Dbcbmbke 7, 1900.] 



SCIENCE. 



881 



A permanent solution of the problem on 

 these lines may be postponed by the fact 

 that, notwithstanding their well-known 

 economic importance, the Phytoptidse re- 

 main a little-known group, even from the 

 systematic standpoint, doubtless owing to 

 their excessively small size. But if the pres- 

 ent hypothesis should prove to have any 

 basis in fact, it will probably be possible to 

 control or even to exterminate the yellows 

 by the local destruction of the wild rela- 

 tives of the peach which may be found to 

 harbor the offending creatures. The extent 

 to which this would be necessary for the 

 protection of peach-orchards will depend 

 upon the agency of transmission. Fortu- 

 nately the mites are without wings, and 

 whether carried by small birds or by in- 

 sects, the distances whence they would or- 

 dinarily be brought are not great. There 

 is thus opened another possibility, at least, 

 of the relief of this important industry from 

 the ravages of maladies which not only 

 cause thousands of dollars of damage an- 

 nually, but which are chargeable with a 

 further public injury in limiting the pro- 

 duction and popular enjoyment of this finest 

 of temperate fruits. 



0. F. Cook. 



"WASHINaXON, D. C. 



SCIENTIFIC BOOKS. 



Legons de ehimie physique. Profess6es al'Uni- 

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

 membre de I'Academie des Sciences de Ber- 

 lin. Professeur ordinaire a I'Universite, et 

 directeur de I'Institut de Physique de Char- 

 lottenbourg. Ouvrage traduit de I'allemand 

 par M. CoEVlSY, Professeur agrege au Lyc6e 

 de Saint-Omer. Paris, Librairie Scientifique, 

 A. Hermann. 1899. 



The book in hand is a French translation of 

 Van't Heft's lectures on ' Selected Chapters in 

 Physical Chemistry ' which he now delivers at 

 the University of Berlin. As the title implies, 

 the book is not a systematic treatise on the 

 whole subject of physical chemistry, but the 



number and importance of the subjects dealt 

 with are so great that the scope of the work is 

 not small. 



The order of arrangement is quite dlfierent 

 from that usually adopted. It is the general 

 custom to take up first the older work in phys 

 ical chemistry which has to do with the rela- 

 tions between composition and physical prop- 

 erties, and constitution and physical properties. 

 This is then followed by the newer physical 

 chemistry, dealing with solutions and energy 

 transformations in the broad sense ; concluding 

 with chapters on chemical dynamics and chem- 

 ical statics. 



Van't Hoff" has, indeed, exactly reversed this 

 order. Part first is on Chemical Dynamics, 

 including chemical equilibrium and reaction 

 velocity. Part second, on Chemical Statics, con- 

 tains chapters on molecular weights, molecular 

 structure and molecular grouping ; and it is not 

 until the third and last part of the book is 

 reached that we find a discussion of the rela- 

 tions between properties and composition ; 

 comprising relations between physical proper- 

 ties and composition on the one hand, and 

 chemical properties and composition on the 

 other. 



In those who have been accustomed to the 

 older order of things this new order of pjfs- 

 entation produces a little discord. This, how- 

 ever, may not argue against the method adopted 

 by Van't Hofi". It should, nevertheless, be 

 noted in this connection that the more gener- 

 ally accepted order of presentation is far more 

 closely in accord with the historical develop- 

 ment of the subject, and there are those who 

 believe in the pedagogical value, at least, of 

 the historical method. 



The newest work of Van't Hoff, like every- 

 thing which he has written, is full of original 

 and brilliant suggestions. There is hardly a 

 subject touched, in the whole book, without 

 new light being thrown upon it and new rela- 

 tions pointed out. No one can read the work 

 without feeling that physical chemistry is here 

 treated by a master hand, which not so much 

 compiles as creates. 



These lectures are, for the most part, rather 

 advanced, as we should naturally expect them 

 to be. They should, therefore, not be placed 



