Les Atomes

Les Atomes ("The Atoms") is a book by the French physicist Jean Perrin, published in Paris by Félix Alcan in 1913. It surveys the evidence for the existence of atoms and molecules and ends with a table of the values of the Avogadro constant that thirteen different phenomena had yielded, which Perrin took to give the existence of the molecule "a probability bordering on certainty".^[1] Perrin received the 1926 Nobel Prize in Physics "for his work on the discontinuous structure of matter, and especially for his discovery of sedimentation equilibrium".^[2] The table, and the argument around it, are described in Molecular reality (history of science).

The preface opens with two kinds of scientific thinking. One generalizes from experiments that anyone can repeat, as Galileo and Sadi Carnot did when they built up the doctrine of energy, and it makes no hypotheses. The other, in Perrin's words, explains "the complications of the visible in terms of invisible simplicity", and he credits "men such as Dalton and Boltzmann" with having given science the doctrine of atoms by that route.^[1] He declines to rank the two methods. Perrin expected a day when atoms would be "directly perceptible at last" and "as easy to observe as are microbes to-day", and adds: "The true spirit of the atomists will then be found in those who have inherited the power to divine another universal structure lying hidden behind a vaster experimental reality than ours."^[1]

The book proceeds as an accumulation. Perrin starts with the chemistry of his day and the physics of gases. He turns to the grains he could see through a microscope, and then to the blue of the sky, black-body radiation, the charge of the electron and radioactive decay, and each route leads to the same number. By the conclusion he can say that if the unknown constant is eliminated from the thirteen equations, twelve equations remain that connect only quantities that can be measured, so that visible reality can be described without mentioning the invisible ones.^[1]

The English edition has a preface, eight chapters and a conclusion, and its text is divided into 120 numbered sections. The chapters are "Chemistry and the Atomic Theory", "Molecular Agitation", "The Brownian Movement: Emulsions", "The Laws of the Brownian Movement", "Fluctuations", "Light and Quanta", "The Atom of Electricity" and "The Genesis and Destruction of Atoms".^[1]

The first chapter opens "some twenty-five centuries ago" with Greek philosophers on the shores of the Mediterranean. Its first section, "Persistence of the component substances in mixtures", turns to sugar dissolved in water and bromine dropped into chloroform, where the color and smell of each survive the mixing, and asks what would explain that if matter were continuous. The chapter then works through chemistry to "the famous hypothesis of Avogadro (1811)", and to the sentence "This number N is what is called Avogadro's Constant or Avogadro's Number".^[1]

The chapters on the Brownian movement are the center of the book, with sections titled "The preparation of a suitable emulsion", "Fractional centrifuging" and "The complete irregularity of the agitation".^[1] The chapter on fluctuations goes through critical opalescence and "The blueness of the sky", each as a visible effect of the molecular structure of a gas or a liquid.

Some sections go beyond the argument. Section 94, "The material part of an atom is concentrated entirely at its centre", reasons from the specific heat of argon, which keeps its value of 3 calories up to about 3,000 °C. A gas of atoms with their mass near the center would, Perrin argued, need so large a quantum of rotational energy to spin that the atoms would practically never rotate, which is what the specific heat shows.^[1] Section 112, "Cosmogony", suggests that heavy atoms may be remade at the centers of stars, where temperature and pressure "favours reciprocal penetration between atomic nuclei, accompanied by energy absorption".^[1]

The book ends with the astronomers: "Nature reveals the same wide grandeur in the atom and the nebula, and each new aid to knowledge shows her vaster and more diverse, more fruitful and more unexpected, and, above all, unfathomably immense."^[1]

Hammick's note records that his translation followed "the 4th Revised Edition" of the French text, and it is dated from Gresham's School, Holt, June 8, 1916. A footnote adds that the book is dedicated to the memory of Noël Bernard, and the preface says that the work is dedicated to a departed friend.^[1]

Philosophers of science have used the book's argument as a case study. The Stanford Encyclopedia of Philosophy lists seven interpretations of it, published between 1983 and 2020, and calls them "often contradictory between them".^[3] George E. Smith and Raghav Seth's Brownian Motion and Molecular Reality (2020) examines the experiments again. It argues that Perrin's values for molecular magnitudes "simply presupposed" that the kinetic energies of his grains matched those of the surrounding molecules, and that other methods were giving different values between 1908 and 1913, but it also calls his measurements exemplars of "theory-mediated measurement".^[4]

• Jean Perrin
• Molecular Reality (book)
• Brownian motion
• Avogadro constant
• Ludwig Boltzmann
• Ernst Mach