Jrom Faraday to J . J . Thomson. 407 



that " the negative ions diffuse more rapidly." This inference 

 was confirmed in 1898 by John Zeleny,* who showed that of 

 the ions produced in air by exposure to X-rays, the positive 

 are decidedly less mobile than the negative. 



The magnitude of the electric charge on the ions of gases 

 was not known with certainty until 1898, when a plan for 

 determining it was successfully executed by J. J. Thomson.f 

 The principles on which this celebrated investigation was based 

 are very ingenious. By measuring the current in a gas which 

 is exposed to Rontgen rays and subjected to a known electro- 

 motive force, it is possible to determine the value of the product 

 nev, where n denotes the number of ions in unit volume of the 

 gas, e the charge on an ion, and v the mean velocity of the 

 positive and negative ions under the electromotive force. As 

 v had been already determined ,J the experiment led to a 

 determination of ne ; so if n could be found, the value of e 

 might be deduced. 



The method employed by Thomson to determine n was 

 founded on the discovery, to which we have already referred, 

 that when X-rays pass through dust-free air, saturated with 

 aqueous vapour, the ions act as nuclei around which the water 

 condenses, so that a cloud is produced by such a degree of 

 saturation as would ordinarily be incapable of producing con- 

 densation. The size of the drops was calculated from measure- 

 ments of the rate at which the cloud sank ; and, by comparing 

 this estimate with the measurement of the mass of water 

 deposited, the number of drops was determined, and hence the 

 number n of ions. The value of e consequently deduced was 

 found to be independent of the nature of the gas in which the 

 ions were produced, being approximately the same in hydrogen 

 as in air, and being apparently in both cases the same as for 

 the charge carried by the hydrogen ion in electrolysis. 



Since the publication of Thomson's papers his general 

 conclusions regarding the magnitudes of e and m/e for gaseous 



* Phil. Mag. xlvi (1898;, p. 120. t Phil. Mag. xlvi (1898), p. 528. 



+ By E. Rutherford, Phil. Mag. xliv (1897), p. 422. 



