August 30, 1900] 



NATURE 



427 



solids naturally furnishes interesting data. A work by M. 

 Mesnager, and another by the author of the present article, were 

 devoted to these two questions. It is interesting to note that 

 the last experimental researches are all in favour of the chemical 

 theory of temporary deformations. 



M. Voigt has devoted a great amount of indefatigable activity 

 to the study of the elasticity of crystals. His summary of this 

 question was a great boon to the second section. It was a con- 

 siderable piece of work, in which, naturally, the mathematical 

 formula was predominant. This work will serve as a base for 

 all those interested in the elasticity and the piezo- electricity of 

 crystals, as well as in questions of symmetry. 



M. van't Hoff was not present at the Congress, but he 

 showed his interest by sending a work on the formation of 

 crystals in a mother- liquor containing a mixture of salts. In 

 this case questions of equilibrium play an important part, the 

 form of the crystals depending, not only upon the solubility of 

 each salt in the mixture, but also upon the quantity of each. 



It was very interesting to learn the ideas arrived at by M. 

 van der Mensbrugghe, after a long career devoted to the study 

 of capillarity. The report presented by Joseph Plateau's son- 

 in-law constitutes a precious document on capillarity, a subject 

 which has been somewhat eclipsed by other subjects, but which 

 has formed the object of investigation of the greatest spirits, 

 and continues to do so. 



We must be short, and can only mention the report by M. 

 Brillouin on gaseous diffusion, by M. Perrin on osmosis, and 

 by M. Bjerknes on hydrodynamical actions at a distance. The 

 latter derives its interest more especially from the fact that a 

 hydrodynamical model may be constructed which possesses all 

 I he characteristics of a world subjected to actions at a distance. 



The third section, presided over by M. Lippmann, dealt with 

 >ptics. The recent researches on the laws of radiation natur- 

 ally formed part of its programme, opened by those inseparable 

 reports, on the theoretical laws of radiation, by M. W. Wien ; 

 on the radiation of solids, by M. Lummer ; and on gaseous 

 radiation, by M. Pringsheim. The practical realisation of the 

 black body, the verification of Stefan's law for a large range of 

 temperature, and certain simple relations between the tempera- 

 ture and the position of the maximum in the spectrum, are the 

 salient facts which the experimental work of recent years has 

 brought out. For gases, a doubtful point is the validity of 

 KirchhofTs law, but according to M. Pringsheim that does no 

 seem to be in any danger if only the purely thermal radiati 

 is considered. 



Recently, the spectrum has been greatly extended in the 

 infra-red. M. Rubens, to whom the greatest progress in this 

 direction is due, had undertaken to give a summary of this 

 question, showing how the dispersion formulae agreed with 

 experiment, and demonstrating experimentally the connection 

 between long light waves and electrical waves. This work again 

 called forth a discussion on the formulae and theories of 

 dispersion opened by M. Carvallo. 



The kinematics of the spectrum has also made great progress 

 since Balmer showed for the first time that the hydrogen rays 

 are represented by a very simple formula. The researches of 

 Kayser and Runge and other physicists, Rydberg among them, 

 have shown that the distribution of the spectrum lines is 

 governed by laws, some of which are clearly established, while 

 others are as yet unknown. Of all this work, M. Rydberg gave 

 an excellent summary. 



The velocity of light has, as we know, given rise to metro- 

 logical work of the first rank, and of extreme difficulty. It fell 

 to the distinguished president of the Congress, M. Cornu, to 

 give a review of this subject,, and during the remarkable speech 

 which he delivered at the Ecole Polylechnique, the physicists 

 from all parts had the privilege of seeing the original apparatus 

 of Fizeau and of Foucault, who were the first to give an approxi- 

 mate value of that velocity by measurements confined to the 

 earth. 



It is this characteristic velocity which for Maxwell was the 

 touchstone of the theory involving the identity of luminous 

 and electrical oscillations. As the instruments become more 

 perfect, and the sources of error disappear, this identity is more 

 and more emphasised. It was very interesting to co-ordinate 

 the numbers furnished by light proper with those furnished by 

 the comparison of units and the direct measurement of the 

 velocity of electric waves. M. Abraham undertook the first 

 part of this work, and MM. Blondlot and Gutton the second. 

 This brings us to the electrical section, presided over by M. 



