684 Radioactivity of Ordinary Materials. 



described above, except that the current was taken between 

 coaxial cylinders, diameters 2*7 and 9' 8 cms. respectively. 

 The outer cylinder was provided with ends of the same 

 material as the walls, so that the air between the walls was 

 not exposed to the radiation of any other substance than that 

 which it was desired to examine. Experiments w r ere made 

 with walls of zinc and of two samples of tinfoil of different 

 activities. 



The results are tabulated below. 



Zinc. 



More active tinfoil. 



Less active tinfoil. 



Pressure (inches). 



Leak. 



Pressure (inches). 



Leak. 



Pressure (inches). Leak. 



3-9 



•66 



•95 



•7 



•9 -26 



107 



1-6 



30 



1-8 



9-8 11 



16-8 



20 



5-41 



34 



144 19 



21-8 



2-7 



106 



60 



202 2-5 



247 



2-6 



166 



fy5 



245 2-6 



29 



26 



231 



60 



301 2-3 



301 



2-8 



29-8 



62 



38-6 2-3 



38-6 



32 



39-2 



5-5 





These measurements are less accurate than might be wished 

 as they were made under somewhat unfavourable conditions, 

 at the Royal Institution. The vibration from the machinery 

 running in the basement made it impossible to take exact 

 readings*. 



It is clear, however, that the current due to radiation from 

 the zinc walls does not reach a limit within the range of 

 pressure experimented upon. With the more active tinfoil 

 the limit is reached at about 10 inches pressure, while 

 with the other specimen of tinfoil 20 inches are required 

 before the current reaches its full value. We may infer that 

 the radiation from different samples of the same material 

 varies, not only in quantity, but in quality also. 



It was desired to compare the radiation from tin and zinc 

 with that from uranium nitrate. Some small fragments of 

 the latter were cemented at intervals over the surface of the 

 cylinders, and the rate of leak taken for various pressures. 

 It was found that the leak began first to diminish con- 

 siderably at 20 inches pressure, although, owing to the 

 presence of a small proportion of penetrating rays in uranium 

 salts, the limit was not very definite. It appears that the 

 a radiation from uranium has about the same penetrating 

 power as the radiation from the second specimen of tinfoil. 



* This remark does not apply to the measurements with the first 

 apparatus, which were much more accurate, 



