REVIEWS 703 



students and engineers who possess a considerable knowledge of mathematics, 

 and it will not be appreciated at all by those who prefer this aspect of the subject 

 to be reduced to a minimum. However, mathematical equations usually become 

 rather cumbersome when they are required to take into account the actual conditions 

 which obtain in practice, and, as might be expected, the feature of the book is the 

 thoroughly practical spirit in which the theory is given. 



It is only ten years since the first edition was published and, in the interval, no 

 very remarkable progress has been made. The sections dealing with armature 

 reaction and the induction motor have been rewritten and considerable changes 

 have been made in the treatment of power transmission. Here Dr. Russell gives 

 the usual solution of the general equation simplified by means of the distortionless 

 wave condition, and then shows how a general solution may be obtained with the 

 help of complex hyperbolic functions. For the convenience of the reader this 

 section is preceded by a chapter on the theory of these functions and a table of 

 their numerical values. The chapter on harmonic analysis is, very largely, a 

 reprint of the paper read by the author before the Physical Society in 191 5. It is 

 shown how, by applying Weddle's rule for areas, any desired harmonic can be 

 calculated independently of the others, thus avoiding the accumulation of errors 

 which is apt to occur when the value of each depends on that preceding it. 



A book of this kind gains much of its importance from its value as a work of 

 reference, and this is much enhanced by the explanatory list of the symbols 

 employed which is given at the beginning ; while another admirable feature is the 

 brief bibliography which concludes each chapter. Altogether a notable work which 

 should find a prominent place on the bookshelf of every electrical engineer. 



D. Orson Wood. 



CHEMISTRY 



Atoms. By Jean Perrin. Authorised Translation from the Fourth Revised 

 Edition by D. Ll. Hammick. [Pp. xiv + 211, with 16 illustrations.] 

 (London : Constable & Co., 1916. Price 6s. net.) 



PROF. Perrin's book in the original was already recognised to be a 

 masterpiece of exposition, and a contribution of the very first importance to 

 scientific literature. We are therefore justly indebted to the translator for 

 having made it even more accessible than it was to English readers through the 

 present excellent translation. The book is one which should make a very wide 

 appeal. We can give no truer estimate of it than by simply saying that it is one 

 of the very few really indispensable books. The following brief notice of its 

 principal contents will suffice to indicate its scope. 



The main feature of >the work is the high philosophic level which is maintained 

 throughout. This is especially true of the introduction, which deserves to be read 

 and pondered deeply. 



The first chapter deals with molecules and atoms in general, with the laws of 

 chemical combination, the gas laws, Avogadro's hypothesis, the periodic system, 

 the elements of the theory of dilute solutions, and the theory of electrolytic dis- 

 sociation. These are familiar matters, but the exact and critical way in which 

 they are treated gives them a new significance. In the second chapter we have 

 a more detailed account of gases from the molecular standpoint, especially the 

 calculation of molecular magnitudes. This leads us on naturally to Perrin's own 

 work on the Brownian movement exhibited by emulsion particles, and the funda- 

 mental importance of the results thus obtained in relation to the assumptions upon 

 which the kinetic theory of fluids rests. Chaps. III. and IV. are devoted to 



