120 Prof. E. Rutherford on Uranium Radiation and 



state of division of the compound, It is possible that the 

 apparently very powerful radiation obtained from pitchblende 

 by Curie * may be partly due to the very fine state of division 

 of the substance rather than to the presence of a new and 

 powerful radiating substance. 



The rate of leak due to the /3 radiation is, as a rule, small 

 compared with that produced by the a radiation. It is 

 difficult, however, to compare the relative intensities of the 

 two kinds. The a radiation is strongly absorbed by gases 

 (§8), while the /3 radiation is only slightly so. It will be. 

 shown later (§ 8) that the absorption of the radiation by the 

 gas is approximately proportional to the number of ions 

 produced. . If therefore the j3 radiation is only slightly 

 absorbed by the gas, the number of ions produced by it is 

 small, i. e. the rate of leak is small. The comparative rates 

 of leak due to the a and /3 radiation is thus dependent on the 

 relative absorption of the radiations by the gas as well as on 

 the relative intensity. 



The photographic actions of the a and /3 radiations have 

 also been compared. A thin uniform layer of uranium oxide 

 was sprinkled over a glass plate ; one half of the plate was 

 covered by a piece of aluminium of sufficient thickness to prac- 

 tically absorb the a radiation. The photographic plate was fixed 

 about 4 mm. from the uranium surface. The plate was exposed 

 48 hours, and, on developing, it was found that the darkening of 

 the two halves w T as not greatly different. On the one half of the 

 plate the action was due to the j3 radiation alone, and on the 

 other due to the a\ and ft radiations together. Except when 

 the photographic plate is close to the uranium surface, the 

 photographic action is due principally to the j3 radiation. 



§ 6. Transparency of Substances to the two Types of 

 Radiation. 



If the intensity of the radiation in traversing a substance 

 diminishes according to the ordinary absorption law, the 

 ratio r of the intensity of the radiation after passing through 

 a distance d of the substance to the intensity when the 

 substance is removed is given by 



r = e , 



where X is the coefficient of absorption and e=2*7. 



In the following table a few values of X are given for the 

 a and @ radiations, assuming in each case that the radiation is 



* C. R. July 1898, p. 175. 



