JEAN BAPTISTE PERRIN

1870-1942

Jean P errin , who died in New York on the 17 April 1942, had been Professor of Physical Chemistry at the University of Paris for about thirty years and during that time he made valuable contributions to the knowledge of atomic physics.

He was elected a member of the French Academy of Science in 1923 and was President of the Academy in 1938. In that year also he was President of the French Government Department of Scientific Research.

His early experiments on cathode rays were made when the attention of physicists was drawn to the phenomena connected with discharge tubes by the discoveries of Rontgen, but the work for which he is best known is the remark­able series of investigations of the motion of small particles suspended in a liquid, known as the Brownian motion, for which he. was awarded the Nobel prize in 1926. He wrote a book on physical chemistry atomes, and two on general physics, Les elements de la physique and Grains de matiere et de lumiere. He also contributed to the general knowledge of science by encouraging research and took an active part in extending the opportunities for students to undertake scientific and industrial research.

His experiments on cathode rays were made in 1896. At that time the theory that the rays consisted of negatively charged particles was not universally accepted. It was given to explain the effect of a magnetic force on the curvature of the path of the rays, but the experiments which had been made to determine the charge were inconclusive and some physicists maintained that the rays were a form of motion of the ether. The experiments made by Hertz and Lenard which showed that the rays passed through thin sheets of glass or of aluminium were interpreted as showing that the rays were of the nature of light and were not material particles. In his investigations Perrin arranged a discharge tube so that the rays were received in a long hollow cylinder which was screened by a larger co-axial cylinder from external charges. With this arrangement the electrons set free by the impacts of the rays from its inner surface did not escape from the smaller cylinder and it became negatively charged when the cathode rays entered it.

He also made experiments which showed that cathode rays may be retarded or accelerated or deviated by an electric field. When the rays passed through a gauze and impinged on a plate covered with a fluorescent powder the fluorescence was diminished by charging the plate to a negative potential, and for a certain difference of potential V between the plate and the gauze the fluorescence was extinguished. If e be the charge on an electron the kinetic energy mz>2/2 of the electrons passing through the gauze is eV.

302 Obituary NoticesThese experiments together with those on the curvature of the rays in a

magnetic field showed conclusively that cathode rays were streams of negatively charged particles which are set free from the negative electrode of a discharge tube.

Perrin’s best known work is the very complete series of experiments he made on the motions and on the distribution of small particles in a liquid, from which he deduced the number of molecules in a cubic centimetre of a gas at normal pressure and temperature. The theory by which this number was calculated is that of the interdiffusion of gases given by Maxwell. It was assumed that this theory also applies to the diffusion of particles in a liquid of masses much larger than that of a molecule, and that the mean energy of agitation of the particles is the same as that of a molecule of a gas at the same temperature as the liquid. Perrin deduced the mean energy of the particles by two methods. Both methods depend on the determination of the difference of the mass m of a particle and the mass m! of the displaced water. The difference ( —m') was found directly from measurements of the specific gravity of a solution containing a known number of particles.

When the water containing the particles (called an emulsion) is placed in a shallow vessel, the particles move under the action of gravity towards the bottom of the vessel. They do not rest on the bottom but rebound from the surface, and a steady state is reached in which the mean velocity in the vertical direction is zero, and the number of particles n per cubic centimetre at a distance z from the bottom remains constant. Under these conditions the differential equation for n as given by Maxwell’s equations becomes —m) pbeing the partial pressure mnv2/3 of the particles. Thus, if and be the num­bers per cubic centimetre at distances zand from the bottom of the vessel the equation for the ratio ttjn2 becomes The ratio of the numbers nx and n2 in horizontal layers at two different heights was measured by a microscope and it was found that log(w1/w2) was proportional to the difference °f the heights. The difference of theheights of the layers at which n2 is one half was of the order of a few thousandths of a millimetre. Having thus determined the mean energy of a molecule, the number N per cubic centimetre of a gas at pressure tz is given by the equation Tz=mN-v2/3.

The emulsion formed a sort of miniature atmosphere in which the change in the density with the height is of the same form as the change in the density of a molecular atmosphere. In air the density is reduced to one-half at a height of about 6 kilometres so that the ratio of the mass of a particle in an emulsion to that of a molecule of air is of the order of the ratio of 6 x 103 metres to 10'3 millimetre.

Perrin also made measurements of the rate of diffusion K of particles suspended in a liquid and the mean velocity W of particles moving in a vertical tube under the action of gravity. From these experiments the mean energy is found by the equation W jK—3(m—m')glmv2.

The experiments were made under widely different conditions. The size

of the particles was changed, also the liquid in which they were suspended. In a mixture of glycerine and water the mass m of the particle was less than the mass m' of the displaced liquid so that the particles tended to rise to the surface and in the steady state the number n diminished with the distance z below the surface.

