206 Prof. J. S. Townsend on 



for positive ions was very large, that possibly some of the 

 ions had double charges, and " in order to come to a definite 

 conclusion with regard to the charges 3i further experiments 

 would be required (Phil. Trans, vol. cxcv. 1900, p. 277). 



The reason why a definite conclusion was not possible in 

 the case of the point discharge was that the rate of diffusion 

 could have been influenced by the presence of the nitric acid 

 produced by the discharge, which would condense on the 

 ions. I was aware at the time that ions with different rates 

 of diffusion could easily be obtained in this case, with both 

 positive and negative ions. The largest values of the rates 

 of diffusion were therefore compared with the velocities, and 

 the smallest values obtained for Nx^X 10 10 were much 

 larger for positive ions than for negative ions, in the ratio 

 1-66 to 1-46. 



The experiments with Rontgen rays do not involve so 

 many difficulties. The velocities and rates of diffusion were 

 found under very similar conditions, and the high values of 

 the product Nxg obtained for positive ions cannot possibly 

 be attributed to the cause suggested by Professor Millikan, 

 namely, that the rate of diffusion is slow owing to the pre- 

 sence of positive ions of comparatively large mass with single 

 charges. Such ions would affect the determination of the 

 velocities by Zeleny's method to a great extent, as by that 

 method it is the velocity of the slowest of the ions that would 

 be measured, and if the mean rate of diffusion is compared 

 with that velocity the number obtained for ~N xe would be 

 too small. 



With regard to some experiments I have made recently 

 (in which no error due to impurities can come in, as the 

 values of N x e can be found from a single experiment in- 

 volving only the determination of the ratio of two charges), 

 Professor Millikan suggests that the results are not con- 

 sistent as a high degree of accuracy was not obtained by 

 Mr. Haselfoot in some experiments of a similar kind, which 

 he made with rays from a radioactive substance (C. E. 

 Haselfoot, ' Proceedings of the Royal Society/ vol. lxxxii. 

 1909). 



When I first published this method I pointed out that the 

 results were reliable inasmuch as the same values of N x e 

 were obtained in a set of experiments with different forces, 

 pressures, and intensities of ionization (Proc. Roy. Soc. vols. 

 Ixxx. & lxxxi. 1908). The actual numbers of all these ex- 

 periments were not given, but it may be of interest to quote 

 them. The observations were as follows : — 



