REVIEWS 379 



adequate reference. The electrical mechanism by which light is emitted from the 

 atom or molecule is, however, not so adequately treated. While Stark's theory 

 that positively charged atoms emit the line spectra can be reconciled with Wien's 

 observations on canal rays, there is no good confirmation of it, and in a paper not 

 mentioned by the author Baerwald (Annalen der Physik, 34, p. 883, 191 1) from 

 modified experiments on the Doppler effect in canal rays comes to the conclusion 

 that the carriers of the series cannot be positively charged, but are in all probability 

 neutral atoms which emit light at the moment of neutralisation by an electron, in 

 accordance with the theory developed by Lenard in his work on phosphorescence 

 and elsewhere, and adopted by Wien for canal rays. There also seems little doubt 

 that line spectra are to be attributed to atoms, band spectra to molecules, which 

 hypothesis will account for the emission sometimes of lines, sometimes of bands by 

 the same element according to conditions, a fact which the author describes as 

 unexplained. 



In the third part of the book a chapter is devoted to the structure of the atom, 

 in which, we think, an unnecessary amount of attention is given to Stark's theory, 

 which has not proved particularly valuable, and which for those interested is 

 easily accessible elsewhere (in Stark's Atomdynamik) : there is no mention of 

 Nicholson's work. The last chapter is on the principle of relativity. The author 

 begins by giving the Einstein transformations, and does not state the physical 

 reasons which led up to them, or the physical assumptions underlying them, until 

 he has deduced their chief results ; this seems rather unsatisfactory for those 

 approaching the subject for the first time. Again, we do not think he gives quite 

 a fair account of the obstacles in the way of acceptation of the principle in its 

 present form, at any rate ; no mention is made of the difficulties presented by the 

 dynamics of rigid body rotation. But the most important applications to electro- 

 dynamics are fully and clearly presented : we only trust that Dr. Campbell's 

 evident contempt for the yet unconverted will not offend intending converts. 



The book is full of matter of extraordinary interest, the treatment is always 

 vigorous, and such small faults as we have found are quite insufficient to warrant 

 us treating it as anything but a very successful attempt to deal with the difficult 

 task of giving an account of electrical theory as it stood at the beginning of this 

 year. The specialist may find small omissions in his particular branch, but he will 

 not find any very serious fault ; in general he will find the book stimulating, 

 informative, and an excellent preliminary when he wishes to read up any other 

 branch. To the student and scientist engaged in other departments of science 

 who have not time for much reference to original papers, the book will be 

 invaluable. 



E. N. DA C. A. 



Mathematical Physics. Vol. I. Electricity and Magnetism. By C. W. C. Bar- 

 low. [Pp. vi + 312.] (University Tutorial Press.) 



As the book does not, as far as we can see, pretend to be more than a 

 cram-book for examinations, it is not necessary to point out that it is not always 

 particularly clear on the fundamental conceptions which underlie the mathe- 

 matical theory of electricity. It has many examples, with answers, and will, we 

 think, answer its purpose. 



E. N. DA C. A. 



