. Thomson's measurement of the ratio of their charge to their mass, which identified the electron, followed in 1897.

Perrin took his doctorate in 1897 and was put in charge of teaching physical chemistry at the Sorbonne. The course became a chair in 1910, and he held it until his retirement in 1940.^[2] In 1901 he proposed in the Revue scientifique that an atom might resemble a planetary system. Louis de Broglie observed in 1962 that experiments by Ernest Rutherford and calculations by Niels Bohr confirmed the model about ten years later. De Broglie added that in 1919 Perrin had argued that the fusion of hydrogen into helium could supply the heat of the Sun.^[2]

From 1918 to 1936 his laboratory also worked on fluorescence and photochemical reactions.^[2] Perrin suggested that a molecule excited by light could pass its excitation to a neighbor by resonance, without emitting a photon, and Theodor Förster worked the idea into a quantitative theory in 1948.^[6] The diagram of a molecule's energy levels and the transitions between them that fluorescence textbooks print is named for Aleksander Jabłoński and, in many of them, for Perrin as well.^[7]

Einstein and Marian Smoluchowski published formulas for the Brownian motion of a particle in 1905 and 1906. Perrin heard of them through Paul Langevin, and he saw that they could not be tested without grains of equal and known size, so he began by making some from gamboge and mastic resin. He then followed two lines. In one, a drop of emulsion settled in a sealed cell and the grains were counted at successive heights, as if they were the molecules of a very heavy gas in a very shallow atmosphere. In the other, single grains were tracked under the microscope.^[8] The numbers are in the article on molecular reality. In one of his series the grain count fell by about half every 30 micrometers of height, where the density of air halves in roughly 5.5 kilometers, so the grains formed an atmosphere roughly 180 million times shallower than ours. Comparing the two heights gave the weight of a grain as a multiple of the weight of an air molecule, and the grain could be weighed. The distribution of the grains gave a value for Avogadro's constant, their displacements gave another, and both agreed with the value from the kinetic theory of gases. Wilhelm Ostwald, who had built a chemistry without atoms, signed a preface in November 1908 conceding that experimental evidence for them now existed.^[9]

The full memoir appeared in the Annales de chimie et de physique in 1909, and Frederick Soddy's English translation (1910) is titled Brownian Movement and Molecular Reality.^[10]

Les Atomes grew out of four papers of 1908 into a book for readers outside physics.^[4] The French publisher was Félix Alcan in Paris. D. Ll. Hammick's translation, Atoms, appeared from Constable in 1916 and is the usual source of English quotations.^[8] Its last chapter set out thirteen phenomena that each gave a value for the number of molecules in a mole, from the viscosity of gases to the blue of the sky to the helium produced by radium. De Broglie wrote that the book convinced the most scrupulous thinkers, made the atomic theory known to a large public, and brought many young people into research.^[2]

The preface contains an argument that has little to do with atoms. Perrin asked the reader to consider one of the white flakes that form when a soap solution is salted. From a distance its outline looks sharp. Closer up, he wrote, "the eye no longer succeeds in drawing a tangent at any point on it", and a magnifying glass or microscope only brings new irregularities into view, so that the flake never presents the smooth outline of a steel ball. If the ball illustrates classical continuity, he proposed, the flake illustrates a continuous function that has no derivative. A map of the coast of Brittany has a tangent everywhere, because a map is drawn that way, but the shore itself, looked at from various distances, behaves like the flake. So does the path of a Brownian grain. Measure the direction from one position to the next over shorter and shorter intervals, and the direction changes without pattern.^[8]

His conclusion was that the mathematicians' curves without tangents, which most physicists took for a game, were closer to nature than the smooth curves that physicists drew. The word fractal was coined by Benoît Mandelbrot in 1975.

Perrin's career in science policy began with petitions. The Chamber of Deputies created a Caisse nationale des sciences in 1930 to help aged scientists, with a budget of 500,000 francs. At Perrin's request the prime minister, Édouard Herriot, had five million francs moved into it from the defense budget. A Conseil supérieur de la recherche followed in 1933 and a Caisse nationale de la recherche scientifique in 1935.^[2]

In June 1936 the government of Léon Blum created the first under-secretaryship for scientific research. Irène Joliot-Curie held it until October, when she resigned, and Perrin succeeded her.^[2] Jean Zay, the minister of national education, later recalled Perrin in ministerial committees: his head would sink and his eyes close, yet he slept so little that he would speak up at the decisive moment, having followed everything that was said.^[2] The CNRS itself was founded by a decree that President Albert Lebrun signed on October 19, 1939, weeks after France entered the war, and under the decree its first mission was the country's "scientific mobilisation".^[11]

The Palais de la Découverte took shape in 1937, during the Paris International Exposition, which opened on May 24. Perrin did not want a museum of collections. In his preface to the exhibition catalogue he wrote that the aim was to familiarize visitors with the fundamental research by which science was made, "en répétant journellement les grandes expériences" (by repeating the great experiments every day), without lowering their level.^[a] Demonstrators performed the experiments in front of the public, and science was presented as a process of discovery, not as a stock of results. On July 21, 1937 President Lebrun visited and listened as Perrin explained an apparatus that demonstrated centrifugal force.^[3]

Perrin left France after the defeat of 1940 and spent his last years in the United States. He died in New York on April 17, 1942, aged 71.^[4] His ashes were returned to Paris after the war and placed in the Panthéon in 1948. On May 4, 1962, twenty years after his death, the Académie des sciences held a national tribute to him there.^[2] He had married Henriette Duportal in 1897, and their son Francis Perrin also became a physicist.

Perrin predicted in the preface to Les Atomes that "a time may perhaps come when atoms, directly perceptible at last, will be as easy to observe as are microbes to-day."^[8] His phrase "molecular reality" is the title of a 1972 history of his work by Mary Jo Nye, and since 2021 it has been part of the name of Molecular Reality Corporation.^[12][b] The company streams the ionic current through a 200-micron laser-drilled pore to a web page where anyone can watch it.^[13]

• Molecular reality (history of science)
• Les Atomes
• Brownian motion
• Avogadro constant
• Ernst Mach
• Ludwig Boltzmann
• James Clerk Maxwell
• Live molecular stream