754 Prof. R. A. Millikan and Mr. H. Fletcher oti the 



By this method Townsend found that ne for the negative 

 ions was accurately 1*23 X 10 10 , but for the positive ions it 

 was 241 x 10 10 . From these results the conclusion was 

 drawn that in X-ray ionization all of the positive ions are 

 bivalent, i. <?., presumably, that the act of ionization by X rays 

 consists in the detachment from a neutral molecule of two 

 elementary electrical charges. 



Townsend accounted for the fact that his early experiments 

 had not shown this high value of ne for the positive ions by 

 the assumption that by the time the doubly charged positive 

 ions in these experiments had reached the tubes in which D 

 was measured, most of them had become singly charged 

 through drawing to themselves the singly charged negative 

 ions with which they were mixed. This hypothesis found 

 some justification in the fact that in the early experiments 

 the mean value of ne for the positive ions had indeed come 

 out some 15 per cent, or 20 per cent, higher than 1*23 X 10 10 

 — a discrepancy which had at first been regarded as attribu- 

 table to experimental errors, and which in fact might well 

 be attributed to such errors in view of the discordance 

 between the observations on different gases. 



Franck and Westphal*, however, have recently redeter- 

 mined ne by a slight modification of Townsend's original 

 method, measuring both K and D independently, and have 

 not only found exactly the same value of D when the positive 

 and negative ions are separated by means of an electric field 

 so as to render such recombination as Townsend suggested 

 impossible as when they were not so separated, but they 

 have also found ne for the positive ions produced by X rays 

 but 1*4 x 10 10 instead of 2*41 x 10 10 . Since this is in fair 

 agreement with Townsend's original mean, the authors con- 

 clude that only a small fraction — about 9 per cent. — of the 

 positive ions formed by X rays are doubles, or other multiples, 

 and the rest singles. The authors attempt to reconcile the 

 discrepancy between their result and Townsend's later finding 

 b}^ pointing out that the singly charged positives formed 

 between A and B (fig. 1) would have twice the diffusion 

 coefficient of the doubly charged positives, and would there- 

 fore largely be lost to the wires of the gauze g in passing 

 through it, while a much larger proportion of the slowly 

 diffusing positives would pass through the meshes. In other 

 words, the gauze would strain out the doubles from the 

 mixture by the process of fractional diffusion. In support 

 of this point of view they placed several very narrow-meshed 

 pieces of gauze in the path of the ions which were moving 



* J. Franck and W. Westphal, Verh. D. Phys. Ges: Mar'z 5, 1909. 



