the Fluorescent Ronlgen Radiations. 403 



in the absorption of X-rays with variation of penetrating 

 power is determined solely by the fluorescent or characteristic 

 radiation of the absorbing substance. Thus as the general 

 absorbability of a radiation decreases — that is as its penetrating- 

 power increases — it becomes less and less absorbed in a sub- 

 stance R, say, its absorption being approximately proportional 

 to its absorption in any other substance when the radiations 

 used are not near on the more penetrating side of the radia- 

 tion characteristic of this second substance. This approximate 

 proportionality holds until the radiation absorbed becomes 

 just more penetrating than the fluorescent radiation charac- 

 teristic of R. Then as the radiation becomes more penetrating- 

 it is absorbed more in R than would be given by this law, 

 the additional absorption being accompanied by the emission 

 of fluorescent radiation from R, by an increased corpuscular 

 radiation from R, and if R is in a gaseous state by an addi- 

 tional ionization in R. The relation between the absorption 

 in R and in any other substance whose characteristic radia- 

 tions are not near on the more absorbable side to the radiations 

 experimented upon is shown in fig. 2. 



Fig 2. 



/'V., 



\SPEcrffAL un£ of Fluorescent Radjat/on from /?'. 



ZlBSORBABiL 1 7Y (MEASURED /Ft Al . ) 



The special point to be observed is the accuracy of this 

 relation in all cases experimented upon. The simple law of 

 approximate proportionality is obeyed until the absorbability 

 of the primary radiation is just that of the radiation charac- 

 teristic of the absorbing substance ; the slightest increase in 



