Question of Valency in Gaseous Ionization. 755 



to the diffusion tubes, and found, indeed, that when the 

 current of air was made exceeding slow, the diffusion co- 

 efficient of the ions which got through became smaller as the 

 number o£ sheets of gauze was increased. Thus they found 

 Avithout the insertion of gauze D = *029; with one sheet of 

 thick gauze D = '020 ; with three sheets D = '0175. 



It is to be noted, however, that these experiments actually 

 show only that some of the ions in the ionized gas had 

 smaller diffusion coefficients than others. They do not show 

 directly that the cause of this difference in K is a difference 

 in the charge carried. Indeed, in later experiments* Franck 

 and Westphal find that the diffusion coefficient of the nega- 

 tive as well as the positive ions produced by po:nt discharge 

 decreases to half value upon the reduction of the speed 

 of the air current or the insertion of gauze, and in this case 

 they do not draw the conclusion of double charges, but 

 merely infer the presence of a certain number of Langevin's 

 large, slowly-diffusing, ionsj. It is obvious, then, that the 

 assumption of doubly charged ions is not necessitated by any 

 of Franck and WestphaPs results. Indeed, it seems altogether 

 likely, from Langevin's and Pollock's work J, that a certain 

 number of large or "intermediate" ions may have been 

 present in their experiments with X rays. In their experi- 

 ments on the ionization produced by a rays, (3 rays, and 

 7 rays, they themselves find no evidence w'hatever for the 

 existence of donbly charged ions. 



These experiments of Franck and Westphal's, while in 

 good agreement with Townsend's original work, do not seem 

 to us to be easily reconcilable with his later experiments, 

 since they certainly make untenable his explanation of the 

 cause of the difference between his earlier and later findings, 

 and yet do not seem to us to replace it by a thoroughly 

 adequate one, for it does not appear likely that Townsend's 

 one sheet of gauze could have produced such complete 

 separation of the doubly and singly charged ions as he found. 

 The two sets of experiments are also difficult to reconcile in 

 that Franck and Westphal find that the hardness of the rays 

 has no effect upon the proportion of doubles and singles, 

 while Townsend § in a later paper finds that very soft 

 X rays produce singles while hard ones produce only 

 doubles. 



This later paper deals with the action of secondary X rays 



* J. Franck and W. Westphal, Verh. D. Thy*. Ges. Juli 2, 1009. 



+ Langevin, C. R. 1140. p. 232 (1905). 



\ Pollock, 'Science,' vol. xxix. p. 919 (1909). 



§ T. Townsend, Proc. Eoval Soc. vol. Ixxxi. p. 464 (1908). 



