﻿Absorption of the fi Rays from Uranium. 381 



An examination of this table will show that though the 

 ratio Xjp is not subject to large variations, yet there is a 

 decided alteration as we pass from one element to another ; 

 and that consequently the stopping-power of a corpuscle is 

 not the same in atoms of different substances. 



On theoretical grounds also we might expect the nature of 

 the arrangement of the corpuscles within the atom to have 

 some effect upon the absorbing power. Thus Prof. J. J. 

 Thomson * has shown that the absorption of an atom in 

 which all the corpuscles were rigidly connected, would be 

 twice that of an atom containing the same number of freely 

 moving corpuscles. It is also quite possible that the presence 

 of so many charged corpuscles within the space of the atom 

 may produce a constraint in the aether analogous to specific 

 inductive capacity. This would affect the magnitude of the 

 forces between the fixed and the (3 corpuscles, and the extent 

 of the alteration would depend again upon the number and 

 arrangement of the corpuscles within the atom. 



Thus differences in the value of the ratio X/p for different 

 elements were to be expected, and it was considered desirable 

 to make an extended series of observations upon as many 

 elements as possible, with the object of investigating these 

 differences and their relation to the atomic weight. 



Experimental. 



Uranium oxide was used as the source of the /3 radiation, 

 the homogeneity of the rays and their fairly penetrating- 

 character more than compensating for the smallness of their 

 amount as compared with the radiation from preparations of 

 radium. The relative intensity of the radiation before and 

 after passing through the material under investigation was 

 measured by the relative ionization produced in the air of a 

 test-vessel. This is a measure of the energy of the rays, and 

 depends not only upon the number of corpuscles but also 

 upon their velocity. Lenard, however, has shown that there 

 is no appreciable change in the velocity of a ft corpuscle 

 during its passage through matter, at any rate for small thick- 

 nesses of material ; and this result is supported by results to 

 be described later in this paper. It has been shown theo- 

 retically t (and the result is borne out by a comparison of the 

 values obtained for the absorption-coefficient for rays of 

 different velocities) that the penetrating-power of the ft rays 

 varies as the fourth power of their velocity. Thus if there 

 were any appreciable diminution in the velocity of the rays 



* Phil. Mag. vol. xi. p. 779 (June 1906). 



t J. J. Thomson, ' Conduction of Electricity through Gases/ p. 315. 



