128 Prof. Bumstead, On the absence, etc. 





Lead. 



High 



speed. 





Disc at rest 





Disc in motion 



(1) 



Balance 





— 



(2) 



— 





0-20 



(3) 



0-22 





— 



(4) 



— 





0-35 



(5) 



0-35 





— 





Tin. 



High 



speed. 





Disc at rest 





Disc in motion 



(1) 



0-15 





— 



(2) 



— 





0-11 



(3) 



0-10 





— 



Some comparisons will serve to give an idea of the delicacy of 

 the test represented by these numbers. When the radium was 

 absent and both vessels charged alike, the leak due to " spon- 

 taneous" ionization was 0'87 volts in 10 minutes. With the 

 radium in place and the compensating cylinder disconnected, the 

 leak in the other cylinder due to the rays which had penetrated 

 55 cm. of lead was at the rate of 42' volts in 10 minutes. With 

 the radium in place and the two cylinders balanced, 0*250 grains 

 of potassium uranium sulphate were spread as uniformly as possible 

 over a circle of the same diameter as the aluminium window and 

 placed on the disc immediately below the window. This gave a 

 leak of 1'82 volts in 10 minutes. Thus a distribution of uranium 

 salt, over the disc, whose surface-density was 1 milligram per 

 square centimetre would give a leak under the experimental con- 

 ditions of 0T7 volts in 10 minutes; and the, results quoted above 

 show clearly that the difference between the effects with the disc 

 at rest and in motion is much less than this. 



With the four substances tested therefore the result is nega- 

 tive. If they retain the power of giving out any rays capable of 

 penetrating - 7 cm. of air and 00005 cm. of aluminium, for 0'009 

 seconds after exposure to the /3 and 7 rays from 30 milligrams of 

 radium, these rays must be considerably less intense than those 

 due to a layer of uranium salt whose surface-density is 1 milli- 

 gram per square centimetre. 



In conclusion, I wish to thank Professor Thomson for the 

 privilege of working in the Cavendish Laboratory, and for many 

 valuable suggestions during the course of these experiments. 



