290 



SCIENCE. 



[N. S. Vol. IX. No. 217. 



on the occasion of the sesqui-centennial cele- 

 bration at Princeton, iu October, 1896. As the 

 subject is one that is rapidly developing, the 

 author iias added the results of numerous inves- 

 tigations that have been published since that 

 time ; so that the present volume gives an ex- 

 cellent presentation of the subject as it now 

 stands. 



Although the electrical discharge in gases 

 has been investigated in its various phases ever 

 since the study of electricity itself began, it is 

 only in the last five or six years that our knowl- 

 edge of the subject has begun to take systematic 

 and satisfactory form. Careful observations had 

 been made by hundreds of physicists, and the 

 scientific literature abounded with descriptions 

 of phenomena of great interest and undoubted 

 scientific importance. But our knowledge of 

 the subject consisted of a mass of isolated facts ; 

 no satisfactory underlying theory had been 

 found by which these facts could be correlated. 

 The development of such a theory is largely 

 due to Professor Thomson, and I know of no 

 place where it is so satisfactorily treated as in 

 the volume before us. The book is not one re- 

 quiring the preparation of a specialist in order 

 that it may be understood ; the greater part can 

 be read with pleasure and profit by one having 

 only an elementary knowledge of electrical 

 science. On the other hand, I should not speak 

 of the book as containing merely a ' popular ' 

 account of the subject, especially if the word 

 ' popular ' is to be regarded as having the same 

 significance as inaccurate. Writers of popular 

 science are, unfortunately, only too prone to 

 look upon the two words as synonymous. Pro- 

 fessor Thomson, however, possesses the rare 

 power of writing upon a difiicult subject with 

 scientific accuracy, and at the same time in such 

 a way as to be intelligible to the lay reader. 



The contents of the book are arranged under 

 three chief sub-divisions, namely : the Discharge 

 of Electricity through Gases; Photoelectric 

 Effects, and Cathode Kays. This classification 

 is not wholly satisfactory, for each sub-division 

 contains a great deal more than is indicated by 

 its title. But the numerous sub-headings, to 

 which reference is made in the table of con- 

 tents, make it a matter of no great difficulty to 

 find any special topic sought. A fairly com- 



plete series of references to original sources con- 

 stitutes a feature that cannot be too highly 

 commended. 



To one unfamiliar with the subject the first 

 twenty pages will probably prove the most 

 difficult portion of the book. The topics there 

 discussed are the various methods by which a 

 gas may be electrified : for example, by chemical 

 processes, by electrolysis, and by the splashing 

 of liquids. The essential peculiarities of the 

 conducting power of gases are first brought into 

 prominence in the account of the effect of 

 Rontgen rays in giving to a gas the power of 

 conducting electricity. Only a few weeks after 

 the discovery of the X-rays it was found that 

 an electrified body rapidly lost its charge when 

 exposed to these rays. This property of the 

 new rays was independently and almost simul- 

 taneously discovered by at least five diflferent 

 physicists, Professor Thomson being one of 

 these, and has since been the subject of numer- 

 ous investigations. The subject is one iu which 

 experimental errors are especially difficult to 

 avoid, and a great deal of confusion naturally 

 existed at first regarding the laws of the phe- 

 nomenon and the conditions of its occurrence. 

 Practically all of the more reliable experiments 

 are now seen to support the view that the dis- 

 charge of an electrified body by the Rontgen 

 rays is due to the fact that the gas surrounding 

 the body is made a conductor by the action of 

 these rays. It is thought that the condition 

 developed in the gas is somewhat similar to 

 that in an electrolyte, i. e., ions are formed, 

 some carrying positive charges and others nega- 

 tive charges. A charged body placed in the 

 ' ionized ' gas would attract one kind of ions 

 and repel the other. Upon coming into con- 

 tact with the charged body the ions are sup- 

 posed to give up their charges and cease to 

 exist as ions. The gas is thus rendered capable 

 of conducting electricity iu much the same 

 manner that an electrolyte conducts. But an 

 essential difference exists between the two cases, 

 due to the fact that the ionized condition of the 

 gas is only temporary ; in less than a second 

 after the Rontgen rays have ceased the ions 

 have recombined and the gas is as good an in- 

 sulator as ever. 



Conducting power may be imparted to a gas 



