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LI. On the Absorption of Rontgen Radiation. 

 By W. J. Humphkeys*. 



THE present investigation was undertaken for the purpose 

 of determining whether the absorption of Rontgen radia- 

 tion depends only upon the kind of elements and their 

 amounts passed through, or whether it is dependent to some 

 extent also upon the manner in which these elements are 

 combined, That is, to determine whether the absorption 

 depends at all upon the grouping of the atoms into molecules 

 or only upon the atoms themselves ; in short, whether the 

 phenomenon is an atomic or a molecular one. 



As everyone knows, the transparency or opacity of a sub- 

 stance to ordinary light cannot be predicted from the pro- 

 perties in this respect of its constituents. Even allotropic 

 forms of a single element, as admirably illustrated by graphite 

 and the diamond, may differ to the greatest extent in respect 

 to their powers of transmitting ordinary light, and therefore 

 it seems certain that the absorption of light-rays is at least 

 largely a molecular phenomenon. The absorption of the 

 Rontgen radiation, on the contrary, judging from the expe- 

 riments presently to be described, is chiefly, if not entirely, 

 an atomic phenomenon, and consequently the absorption of 

 Rontgen rays due to a compound differs but little, if at all, 

 from the sum of those of its constituents. 



The experiments were arranged as follows : — Above a pro- 

 tected dry plate was placed a slab of uniform thickness of 

 some compound, and close beside this slab, on top of each 

 other, slabs of equivalent thicknesses of its constituent ele- 

 ments. Above these in turn, or rather directly above a point 

 halfway between them, was arranged the source of the Ront- 

 gen radiation. According to this arrangement, the average 

 amount of any element met by a ray in passing through the 

 compound is the same as that met by a ray (equally inclined 

 to the normal) in passing through the pile of its constituents. 

 Consequently if, after an exposure of suitable length, the 

 photographic plate is unequally affected beneath the compound 

 and the equivalent pile of its elements, one is forced to con- 

 clude that for some reason the absorption in the one case was 

 different from that in the other, provided of course that the 

 radiation was equally intense in the two directions, and also, 

 as is probably always the case, there was but little loss due to 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 44. No. 270. Nov. 1897. 2 G 



