Nolan — Ionic Mobilities in Air and Hydrocjen. 91 



reference in the second paper (p. ;i2, Nolan and Harris, Proc. R.I.A., 1922), 

 it is difficult to believe that Blackwood can have carefully read the papers 

 which he sets out to criticize. Blackwood states that, using gasometers to 

 produce the air-stream, he at first found singularities on the current- voltage 

 -curve, but that these disappeared when the gasometers were replaced by a 

 pumping arrangement. He considers that the pumping arrangement gave a 

 more uniform flow than the gasometers. It is possible that the converse was the 

 ■case, as the author, who has used both methods with identical results as far 

 .as mobilities are concerned, found that the gasometer method was the steadier. 

 Blackwood has modified the author's original experiment by using polonium 

 as the source of ionization, and thus getting rid of effects due to volume 

 ionization. Curves obtained by this method are reproduced, from which 

 ^mobility values of 1'96 and 2-13 are deduced. These results, " within 30 and 

 42 per cent, of the accepted value for moist air," are considered by Blackwood 

 as "sufficient to show that the method indicates the existence of only one 

 size, of ion of approximately normal mobility." It is difficult to make any 

 such deduction from the curves reproduced. The latter, in so far as evidence 

 bearing on this point can be obtained from them, are not inconsistent with 

 the present writer's two contentions : ( 1 ) that the normal ion is complex and 

 not simple, and (2) that among the components are small numbers of ions of 

 Mgh mobility value. As it is stated that the two curves upon which' 

 Blackwood relies were the only ones obtained, it cannot be said that the 

 .amount of proof which he brings forward is very strong. He appears to 

 consider that the failure of the present writer and Harris to detect the very 

 mobile ions in undried air by the alternating-field method is in some way a 

 confirmation of his present claim, i.e., that there is only one class of ion ; in 

 spite of the fact that these experiments showed the complex character of the 

 •ordinary ion, confirming the results got by the other method. As recorded 

 already in this paper, the writer has since obtained by the alternating-field 

 method ample evidence of the existence of the mobile ions in small quantities 

 in ordinary undried air. Blackwood also quotes Erikson (whose work has 

 been already referred to) in support of his views, although, if he accepts 

 Erikson's results, he must admit the existence of two positive ions. 



The general tendency of the results given in this paper is clear. A 

 detailed discussion must be postponed until the fnrther information is 

 available which the author hopes to obtain from an examination of very dry 

 and pure hydrogen. It is also hoped to obtain more accurate values for the 

 mobilities of the faster ions described in this paper, when, as is expected, 

 they will be brought into greater prominence by extreme drying. 



