914 



SCIENCE. 



[N. S. Vol. VIII. No. 208. 



result should be given in the corrected form 

 which was accepted by Joule after Rowland's 

 exhaustive experiments in Baltimore. Joule's 

 equivalent is now quite generallj' quoted as 427 

 rather than 424 kilogram-meters at ordinary 

 laboratory temperature. 



The difficulty of conveying clear ideas with- 

 out mathematical methods is particularly felt 

 in the attempt to define elasticity and to em- 

 ploy this word intelligibly in the formula for the 

 velocity of propagation of a wave. To say that 

 this velocity varies as the square root of the 

 quotient of elasticity by density conveys no idea, 

 unless modulus of elasticity has been previously 

 defined and abundantly illustrated. The ordi- 

 nary student regards india-rubber, a highly 

 compressible solid, as the type of elasticity, 

 while inreality its modulus of elasticity is excep- 

 tionally small. The stretch modulus is defined 

 in an appendix to the present volume ; but it is 

 not concerned in the propagation of a sound 

 wave through air, and it is in this connection 

 that the formula is given. To account for the 

 high velocity of sound in solids and liquids by 

 reference to their superior incompressibility is 

 inadequate unless the relation between compres- 

 sibility and the volume modulus of elasticity 

 has been already rnade plain. The elementary 

 student has scarcely any alternative but to 

 memorize words in this connection, and to 

 trust to the future for the ideas they are intended 

 to convey, however faithful the author of the 

 textbook may have been to put into English 

 what is always beyond the youthful reader. 



The presentation of the subject of electric 

 potential is unusually well given ; it is, indeed, 

 as good as could possibly be expected without 

 mathematics. The general treatment of electric- 

 ity is clear and up to date, several pages being 

 devoted to X-rays and the phenomena of alter- 

 nating currents of high potential and high fre- 

 quency. 



The book is not free from typographical 

 errors, but these are in no case serious. There 

 are occasional statements of minor importance 

 to which exception may be taken, but the 

 author is generally accurate and reliable, and 

 his skill in the art of presentation is unques- 

 tioned. Among the welcome features are wood- 

 cut reproductions of the portraits of Archimedes, 



Galileo, Newton, Franklin, Faraday and Lord 

 Kelvin. W. Le Conte Stevens. 



Washington and Lee University, 

 Lexington, Va. 



Legons de chinvie physique. Profess6es a L'Uni- 

 versit6 de Berlin. Par J. H. Van't Hoff, 

 Membre de l'Acad6mie des Sciences de Ber- 

 lin, Professeur ordinaire a L' University, et 

 Directeur de L'institut de Physique de Char- 

 lottenburg. Translated from the German by 

 M. CoEVisY, Professeur agrege, au Lycee de 

 Saint-Omer. Premiere partie. La Dynamique 

 Chimique. Libi-airie Scientifique. Paris, A. 

 Hermann. 1898. 



This work, as the title implies, is a trans- 

 lation of Van't HofF's 'Vorlesungen iiber theo- 

 retische und physikalishe Chemie,' or of that 

 part of it which has thus far appeared — Chem- 

 ical Dynamics. The book, as the author states, 

 " is based upon the lectures which I give at the 

 University of Berlin, on ' Selected Chapters in 

 Physical Chemistry.' Indeed, it contains more 

 than these lectures, since, in the limited time at 

 my disposal, I was able to take up only some of 

 the more important points, in order to cover the 

 entire field in one lecture a week during four 

 semesters." The method of treatment is that 

 adopted by Lothar Meyer in the later editions of 

 his ' Blodern Theories of Chemistry.' The whole 

 subject is treated under the general heads of 

 Statics and Dynamics ; Statics dealing with ho- 

 mogeneous substances, with views as to thestruc- 

 ture of matter, with the molecular and atomic 

 conception, and with the determination of con- 

 stitution ; Dynamics, with the reciprocal trans- 

 formation of several substances, with aflSnity, 

 reaction, velocity and chemical equilibrium. A 

 third part is added, on the relation between phys- 

 ical and chemical properties and composition. 



The order is, however, reversed. Chemical 

 Dynamics, having been placed on a surer basis 

 by thermodynamics, has acquired greater prom- 

 inence, and is dealt with at the beginning. We 

 have then: First, Chemical Dynamics ; second, 

 Chemical Statics, and third. Relations between 

 Properties and Composition. 



The advantage of this order is that in the 

 first part of the work only the molecular con- 

 ception enters, while the atomic hypothesis and 

 the problem of configuration do not appear 



