426 



NATURE 



[August 30, 1900 



The cause of this unexpected success must no doubt be sought 

 in the idea underlying the plan of the Congress, worked out as it 

 was with the greatest care by a committee of the Societe Fran- 

 9aise de Physique. That committee deliberately rejected the 

 method of simply presenting personal memoirs, or notes on 

 limited subjects, and concentrated all its efforts upon the pre- 

 paration of a well-arranged summary of the actual state of 

 physical science, in the branches in which, within the last few 

 years, the greatest progress has been made, and the actual stage 

 of progr ess of which at the end of the nineteenth century it was 

 considered most important to investigate. Once the list of 

 subjects was completed, the work was divided among the 

 physicists who seemed best qualified to give a complete re- 

 presentation ot their special subject. This plan gave rise to 

 a series of reports,^ many of which are works of a very high 

 value, and which, in their entirety, constitute the most complete 

 representation of any science at a given epoch yet made. These 

 reports number about 80. To summarise these here would be, 

 so to speak, repeating the work of the Congress on a small 

 scale, and that could not be thought of. I shall, therefore, con- 

 fine myself to referring to them by groups which are obviously 

 related and mutually supplement each other. 



For this considerable task, a preface was necessary. M. H. 

 Poincare provided such a preface, and brought it before the 

 Congress amid great applause, showing how mathematical 

 generalisation could render experimental work infinitely more 

 fertile. Experimental physics is a library. Mathematical 

 physics arranges it and prepares the catalogue. It does not 

 enrich it, but if it is well prepared it enables one to draw a 

 greater profit from the former. The celebrated mathematician 

 then showed how hypotheses have succeeded each other, in 

 the form of physical images or simply mathematical images, 

 "where the symbol often remains true even when the mechanism 

 is no longer accepted. Mathematical analysis also alone gives 

 the true sense of the simplicity hidden under complexity, as in 

 the case of Newton's law — which is always rediscovered in the 

 most complicated movements of the heavenly bodies — or the 

 kinetic theory of gases, where the law of large numbers hides 

 the isolated individuals, only permitting the appearance of an 

 aggregate for which the laws of Mariotte- Boyle and Gay-Lussac, 

 long considered simple, are only the destruction of the action of 

 individual molecules. Starting from these now well-known facts, 

 M. Poincare showed how the same methods and ideas apply to 

 theories now being evolved concerning the interaction of mutter 

 and eiher. His speech will no doubt be read and studied for a 

 long time to come, and will remain one of the most perfect 

 expressions of the state of mind of the masters of modern science. 



To the organisers of the Congress, Lord Kelvin's promise of 

 a personal contribution of work had been a powerful and valued 

 encouragement. But what they hardly dared hope for was to 

 see him, after the fatigues of a voyage, take a very active part in 

 the Congress, and to see him hold spell-bound by the charm of 

 his discourse a respectfully attentive audience bent upon seizing 

 every thought of the great physicist. M. Poincare's speech 

 gave him an occasion for a brilliant improvisation on the con- 

 stitution of the ether ; and he also dealt with the subject in a 

 paper on the waves produced in an elastic solid by the motion 

 of a body acting upon it by attraction and repulsion. But it 

 was not only in that speech that the illustrious honorary president 

 of the Congress showed the interest he felt in the assembly. 

 Presiding every day at sectional meetings, he clothed both reports 

 and debates with a very special authority. 



To facilitate work, the Congress had been divided beforehand 

 into seven sections, the work of which I propose briefly to 

 review. 



In the measurement section, presided over by M. Benoit, the 

 chief work was that of determining the actual state of metrology 

 properly so called. After a very complete recapitulation, by the 

 president, of the history of standards and methods employed in 

 the measurement of length and the progress so far made, detailed 

 attention was devoted to the complete metrological definition of 

 standards and their legal definition ; the legal status of the 

 electrical units ; and some improvements which might con- 

 veniently be made in a number of insufficient definitions, or 

 definitions referring to conceptions recently introduced into 

 science, such as the different abscissae of the spectrum, &c. Some 

 resolutions were passed, such as that recommending the adoption 



1 These reports presented to the Congress have been translated into 

 French. They were printed for purposes of discussion, and will be shortly 

 published in three volumes. 



