REVIEWS 519 



advantage in a subject so comprehensive as the development of optical theories 

 from the earliest times to the present day. The book in fact suffers considerably 

 from undue compression, and the greater portion of it will be very difficult reading 

 except for a student well acquainted with the subject. To the average student, 

 Dr. Whittaker's work, with its fuller treatment and comparative avoidance of 

 mathematics, will form both easier and more interesting reading. The author 

 would have been wise to omit much of the mathematics ; unless an argument can 

 be given in full, long formulas, whose derivation is not given, have but little 

 interest. 



The preceding remarks are not intended to convey the impression that the 

 book has no merits. The author has been at considerable pains to make clear the 

 relationship between the various theories, and how we have been led up to 

 the present position. The most valuable part of the book is the final chapter, 

 with its ten pages of summary', which forms an admirable and succinct account of 

 the theories discussed in the preceding chapter. There are several misprints and 

 omissions, and the frequent references forward are annoying. In §54 it is assumed 

 that the ray is normal to the wave-surface, which is not necessarily true in 

 crystalline media. In §100, the sign of the right-hand side of the first equation is 

 incorrect. In §132,477 is omitted in the expression for/. The nature of a dis- 

 placement current is not well explained, and the distinction between displacement 

 in a polarised medium and in free ether, which is so fundamental in Maxwell's 

 theory, is not mentioned. The whole treatment of the question of energy in the 

 electromagnetic field is confused, and it is difficult to see what assumptions are 

 made in the various steps. 



There is a great need for a full, rigid, and consistent treatment of this important 

 subject ; an adequate treatment is not to be found in any existing volume. 

 Closely bound up with this looseness of the discussion of energy is the failure to 

 maintain throughout the book the fundamental distinction between the magnetic 

 force and the magnetic induction. In several places the author does not seem 

 sure which he should be dealing with. For the student fully to understand the 

 subject it is essential that such distinctions should be made absolutely clear. 



H. S. J. 



CHEMISTRY 



A Class-book of Organic Chemistry. By J. B. Cohen, Ph.D., B.Sc, F.R.S. 

 [Pp. viii + 344, with illustrations]. London : Macmillan & Co., 191 7. 

 Price 4s. bd. net.). 



In this little book the author has set aside the usual conventional order of treat- 

 ment of the subject, and in simple language gives a clear and readable intro- 

 ductory account of organic chemistry. The book is meant for first-year medical 

 students and for senior science students in schools, and is divided into three parts. 

 The outstanding feature is the way in which the subject-matter is throughout pro- 

 vided with practical illustrations and experiments, with the result that we are 

 introduced practically at the outset to the process of fermentation and enzyme 

 action, with a view to supplying material for illustrating the purification of organic 

 compounds by distillation, while grape sugar is chosen for demonstrating methods 

 of combustion, and molecular weight determination. The determination of the 

 constitutional formula of an organic compound is explained in the second chapter 

 by a study of the reactions of ethyl alcohol, which leads successively to the 

 description of ethyl acetate, ethyl ether, acetic aldehyde, and acetic acid. The 

 first part is brought to a close by a description of methyl alcohol and its oxidation 



