Chemistry and Physics. 333 



6. The Electron ; by Robert Andrews Millikan. Pp. xii, 

 268, with 33 figures. Chicago, 1917 (The University of Chicago 

 Press). — "The purpose of this volume is to present the evidence 

 for the atomic structure of electricity, to describe some of the 

 most significant properties of the elementary electrical unit, the 

 electron, and to discuss the bearing of these properties upon 

 the two most important problems of modern physics : the struc- 

 ture of the atom and the nature of electromagnetic radiation." 

 In order to extend the domain of usefulness of the text beyond 

 that of the physicist, the material has been presented in a semi- 

 popular style and all mathematical proofs have been collected in 

 six of the appendixes. 



The first chapter deals with the early views of electricity, 

 special attention being given to the growth of the atomic theory 

 of matter and the historical development of electrical theories. 

 The extension of the laws of electrolysis to the conduction of 

 electricity through gases is discussed in the second chapter. The 

 next chapter is devoted to an account of the early attempts at the 

 direct determination of the electronic charge e. The methods of 

 J. S. Townsend, J. J. Thomson, H. A. Wilson, and the balanced- 

 droplet method of R. A. Millikan are here taken up in the order 

 named. The next two chapters pertain respectively to the general 

 proof of the atomic nature of electricity and the exact evalua- 

 tion of e. These two chapters are especially illuminating and 

 important, and they contain a very lucid account of the accurate 

 and beautiful experimental investigations of the author. The 

 contents of the remaining chapters may be inferred from their 

 respective titles, which are: "The Mechanism of Ionization of 

 Gases by JT-Rays and Radium Rays, Brownian Movements in 

 Gases, The Existence of a Sub-Electron ? The Structure of the 

 Atom," and "The Nature of Radiant Energy." The volume 

 closes with eight appendixes, and author and subject indexes. 



The book merits the attention of all physicists and students of 

 physics because of its authoritative nature, broad perspective, and 

 general accuracy. It should also be added that the author's 

 exceptional clearness of presentation and well-controlled enthusi- 

 asm have imparted to the text a freshness and vigor which appre- 

 ciably increase the pleasure of reading the volume. h. s. u. 



7. Failure of JPoissoii's Equation. — In a short mathematical 

 paper, Ganesh Prasad gives some interesting special volume dis- 

 tributions for which Poisson's classical equation fails. Let p^ 

 denote the density at any point F {x, y, z) within the solid. Then 

 Poisson's equation fails at P when v'^T^is either meaningless or 

 has a value different from — 4 irp^, V being the potential due to 

 a small sphere of radius a and center P. 



Case I. Let the density of the sphere at any point Q (^, 77, t) 

 inside it be 



cos'^ 

 P = 



l^^T" 



