the Cause and Nature of Radioactivity. 573 



trodes E, F. H in each case, keeping the current of air 

 constant. If the ratio of the currents is the same we may 

 conclude that the radiating power of each diminishes at the 

 game rate. The comparison of the emanation is thus rendered 

 qualitative as well as quantitative. In most of the experiments 

 the current to the electrode E was about twice that to the 

 electrode H ; the velocity of the current of air along the 

 cylinder was thus about 0*8 cm. a second. 



Comparison of Emanating Poicer. — The experiments in all 

 cases on the amount of emanation from different substances 

 are comparative. The standard of comparison was usually a 

 sample of 10 grams of thoria as obtained from the maker, 

 which gave out a conveniently measurable quantity of ema- 

 nation. Preliminary experiments were made to find the 

 connexion between the weight of thoria and the amount of 

 emanation as tested in the cylinder. The following numbers 

 .show that the amount of emanation is within the limits of 

 accuracy desired directly proportional to the weight of sub- 

 stance : — 





Divisions of scale 



r eight of thoria. 



per second. 



'2 grams. 



1-4:1 



i „ 



2-43 



10 „ 



(5-33 



20 „ 



13-2 



Correction for Natural Leakage. — Even with no emanating 

 material in C the electrometer generally indicates a slight 

 movement on separating the quadrants. This is caused by a 

 small current, chiefly made up of leakage due to conduction 

 over the ebonite, as well as the current produced by the 

 excited radioactivity which has collected on the negative 

 electrode during the course of the day's experiments. It 

 varies from day to day, and is as a rule negligible ; but in 

 case of bodies possessing very low emanating power it is 

 necessary to correct for it. The number ot divisions of the 

 scale per second indicated by the electrometer-needle when 

 no emanating material is present, is subtracted from the 

 number obtained with the specimen being tested. The 

 corrected number indicates the current due to the emanation 

 alone. 



Alternative Method of Comparing Emanating Power. — The 

 apparatus (fig. 1) described in the first paper (Phil. Mag. 

 Sept. 1902) for the comparison of radiations, can also be quite 

 well employed for a comparison of emanating power. In this 

 case, a thick layer of thoria (several grams) is spread over 

 the plate and covered with two thicknesses of ordinary paper. 



