Spontaneous Ionization of Air in Closed Vessels. 551 



existence of this last radiation is very important from its 

 bearing on the question of the radioactivity of ordinary 

 materials. Strntt * had previously found that the ionization 

 of the gas in a vessel varied with the material of which the 

 walls of the vessel were made. He found considerable 

 differences, however, between different samples of the same 

 material, and was inclined to conclude that the intrinsic 

 radiation from the walls of the vessel was due to traces 

 of radioactive impurities. Such very small traces would be 

 sufficient to account for the effects observed, and the distribu- 

 tion of radium is so universal, that the explanation seemed 

 highly probable. An obvious way to attack the problem was 

 to repeat the experiments of Strntt, using different samples of 

 rlie >ame metals, and to compare the results. This was done 

 in the present investigation, and it was found that the order 

 obtained for the various metals tried was exactly the same as 

 that given by Strutt with one exception. Xot only so, but 

 the numbers obtained showed a considerable agreement, 

 'specially when the difficult character of the research is 

 taken into account. Similar experiments were also per- 

 formed in the Cavendish Laboratory by Campbell f, and the 

 order obtained by him was again the same. McLennan' s J 

 results for lead, tin. and zinc further confirm this order ; 

 and so it seems reasonable to conclude that although 

 radioactive impurities may cause differences between the 

 values obtained for different samples of the same metal, those 

 differences only partially conceal the fact that each metal 

 gives out a specific intrinsic radiation, the amount of which 

 varies very much from one metal to another. This is strong 

 evidence in favour of the view that all matter is radioactive, 

 and suggested the desirability of collecting more evidence 

 bearing on the point. A search was accordingly commenced 

 for any traces of emanation from the metals used in the 

 experiment-. This search was unsuccessful in obtaining any 

 direct evidence of the existence of sucb emanations. It was 

 found that if a current of air was passed over heated tin, 

 something was carried off with it which was capable of 

 increasing the ionization in a closed vessel. It was at first 

 supposed that this might be an emanation, but when the gas 

 from the heated tin was drawn through a tube to which a 

 strong electric field was applied, it failed to affect the ionization 

 in the vessel. The immediate cause of the increased ionization 

 was therefore, presumably, free ions. Those may, of course, 

 have been formed by the action of an emanation which itself 

 ' Phil. Mi j. June L908. t Ante, p. 53] 



: L'hyfl. Rev. Lv. L903. 



