682 Hon. R. J. Strutt on Radioactivity 



Before making an experiment the insulation, which is all 

 important, was tested, m was permanently exhausted, and 

 the stopcock p closed, b was also exhausted, for the time, 

 and a charge given to the brass strip k, so that the leaf / 

 diverged. An interval of one hour was allowed in order that 

 any absorption of the charge by the glass insulation h should 

 have time to take place. After that it was found that 

 there was no measurable movement of the leaf over the scale- 

 divisions in two or three hours ; i of a division would have 

 been visible. 



When the vacuum test of insulation had been made, dry 

 air was admitted into the vessel b. It was assumed that the 

 glass insulation, which was satisfactory in a vacuum, re- 

 mained so when air which had been dried by passage 

 through phosphoric anhydride was admitted in contact with 

 it. As soon as the air was admitted into b, a leakage of 

 electricity from the wire a was observed. 



The amount of this leakage, in scale-divisions per hour, 

 with various materials surrounding the charged wire, is given 

 be]ow. 



Tinfoil 3*3 



Ditto, another sample 2'3 



Glass, coated with phosphoric acid 1*3 



Silver, chemically deposited on glass ... 1*6 



Zinc 1-2 



Lead 2*2 



Copper (clean) 2'3 



Ditto, thoroughly oxidized 1*7 



Platinum (various samples) 2*0, 2*9, 3*9 



Aluminium 1*4 



It appears, then, that there are very marked differences in 

 the rate of leak when different materials constitute the w^alls 

 of the vessel. There can, therefore, be little doubt that the 

 greater part, if not the whole, of the observed ionization of 

 air is not spontaneous at all, but due to Becquerel rays from 

 the vessel. 



It is, I think, interesting to find that the phenomena of 

 radioactivity, which have hitherto been regarded as rare and 

 exceptional, are really everywhere present. The rate of leak 

 with various pieces of tinfoil from the same stock was always 

 the same, as nearly as the experiments could show. But, as 

 may be seen in the table, a piece from another stock gave a 

 different amount of leakage. The same holds good for plati- 

 num, one specimen tried being tw T ice as active as another 



