254 Prof. J. J. Thomson on the Theory of Conduction of 



ultra-violet light by Rutherford ; for the ions in flames by 

 McClelland and H. A. Wilson; and for the ions in gases 

 near to incandescent metals or to the arc discharge by 

 McClelland. The velocities in the different cases vary very 

 much ; the velocity of an ion in the same gas is much the 

 same whether the conductivity is due to Rontgen rays, 

 uranium rays, or ultra-violet light; it is much smaller when 

 the conductivity is produced by an arc or by incandescent 

 metal. Thus the mean velocity of the positive and negative 

 ions under a volt per centimetre in air exposed to Rontgen 

 rays was found by Rutherford to be about 1'6 cm./sec, while 

 for gas drawn from the neighbourhood of an arc-discharge 

 in carbonic acid the mean velocity of the positive and negative 

 ions was found by McClelland to be only "0035 cm./sec. 



This difference is caused by the ions acting as nuclei about 

 which condensation, whether of the gas around them or of 

 water-vapour present in the gas, takes place. The power 

 of these ions to act as nuclei for the condensation of water- 

 vapour is strikingly shown by C. T. R. Wilson's * experi- 

 ments on the effects of Rontgen and uranium radiation on 

 the formation of clouds, and also by R. v. HelmhohVsf 

 experiments on the effects produced by ions on a steam jet. 

 If the size of the aopreoation which forms round the ion 

 depends on the circumstances under which the ion is liberated 

 and the substances by which it is surrounded, the velocity 

 which the ion acquires under a given potential gradient will 

 also depend on these circumstances, the larger the mass of 

 the aggregation the smaller will be this velocity. A remark- 

 able result of the determination of the velocities acquired by 

 the ions under the electric field is that the velocity acquired by 

 the negative ion under a given potential gradient is greater 

 than (except in a few exceptional cases when it is equal to) 

 the velocity acquired by the positive ion. Greatly as the 

 velocities of the ions produced in different ways differ from 

 each other, yet they all show this peculiarity. The relative 

 velocities of the negative and positive ions differ very much 

 in the different cases of conduction through gases ; thus in 

 the case of imperfectly dried hydrogen traversed by the 

 Rontgen rays, Zeleny found that the speed of the negative 

 ions was about 25 per cent, greater than that of the positive, 

 while in the case of conduction through hot flames H. A. 

 Wilson found that the velocity of the negative ion was 17 or 

 18 times that of the positive. In the case of the discharge 

 through vacuum-tubes, the measurements which I made of 



* Wilson, Phil. Trans. A, 1897 ; Proceedings of Cambridge Phil. Soc. 

 vol. ix. p. 333. 



t R. v. Helmholtz, Wied. Ann. vol. xxvii. p. 509 (1886). 



