Up to the year 1908 the only experiments which threw any light whatever upon the question as to what the act of ionization of a gas consists in were those performed by Townsend[56] in 1900. He had concluded from the theory given on p. 34 and from his measurements on the diffusion coefficients and the mobilities of gaseous ions that both positive and negative ions in gases carry unit charges. This conclusion was drawn from the fact that the value of in the equation came out about , as it does in the electrolysis of hydrogen. In 1908, however, Townsend[57] devised a method of measuring directly the ratio and revised his original conclusions. His method consisted essentially in driving ions by means of an electric field from the region between two plates and (Fig. 11), where they had been produced by the direct action of X-rays, through the gauze in , and observing what fraction of these ions was driven by a field established between the plates and to the central disk and what fraction drifted by virtue of diffusion to the guard-ring . [Pg 126] By this method Townsend found that for the negative ions was accurately , but for the positive ions it was . From these results the conclusion was drawn that in X-ray ionization all of the positive ions are bivalent, i.e., presumably, that the act of ionization by X-rays consists in the detachment from a neutral molecule of two elementary electrical charges. Fig. 11 Townsend accounted for the fact that his early experiments had not shown this high value of for the positive ions by the assumption that by the time the doubly charged positive ions in these experiments had reached the tubes in which was measured, most of them had become singly charged through drawing to themselves the singly charged negative ions with which they were mixed. This hypothesis found some justification in the fact that in the early experiments the mean value of for the positive ions had indeed come out some 15 or 20 per cent higher than —a discrepancy which had at first been regarded as attributable to experimental errors, and which in fact might well be attributed to such errors in view of the discordance between the observations on different gases.