Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/427

 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 The principles on which this celebrated investigation was based are very ingenious. By measuring the current in a gas which is exposed to Röntgen rays and subjected to a known electromotive force, it is possible to determine the value of the product, where denotes the number of ions in unit volume of the gas,  the charge on an ion, and  the mean velocity of the positive and negative ions under the electromotive force. As had been already determined, the experiment led to a determination of ; so if  could be found, the value of  might be deduced.

The method employed by Thomson to determine 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 condensation. The size of the drops was calculated from measurements 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 of ions. The value of 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 and  for gaseous