Mobilities of Ions in Air at High Pressures. 485 



by ice-crystals formed from condensed atmospheric water- 

 vapour, have been two disturbing factors which we have not 

 as yet been able satisfactorily to eliminate. It has been 

 difficult, too, to reduce the size of the ionization chamber 

 of the measuring apparatus to dimensions small enough to 

 permit of its use in a mass of liquid air small enough to be 

 jacketed and kept at a low temperature by an outside 

 vessel of liquid air maintained at a low temperature by 

 rapid evaporation. 



With regard to measurements on the mobilities of ions in 

 air at high pressures, however, it has been quite different, 

 for it has been found easy to make measurements on the 

 mobilities at all pressures up to as high as approximately 

 one hundred and ninety atmospheres, for such high pressures 

 were obtained quite readily by the use of a liquid-air com- 

 pressor. 



The only experiments which have been made hitherto 

 on the mobilities of ions in air at high pressures appear 

 to be the ones made by Dempster * and those made by 

 Kovarik f . 



In his work Dempster used pressures as high as 100 

 atmospheres, and he found that over the range from 1 atmo- 

 sphere to this limit the mobility of the positive ion made in 

 air, by alpha rays varied inversely as the pressure. He 

 found, however, that the mobility of the negative ion at 

 the higher part of the range did not appear to vary inversely 

 as the pressure, but it decreased less rapidly with the 

 pressure than it should have done if the inverse-pressure 

 law had been valid. Kovarik in his experiments, on the 

 other hand, worked with pressures from 13*3 to 74*6 atmo- 

 spheres, and over the whole of this range he found that 

 the mobilities of both positive and negative ions made in air 

 by alpha rays followed the inverse-pressure law. 



In the present investigation the mobilities of the two 

 kinds of ions were measured in air over a range of pressures 

 commencing at G6*8b' atmospheres and extending to 181'5 

 atmospheres. At the lower pressures of this range the 

 mobilities obtained agreed with the results of Kovarik, but 

 at the higher pressures it was found that the mobilities ot 

 the two kinds of ions began to approach each other in value, 

 and both decreased less rapidly with increases in pressure 

 than they should according to the inverse-pressure law. 



* Dempster, Pliys. Review, vol. xxiv. No. 1, Jan. 1912, p. 53. 

 | Kovarik, Proc. Roy. Soc. A. vol. lxxxvi. p. 154 (1912). 



