944 



SCIENCE 



[N. S. Vol. XLI. No. 1069 



of the real nature of tlie effect of ultra-violet 

 light in facilitating discharge, and in honor 

 of whom the phenomena of photo-electricity 

 are not infrequently called " The HaUwachs 

 Effect," one too whose laboratory has been 

 perhaps the most continuous and prolific con- 

 tributor to the literature on the subject, this 

 book gains an authoritativeness which none 

 of its predecessors can claim. 



In addition, the book has been written with 

 an admirable objectiveness and freedom from 

 bias of either a national or personal kind. All 

 experimental work is reported fairly and fully, 

 and all theories are presented from the points 

 of view of their authors. Professor HaUwachs 

 does not hesitate to state his own convictions, 

 but he never seeks to impose them. If only 

 the scholars and statesmen had carried over 

 into political affairs the method and spirit 

 which HaUwachs here exemplifies, there would 

 have been no great war. 



Professor HaUwachs divides his historical 

 survey of photo-electricity into two periods: 

 the first period extends from the discovery of 

 the photo-electric effect in 188Y to the year 

 1900 ; the second period covers the period from 

 1900 up to the present. This division is prob- 

 ably due to the fact that the interpretation of 

 all electrical phenomena from the standpoint 

 of the electron theory began about 1900. 



The first two hundred pages constitute an 

 exceedingly valuable digest of all the experi- 

 mental work done up to August, 1913. In the 

 11th Chapter, too, this summary is extended so 

 as to include all articles which appeared up to 

 the end of the year 1913. Some of the most in- 

 teresting photo-electric developments which 

 have appeared since 1913, especially those hav- 

 ing to do with the relations of Planck's 7;. to 

 the initial energy of emission and the relation 

 of gases to photo-electric discharge, could not 

 be included at the date at which the book 

 went to press. 



The portions of the book thus far considered 

 make it an invaluable reference book to every 

 student of photo-electricity. But it is in the 

 thirty-five pages included in Chapter 9 that 

 the greatest interest in the book will center, 

 for it is in this chapter, which is entitled 



" Ueber den lichtelektrischen Grundproceas ; 

 Versuche einer speziellen Deutung der Licht- 

 elektrischen Vorgange," that the author reviews 

 and weighs all attempts which have been made 

 thus far at an interpretation of photo-electric 

 results. He does not commit himself to a 

 belief in any theory thus far formulated. 

 He gives, however, a very clear exposition of 

 the strength and weaknesses of the theories 

 which are at present before the scientific world. 



R. A. MiLLIKAN 



Univebsity op Chicago 



Sewage Purification and Disposal. By G. 



Bertram Kershaw. London, Cambridge 



University Press, 1915. Pp. x + 340, 56 



figures. 



Although books on sewage purification and 

 disposal are becoming increasingly numerous 

 this new work of Mr. Kershaw's is a welcome 

 addition to the list. The author was engineer 

 to the Eoyal Commission on Sewage Disposal 

 for many years and has a well-deserved repu- 

 tation not only in England, but in this coun- 

 try. No one has had a better opportunity than 

 he has had to study the various methods of 

 sewage treatment under English conditions. 



The present volume is a recapitulation of 

 some of the previous works of the author and 

 is published as one of the Cambridge Public 

 Health series. It deals chiefly with the meth- 

 ods which have shown their value by experi- 

 ence and does not pretend to cover some of the 

 newer processes which are now in the experi- 

 mental state. Eor this reason it is a more 

 satisfactory book to place in the hands of stu- 

 dents than are those which do not take pains 

 to discriminate between the old and the new, 

 the tried and the untried. Throughout the 

 book there is frequent reference to the cost of 

 processes. The data are valuable, but Amer- 

 ican readers must remember that the condi- 

 tions in England are different from those in 

 the United States, and in general it will be 

 necessary to nearly double the English prices 

 in order to make them applicable to conditions 

 in America. 



The author omits entirely the subject of 

 disposal of sewage by dilution as that subject 



