﻿Absorption of the j3 Rays from Uranium. 387 



n ' In addition to this periodic variation there is also an increase 

 in the value of the ratio \/p with increase of atomic weight. 

 This is clearly brought out by considering groups o£ similar 

 elements. Thus we have : — 



9 9 



Phosphorus 6-1 Copper 6*8 



Arsenic 8*2 Silver 8-3 



Antimony 9-8 Gold 9-5 



\ 



9 



Sulphur 6-6 



Selenium 8'65 



Tellurium 108 



It will thus be seen that passing down any group from 

 elements of lower to elements of higher atomic weight, there 

 is a continuous increase in the value of \/p. The rate of 

 increase, however, seems to fall off as the atomic weight 

 increases, and is distinctly less rapid after passing the second 

 long period. 



Allotropif. 



Several elements occur in two or more distinct forms, 

 differing among themselves in density, appearance, and other 

 physical and chemical properties. It was thought that these 

 different varieties might give slightly different values for the 

 ratio X/p. 



Accordingly a careful comparison was made between pure 

 graphite (p = 2'2o) and pure charcoal (p = 1'85) prepared from 

 grape-sugar ; but no difference whatever could be detected 

 between the values of \/p in the two cases. Similar expe- 

 riments were made with the different modifications of sulphur, 

 with a similar result. It appears therefore that, whatever 

 the nature of allotropy may be, it does not affect the absorbing- 

 power of the atom. 



Absorption of Compounds* 



The variation of the absorption per corpuscle with atomic 

 weight having been established, it remained to decide whether 

 the absorption depended upon the state of molecular combi- 

 nation ; or, in other words, if the forces which link together 

 the atoms of a compound influence in any w T ay the absorption 

 of the constituent atoms. A considerable number of com- 

 pounds have been tested with a view to determining this 

 point. 



2C2 



