REVIEWS 327 



largely upon Eddington's Report on the Relativity Theory of Gravitation, 

 published by the Physical Society. The discussion is very much condensed, 

 and the author would have been well advised had he endeavoured to cover less 

 ground and to simplify the treatment. The theory of tensors is dealt with in 

 some detail ; would it not have been wiser, in such a limited exposition, to 

 have indicated the bases of Einstein's exposition without attempting to give 

 all the mathematics entering into it ? The applications to the three-crucial 

 phenomena could then have been explained in greater detail and the extension 

 of the theory to electro-magnetism need not have been so summarily disposed 

 of. As the volume stands the two parts seem disjointed : the one clear, 

 elementary, and free from mathematics ; the other condensed, difficult for the 

 average student of physical science, and full of mathematics. We hope that 

 in a future edition they will be better harmonised. 



H. S. J. 



A Brief Account of Radio-activity. By F. P. Venable, Ph.D., D.Sc 

 [Pp. vi + 54.] (London : George G. Harrap & Co. Price 3s. 6rf. net.) 



On producing this little book of 52 pages the author has sought to replace 

 the usual page or two on radio-activity which is included in text-books for 

 students of general chemistry. It is written simply, and is devoid of mathe- 

 matical notation, for which reasons it should appeal to that larger class of 

 individuals who are generally interested in science. 



On the whole, there is a general lack of orderly arrangement in the presenta- 

 tion of the facts. To give instances, one is merely informed in Chapter I of 

 the " discharging action " of radio-active salts. Chapter II opens with a 

 statement to the effect that the radiations possess the power of forming 

 ions, which leads to a brief description of ionisation in gases, whilst in the 

 latter part of the same chapter one is informed that " radiations from radio- 

 active bodies can discharge both positively and negatively electrified bodies." 

 Chapter III describes changes in radio-active bodies, although one must 

 read Chapter IV to ascertain the nature of the alpha particle. 



A chapter on the Structure of the Atom begins with a section on the pro- 

 perties of radium, and another follows on the energy evolved by that element. 

 One is glad to find brief descriptions of the Thomson and of the Rutherford 

 models of the atom included in the chapter. 



Short as the account is, instances of repetition are by no means lacking. 



Several well-known diagrams and tables appear in the text with advantage, 

 whilst many of the recent advances in radio-activity are briefiy narrated. 

 The book should do much to stimulate interest in a most fascinating subject. 



L. H. C. 



The Thermionic Vacuum Tube. By H. J. van der Bijl, M.A., Ph.D. 

 [Pp. xix + 391.] (London and New York: McGraw-Hill Book 

 Company, 1920.) 



This volume will provide somewhat startling reading for the general physicist 

 who has not had the opportunity of following the recent applications of ther- 

 mion ics. The subject is of phenomenal growth during the past eight years, but 

 military necessity until recently forbade publication of most of the details. 

 The last two years, however, have coincided with a pause in the development 

 of the subject, and volumes wholly devoted to the thermionic tube and its 

 applications are now appearing in France, England, America, and Germany. 

 Dr. van der Bijl's work is a particularly attractive survey of the work done 

 in America. Happily he has realised that more or less parallel investigations 

 were carried out in all four countries, and has not seriously attempted to 

 allot responsibility here and there for particular developments. The treat- 

 ment throughout is essentially the practical one of a telephone engineer. 



