90 Proceedings of the Royal Irish Academy. 



TJnder the first head we have the author's evidence by three different 

 methods for air and the present ■work on hydrogen. We find also that 

 Erikson,- using a method resembling the author's air-stream method, and 

 Wahlin,- using an altemating-field method, are prepared to admit the co- 

 existence in air of positive ions of mobility about I'SO and 1'35. Erikson 

 believes that these represent ions composed of one molecule and two 

 molecules respectively. How then are we to explain the mobility found for 

 the positively charged recoil-atom ? ^ 



With regard to the high-mobility ions, we have, fii-st, the work of 

 Altberg.- quoted in a previous paper. Using Zeleny's method of bringing 

 the ions in an air-stream to rest by an opposing field, and using X-rays as 

 the source of ionization, Altberg finds for the mobility of the fastest ion 

 in air the value lO-l. He does not distinguish different groups, but finds 

 that there are ions present of mobilities up to this value. Then we have the 

 work of Haines on hydrogen. Yen,^ on repeating Haines' work, was unable 

 to find the mobile ions, but we cannot be sure that his expeiimental 

 conditions were favourable. (We may note in passing that for the mobility 

 of the positive ion in air, Ten finds a set of values ranging from I'lO to 

 1*22, with a mean value 1'14, using low voltages and low frequencies; 

 while for high voltages and high frequencies he finds values fi-om 1-55 to 

 I"66, mean value 1-61. He gives, without comment, a mean of 114 and 1"61 

 as the mobility of this ion.) Finally we have the work of the author by two 

 methods for air and the work on hydrogen now reported, which agrees with 

 the results of Haines. 



Funher Criticism hy Blackwood. 



When this paper was just completed, the 3!fovember number of the 

 Physical Eeview reached Dublin (December 4). This contains a paper by 

 Blackwood, which is an extension of the biief paper already referred to. 

 The criticism based on the alleged neglect by the present writer of the general 

 volume ionization in his original air-stream method is repeated. In view of 

 the quotation already given from the author's first paper, and of a similar 



' Eriksun, Physical Review, Angast, 1922. 

 - Wahlin, Physical Review, September, 1922. 



3 Rutherford. PhiL Mag.. 5, p. 95. 19<33, and Franck, Verb. d. D.P.G., 11. p. 397, 

 1909- 



* Altberg. Ann d. Physik, 37, p. 849, 1912. 

 » Ten, Physical Review, si, p. 337, 1918. 



