286 SCIENCE PROGRESS 



PHYSICS. By James Rice, M.A., University, Liverpool. 

 Ionisation. — In the Phil. Mag. for April and May, Prof. J. A. 

 Pollock publishes the results of some work on the nature of 

 the large ions in the air, which were discovered by Langevin 

 in 1 90s and have a mobility approximately equal to 1/3000 ; 

 and also on the nature of a new type of ion in the air, with 

 a mobility intermediate between that of the Langevin ion 

 and the small gas ion of mobility 1*5. He summarises a good 

 deal of information concerning the Langevin ion, which he 

 considers to be a collection of water molecules surrounding 

 a dust particle, the whole being electrified by the attachment 

 of a small ion, and so affording an interesting example of the 

 adsorption of water vapour at a rigid surface. The applica- 

 tion of a thermodynamic argument leads to the conclusion 

 that the adsorbed moisture is in the liquid state with a latent 

 heat and density little different from those of water. This 

 ion, it would appear, has a mobility which at constant atmo- 

 spheric pressure is a function of the relative humidity only, 

 and a diameter varying from 3 to 4 micromillimetres according 

 to atmospheric conditions. Regarding the second type of ion, 

 it would seem that it cannot exist in the air if the vapour 

 pressure is above 17 mms., and its mobility (on the average 

 about 1/50) depends on the vapour pressure rather than on the 

 relative humidity. It is probable that it consists of a rigid 

 nucleus enveloped in a dense atmosphere of water-vapour, 

 the mass of the ion becoming greater as the vapour pressure 

 increases until the absorbed fluid assumes the liquid state and 

 the aggregation develops into the large ion of Langevin, which 

 alone exists above the critical pressure mentioned above. 



Dr. N. Campbell {Phil. Mag. June) gives a preliminary 

 account of some experiments still in progress, on the ionisation 

 produced by the positive particles liberated from heated 

 sodium and aluminium phosphates, the ionisation consisting 

 of the liberation of electrons from the surface of a copper 

 plate on which the positive particles impinge. Curves are 

 given showing the connection between the P.D. through which 

 the particles fall and the number of electrons liberated by 

 each positive particle. This number reaches a maximum at 

 a P.D. between 30,000 and 40,000 volts, the maximum being 

 in the neighbourhood of 3. The suggestion is made that the 

 fall in the number beyond this P.D. is due to the fact that 