In all these different circumstances the number N was found to be nearly the same. The actual results gave numbers between the limits 2’9 x l0 19 and 3 '2 x l0 19 for the number of molecules per c.c. of a gas at 760 mm. pressure and 0° C. Thus the mean value of the number of molecules per cubic centimetre of a gas at 760 mm. pressure and 15° C. deduced from Perrin’s experiments is 2 '9 x 1019. This number is greater than the number deduced from the measure­ment of the atomic charge, but the agreement is as close as can be expected taking into consideration the probable experimental errors and the uncertainties in the simplifying hypotheses involved in the calculations.

The method of deducing N from the atomic charge depends on the results of experiments on the mean velocity w of ions in the direction of an electric force Z and the rate of diffusion k of the ions in a gas. It follows from Maxwell’s general equations that the ratio w/k is NeZjn where e is the charge on an ion and tcthe pressure mNv2/3 of a gas containing molecules per cubic centimetre. The quantity of electricity Newhere N is the number of molecules in a gas at 760 mm. pressure and 0° C. was thus found to be 1‘23 X1010 and taking the atomic charge e to be 4‘8 x l0 " 10 E.S. units as determined by Millikan, the number N is found to be 2* 56 X1019. This estimate of the number N is generally supposed to be more exact than the number deduced by Perrin. But that does not detract from the interest and importance of his experiments which show that there is a remarkable resemblance between the motion of small particles suspended in a liquid and the description of the motion of molecules in Maxwell’s kinetic theory of gases, also that the mean energy of agitation of the molecules is the same as the mean energy of agitation of particles of masses millions of times greater than the mass of a molecule.1

A full account of this work is given in his book on the properties of matter, Les atomes. In another book Grains de matiere et de , published in 1935,he gives an account of modern theories of the constitution of atoms and the corpuscular theory of radiation. The last paragraph of this book, which is as follows, gives an idea as to its contents and refers to the assistance he received from his son—

‘J’ai tente, aux pages precedentes, d’esquisser le progres, sans cesse plus rapide, des recherches tendues vers l’essence des choses. Dans la mesure oil j ’ai pu reussir, j ’ai une grand joie a dire l’aide precieuse que j ’ai trouvee dans mes longues conversations avec mon fils Francis Perrin.’

His son, Francis Perrin, is a distinguished mathematician and physicist. He was a professor of mathematics at the Sorbonne and is now visiting Professor of Mathematics and Physics at the •Columbia University.

1 The experiments were first published in Annales de physique, Paris (1914). An English translation by F. Soddy has been published by Taylor & Francis.

Jean Baptiste Perrin 303

304 Obituary NoticesPerrin’s interests were not confined to extending purely scientific knowledge.

He* realized the importance of applied science, and as adviser to the French Government during the years 1914-1918 he kept in touch with physicists engaged in war work, and was at the centre of the remarkable activity in Paris where discoveries that have become of such general importance were being made. It was at that time that the French three-electrode valve was brought into general use for wireless telegraphy and telephony at the establishment directed by General Ferrie, and the supersonic method of detecting submarines was made practical by Professor Langevin. Perrin himself at that time, devised a method and constructed an apparatus for determining the direction of the sound from an aeroplane.

After the armistice he continued to take an active part in co-ordinating scientific and industrial research. As one of his friends said of him, ‘Mais plus que tout autre il comprenait le benefice que l’industrie peut recueiller du contact permanent avec la science, et c’est pour completer sa mission qu’il organise le service de la Recherche Scientifique Appliquee, disons plutot la ‘recherche dirigee’ pour rester parfaitment fidele a sa pensee’.

He persuaded the government to form a department whose duty it was to assist advanced students who were qualified to do research, and to establish laboratories for them in provincial towns. With a view to interesting the general public in modern discoveries, the Palais de la Decouverte was established in Paris. In 1936 Perrin was appointed Under-Secretary of State for scientific research.

He was as enthusiastic about this work as he had been about his researches on cathode rays and the Brownian mouvement when he was a young man. As he himself said, ‘Je veux que l’Etat puisse acheter dans la classe ouvriere des Ampere des Faraday et des Pasteur’.

He was an inspiring teacher and a most agreeable colleague. It would be impossible to give a better description of his brilliant and happy personality than that contained in his epitaph written by a friend of his who had collaborated with him in France, and is now in England with the Fighting French.

Passant, laisse ton pas s’alanguir, le soleil La bas acheve de descendre.

Viens rever un instant, sans funebre appareil.D ’un grand humain voici la cendre.

II n’avait d’autre foi que sa ferme raison Et ne hai'ssait que la haine

II aimait que l’on chante et rie en sa maison Apres le travail et la peine.

Son genie etait vaste, aventureux, divers.Pas d’audace qu’il n’ait osee

II comptait des milliards de milliards d’univers Dans une goutte de rosee.

II croyait a la Vie au Progres au Bonheur Et donnait avec allegresse

A tous, autour de lui, les fruits de son labeur Et le tr^sor de sa tendresse.

II aimait sons pays, mais quand la liberte Y fut dans la honte egorgee.

II prefera l’exil ou dans l’adversite Sons ame restait inchangee.

II est mort dans la lutte en nous criant ‘Espoir!La France renaitra plus belle

Dans un monde plus juste, apres le gouffre noir,Car la lumiere est eternelle.’

Jean Baptiste Perrin 305

J. S. T ownsend