792 Mr. G. W. Todd on the Mobility of Positive Ions ■ 



a wide range of pressures Rutherford *, Langevin t, and 

 Wellisch % nave shown that the product of pressure and 

 mobility is constant for a particular gas. Langevin took 

 observations down to a pressure of 7*5 cm. of mercury, and 

 found in the case of the negative ions in air and carbon 

 dioxide when the pressure was below 20 cm., that the 

 product of pressure and mobility increased rapidly with 

 diminution of pressure, while in the case of the positive ions 

 there was only a tendency to increase. Kovarik § has 

 determined the mobilities of the negative ions in air and 

 carbon dioxide down to pressures of 1 cm., and he finds that 

 the product of the pressure and mobility continues to increase 

 very rapidly. 



The writer has shown || that the mobility of the positive 

 ions produced by means of X-rays in gases at low pressures 

 obeys the inverse pressure law down to a few milimetres of 

 mercury. The experiments could not be carried further 

 owing to the fact that the ionization produced by X-rays 

 becomes very feeble at low pressures. 



From his experiments, Langevin came to the conclusion 

 that the abnormal increase in the mobility at low pressures 

 was due to the dissociation of groups of molecules, of which 

 he considered the ion to consist at higher pressures. There 

 is evidence also, at any rate for the negative ion, for the view 

 that the ion consists of a single molecule with a corpuscle 

 attached, and that the latter at low pressures is continually 

 migrating from one molecule to another, thus existing for a 

 fraction of time in the free state. 



It would be interesting to see whether the positive ion 

 behaves in a manner similar to the negative ion at low 

 pressures. The difficulty in such an investigation is in 

 obtaining a sufficient number of ions to work with at low 

 pressures. It is well known, however, that certain salts 

 when strongly heated yield a copious supply of positive 

 electricity, and this fact solved the difficulty. The experi- 

 ments described in the paper last quoted were continued, 

 using as a source of positive ions strongly heated aluminium 

 phosphate — an especially good salt for the purpose — with the 

 object of measuring mobilities at much lower pressures. 



* Camb. Phil. Soc. Proc. ix. p. 410. 



t Annal»s de Ckimie, vii. p. 28 (1903). 



t Phil. Trans. A. ccix. p. 249. 



§ Phys. Rev. xxx. 4, p. 420. 



|| Camb. Phil. Soc. Proc. xvi. p. 21. 



