75G Prof. R. A. Millikan and Mr. fl. Fletcher on the 



and reports experiments by bis later method, in which, when 

 the secondary radiator was a brass plate covered with a coat 

 of vaseline, singles only were formed, while when the plate 

 was freshly cleaned a considerable proportion of the ions 

 were doubles. This is explained by the assumption that the 

 vaseline absorbs the very soft secondaries which are alone 

 responsible for the singles. According, however, to Hazel- 

 foot's* observation Townsend's later method does not appear 

 to yield very consistent results for varying values of the 

 field between B and C, the value of ne for the positives 

 formed by radium coming out 1*37 for a 4 volt P.D. between 

 B and C, and 1*24 for a 2 volt P.D. 



In summarizing then the work of all preceding observers 

 in this field it may be said that although both Townsend and 

 Franck and Westphal draw the conclusion that doubly 

 charged ions exist in gases ionized by X rays, there are 

 such contradictions and uncertainties in their work as to 

 leave the question still an open one. In gases ionized by other 

 agencies than X rays no one has yet found any evidence for 

 the existence of ions carrying more than a single charge. 



§ 2. A study of the act of ionization by primary X rays through 

 the capture upon oil drops of the products of the ionization. 



The method of studying gaseous ionization recently devised 

 by one of usf seemed capable of furnishing a direct and 

 unmistakable answer to the question as to whether or not 

 the phenomenon of valency exists at all in gaseous ionization, 

 and if so to what extent. The conclusion to which we come 

 as a result of a number of months of study of the subject, is 

 that under no circumstances which we have as yet been 

 able to investigate does the act of ionization of air consist in 

 the detachment of more than one elementary charge from a 

 neutral molecule. This conclusion is contrary to an opinion 

 expressed by one of us in the first report made upon the 

 catching of individual ions by oil drops, for although it was 

 stated in this report that " the results already obtained de- 

 monstrated that the great majority of atmospheric ions of 

 both positive and negative sign carried but one single 

 elementarv electrical charge/' nevertheless the statement 

 was added that strong evidence had been obtained that 

 doubles and triples were in occasional instances formed. The 

 evidence here referred to was found in the fact that although 



* C E. Hazelfoot, Proc. Royal Soc. vol. lxxxii. p. 18 (1909). 

 t Millikan, ' Science,' Sept. 30, 1910. See also Phys. Review, April 

 1911, for a more detailed account of this work. 



