produced by the Action of Thorium Compounds. 173 



This method can also be used to compare the radiations 

 from the various metals when made radio-active. In this way 

 it was found that all the substances tried, viz., Cu, Pb, Pt, 

 Al, Zn, brass, cardboard, paper, which had been made radio- 

 active, gave out radiations of the same penetrating power. 

 It was also found that the same type of radiation was given 

 out from polished and dull surfaces, and that it was unaffected 

 by the concentration of the radioactivity. 



Since the same radiation is given out by all the metals and 

 non-metallic substances like cardboard or paper, under varying 

 conditions, we may conclude that either the substance itself 

 which has been made radio-active plays no direct part in 

 determining the kind of radiation, or that all exert exactly 

 the same action. 



The "excited" radiation is also of a more penetrating 

 character than that given out by uranium, thorium, and the 

 pitchblende derivatives radium and polonium. 



Absorption of the Radiation in Air. 



The absorption of the induced radiation in air was also 

 determined. The method employed was similar to one pre- 

 viously used and described by the author * for determining 

 the absorption of uranium radiation by different gases. A 

 similar apparatus has been employed by Owens f for thorium 

 radiation. 



Two insulated parallel plates, kept a fixed distance apart, 

 could be moved by means of a screw to different distances 

 from the parallel radio-active surface. The radiation from 

 the active surface passed through a circular opening in the 

 lower plate, covered with thin aluminium-foil, and was stopped 

 by the upper plate. The current between the two fixed 

 plates for a large voltage was determined for different dis- 

 tances from the radio-active plate. If the radius of the 

 active surface is large compared with the distance of the lower 

 of the pair of plates from it, the current between the plates 

 for a distance x of the lower plate from the active surface 

 varies as e~ kx , where X is the coefficient of absorption of the 

 radiation in the gas. 



The following table gives the results obtained for the 

 radiation from a lead surface which had been made strongly 

 radio-active ; — 



* Phil. Mag. Jan. 1899, p. 124. 

 f Ibid. Oct. 1899, p. 378. 



