produced by the Action of Thorium Compounds. 163 



fully scrubbed with sand- or emery-paper, the radio-active 

 power was to a great extent destroyed. It was found possible 

 to make the plate B active, even if the thorium oxide were 

 covered with 30 layers of foolscap paper tightly waxed down 

 so as to prevent the escape of dust particles. 



If the plate C was charged — and B -h , the plate B no 

 longer became radio-active, but the top layer of paper over 

 the thorium was found to be active on its upper side to about 

 the same extent as the plate B in the previous case ; i. <?., the 

 negatively charged surface was made active in both cases. 



All the compounds of thorium examined have the power of 

 causing radioactivity in substances. The oxide, however, 

 gives far the largest effects, and has consequently been used in 

 most of the experiments. 



The thorium compounds used were supplied by Messrs. 

 Eimer & Amend, New York. The oxide was obtained by 

 igniting the nitrate which had been manufactured from 

 monazite sand. If the oxide is heated for some hours to a 

 whi.te heat in a platinum crucible, it loses its power of 

 exciting radioactivity in substances to a very large extent. 



Comparison of Intensities of Radiation. 



The intensity of the radiation, excited in substances in the 

 manner described, was in all cases compared by the electrical 

 method. In general, for the purposes of measurement, the 

 radioactivity was excited in flat plates or circular cylinders. 



For flat plates the testing apparatus was similar to fig. I, 

 The brass plates corresponding to B and C were 5 cms. apart, 

 with a potential-difference of 50 volts between them. The 

 current between the plates, measured by the rate of movement 

 of the electrometer-needle, was taken as proportional to the 

 intensity of the radiation at the surface. With radio-active 

 cylinders, the active cylinder was placed in a larger cylinder 

 and concentric with it. The current for 50 volts between the 

 cylinders was taken as a measure of the intensity of the 

 radiation at the surface. 



For experiments, extending in some cases over several days 

 or weeks, it was necessary that for each observation the electro- 

 meter should be of the same degree of sensitiveness. This was 

 roughly ensured by the Thomson Beplenisher and Gauge, 

 attached to the electrometer. For small variations from the 

 standard sensitiveness, the values of the current were corrected 

 by observing the number of divisions on the electrometer-scale 

 corresponding to the E M.F. of a Clark cell. 



As in the course of this paper it will be necessary to compare 



M 2 