NO. 1609, VOX. 62] 



Potier, and in his absence by M. Bouty. The line of demarca- 

 tion, however, is becoming more and more difficult to draw. 

 The extremely interesting work of M. Lebedef on the pressure 

 produced by radiations, has its origin in the great work of Max- 

 well ; but it might also arise from pure thermodynamics, as 

 shown by Bartoli and Boltzmann. As regards Hertzian waves, 

 treated in a masterly manner by M. Righi, they approach so 

 closely to the work of M. Rubens, that the small interval which 

 still separates them is probably the only reason — and a very 

 artificial one — for keeping them separate at all. In a supple- 

 mentary note, M. Branly gave an account of some of his own 

 researches on coherers. The reports just mentioned furnished 

 the experimental side of an idea, the theoretical aspect of which 

 was treated of in a paper by Prof. Poynting on the propagation 

 of electrical energy. 



We encounter another group of questions in the gaseous 

 dielectrics, studied by M. Bouty, as well as electrolysis and 

 ionisation, which have made such vast progress during the last 

 decade, and which were dealt with by M. Arrhenius, one of the 

 promoters of the new ideas, in a paper which will remain a 

 model of clearness. Finally, we have M. Christiansen's theories of 

 contact electricity, M. L. Poincare's theories of the electric cell, 

 and the exposition of Nernst's ideas, which had not been con- 

 templated in the programme of the Congress, but which enabled 

 their founder to give to the meeting a review admirably 

 completing this group of questions. 



The presentation of present ideas on magnetism had been ex- 

 cellently prepared by two fundamental reports, one, by M. du 

 Bois, on the general magnetic properties of bodies, and another, 

 by M. Warburg, on hysteresis, which he was the first to observe, 

 and which, in the hands of Ewing, Hopkinson and others, 

 attained such great importance. Two particular aspects of 

 magnetism, viz. magneto-striction and the E. M.F. of magneti- 

 sation, which could not form part of the general reports, were 

 treated separately by M. Nagaoka and M. Hurmuzescu. 



Although the applications of electricity are almost entirely 

 beyond the subject-matter of the Congress, there are some which 

 are connected so closely with general physics that it seemed 

 very desirable to have them dealt with. This was done by M. 

 von Lang, whose work on the electric arc is well known, while 

 M. Potier gave an exhaustive paper on the theory of polyphase 

 currents, and M. Blondel the description of apparatus for tracing 

 the curves of rapidly varying currents. 



In a few years' time the work of the fifth section — ionisation 

 and magneto-optics, presided over by M. Becquerel — will no 

 doubt fall naturally into one of the preceding sections. But at 

 the present moment they are still so undefined, they open up 

 such new horizons, that it appeared well to collect them in a 

 special section. The idea proved very fruitful, for the section 

 was largely attended, and the discussions at it proved very 

 fascinating. 



M. Lorentz had prepared an admirable report on magneto- 

 optics, with special reference to the Zeeman phenomenon. He 

 expounded both his own ideas and those of M. Voigt. The 

 presence of the latter gave the section the privilege of an 

 exposition at first hand of his latest ideas. 



The absence of Prof. J. J. Thomson could not but be severely 

 felt. But the work which he had sent in, concerning the ratio 

 of the electric charges to the masses carrying them, was read 

 amid great interest after the general exposition made by 

 M. Villard of the state of our knowledge of kathode rays. 



The phenomena of actino-electricity, somewhat forgotten 

 now, though much studied ten years ago, gave rise to a report 

 by MM. Bichat and Swyngedauw. Perhaps increased atten- 

 tion will be devoted to them now that the researches of M. 

 Becquerel and those of M. and Mme. Curie have proved so 

 fertile in the examination of new bodies. 



The speeches in which first M. Becquerel and then M. Curie 

 expounded the disconcerting properties of uranium, polonium 

 and radium rays, were for many a revelation. These extra- 

 ordinary bodies, discovered by their radio-active properties, 

 which were first announced by M. Becquerel, and then followed 

 up with such startling success by M. and Mme. Curie, were 

 known to the majority of those present, but only a few had seen 

 those few decigrammes of material extracted from several tons of 

 the mineral richest in it, pitchblende, and certainly the effects 

 produced surprised by their intensity those who saw them for 

 the first time. 



Several hundred persons at a time could see this light, which 

 appears everlasting, radiated perpetually by radium, the clear 



