lonisation in Dried Air. 461 



from day to day, but in no case was the ratio of increase less 

 than 2. 



At first, it was thought that this effect might be due to the 

 low temperature and dependent upon the metal of which the 

 U-tube was composed. The copper tube was therefore replaced 

 by a glass one of approximately the same section, and the experi- 

 ment was repeated — but without any change in the result. 



The uranium oxide, the ionising agent, was then suspected ; 

 if it gave off an active gas which condensed at the low temperature 

 of liquid air, the results obtained might perhaps be explicable. 

 Obviously this action could be eliminated by employing another 

 ionising agent. The simplest that suggested itself was the natural 

 ionisation of the air. It is well known that under normal con- 

 ditions there is present in the atmosphere a quantity of radium 

 emanation sufficient continually to produce small ions of both 

 signs at the approximate rate of 30 per cubic centimetre. This 

 gave a small but fairly constant source of ionisation to work on. 

 Here again, the drying (or cooling) of the air almost trebled the 

 ionisation, and the uranium was restored to its place. 



The only other possible causes for the phenomenon which 

 suggested themselves were diffusion, recombination, and conversion 

 of large ions into small ones. 



Diffusion was first tested. As was pointed out in the intro- 

 ductory section of this paper, the coefficient of diffusion of negative 

 ions in air is 50 7o greater than that of positive ions, and this 

 should lead to an excess of positive ions, if conditions were such 

 as to allow of such action. Although it seemed extremely unlikely 

 that the large tubing used could account for the effect, the matter 

 was tested directly. U -tubes of two different bores, one greater 

 and one smaller than that originally used were inserted, but with- 

 out any appreciable change in the result. 



The question of conversion of large ions into small ones 

 promised to give an explanation of the phenomenon. These 

 large ions, discovered by Langevin* in 1905, have a mobility 

 of only 3 x 10~* centimetres per second and occur naturally in the 

 atmosphere, there being normally about 2000 of each, sign in a 

 cubic centimetre of air. The dimensions of the testing apparatus 

 were such that only a negligibly small portion of them were 

 caught. In a recently published paper, M. de Broglie-f" has shown 

 that these large ions are produced by the combination of ordinary 

 small ions (whose dimensions are of molecular order) with large 

 " neutral centres," whose linear dimensions are 100 times that 

 of a molecule, that is about the size of ultra-microscopic particles 

 or particles suspended in colloidal solutions, namely about 10 fifi. 



* Langevin, C. R. cxl. 1905, p. 232. 

 t de Broglie, Journ. de Phys. Dec. 1909. 



VOL. XV. PT. V. 30 