NO. 1609, VOL. 62] 



of the mechanical C.G S. units (erg and joule) for the expres- 

 sion of calorimetric quantities, comprising, naturally, the solar 

 constant, to be reduced by the meteorologists to the calorie per 

 minute per sq. cm. Also that in the expression of elastic 

 constants, the C.G.S. unit of pressure, the baiie, be adopted, 

 of which the multiple by 10", the niegabarie, is sufficiently 

 represented by the pressure exercised by a column of mercury 

 75 cm. long at 0° and under normal gravity. The Congress 

 further supported the sectional resolution that national labora- 

 tories be created in countries which do not as yet possess any. 



The interferential methods of measurement brought out an 

 excellent paper by M. Mace de Lepinay ; there were also four 

 contributions relating to thermometry of precision (Chappuis), 

 pyromelry (Barus), the mechanical equivalent of heat (Ames), 

 and a special study of the variation of the specific heat of water 

 (Griffiths). All these showed that great progress has been made 

 in these various departments. Thus at present the divergencies 

 among the various gas thermometers are known over a long in- 

 terval, and it is also known that though the hydrogen and nitrogen 

 thermometers, for instance, may still differ between 0° and 100", 

 their divergence at the higher temperatures is insignificant if 

 care is taken to slightly correct the mean coefficient of expan- 

 sion between 0° and 100°. The difficulty of employing hydrogen 

 at high temperatures lends a great importance to this provision. 

 Pyrometry also is rapidly advancing, and as regards the 

 mechanical equivalent, the great divergences which existed a 

 few years ago have disappeared owing to a more complete cor- 

 rection of thermometric values and a better knowledge of 

 electric standards. 



Some very fine work has also been done in connection with 

 gravitation. The measurement of the Newtonian constant, ad- 

 mirably expounded by Mr. Boys, whose special work in this 

 department is now classical, and the announcement of anomalies 

 of gravitation by Messrs. Bourgeois and Eotvos, gave rise to 

 very interesting discussions. A few years ago these anomalies 

 were placed beyond a doubt, and it is already possible to study 

 the details with the aid of apparatus which, like that of 

 M. Eotvos or that of Messrs. Threlfall and Pollock, indicates 

 the most minute details, whereas the pendulum formerly em- 

 ployed only gave the more considerable anomalies. The 

 Congress expressed a hope that the study of these anomalies will 

 be pursued by the new methods, not only for the sakeot knowing 

 the gravitational acceleration in every place, but also for the 

 better knowledge of the constitution of the globe. 



Finally, M. Leduc presented to the section a report on the 

 electro-chemical equivalentof silver, and M. Gouy another on the 

 standard of E. M. F. It appears from the latter that the cadmium 

 standard is preferable to every other. 



The measurement of the velocity of sound, dealt with by 

 M. Violle, form.s in a manner the transition between the section 

 of measurement and that of mechanical and molecular physics. 

 In the latter, presided over by M. Violle, after a very complete 

 treatise by M. Amagat on the whole of his work, and an admir- 

 able paper by M. van der Waals on the statics of mixed fluids, 

 M. Mathias showed, in a paper well provided with references, 

 how the critical point may be determined by various methods. 

 Specialising further. Prince Galitzine dealt with the re- 

 Jractive index, and, finally, M. Battelli exhibited the relations 

 between the statics of fluids and their calorimetry. Except 

 as regards mixtures, the ideas on these various subjects 

 are well fixed nowadays, and new light can only come from the 

 experimental side. Mixtures are less known, and the paper in 

 which the celebrated Amsterdam physicist condensed our actual 

 knowledge of this question will no doubt powerfully contribute 

 to make them known. Having created the idea of continuity 

 between the liquid and the gaseous states, he has had the satis- 

 faction of seeing it become classical. But it is in another 

 direction that this evolution advances nowadays. Does this 

 continuity also exist between the liquid and tne solid state ? 

 The diffusion of solids, their flow under pressure, the constitu- 

 tion of alloys, so well studied, notably by M. Spring and Sir W. 

 Roberts- Austen, might lead to that belief, especially since M. 

 Schwedoff has proved the rigidity of liquids. M. Tammann 

 raises some doubts concerning this idea, and recommends a 

 careful distinction between the amorphous and the crystalline 

 states. In any case, the presence at the Congress of the eminent 

 physicists mentioned, with the exception of M. Tammann, who 

 was represented by M. Weinberg, contributed greatly to the 

 interest of the subject and to its future progress. 



The study of the permanent or temporary deformations of 



