752 



Dr. C. G. Barkla and Mr. C. A. Sadler on 



It is also seen that in the cases of absorption which result 

 in the emission of the secondary radiation characteristic of 

 the absorbing substance there is considerable deviation from 

 this relationship, the absorption being invariably greater by 

 a considerable amount than would be produced if the simple 

 proportionality still held. There is thus a special absorption 

 and a very large one invariably connected with the emission 

 of the secondary radiation which is characteristic of the 

 absorbing element. 



The point is so important that one curve will be treated in 

 detail as typical of all substances. Beginning with a very 

 solt homogeneous primary radiation, the absorption in a 

 substance B (say) is considerable. As the radiation is made 

 more penetrating the absorption in aluminium and in B 

 diminish proportionate] y, as shown by the portion of the 



Fig. 4. 



//v fil of Pft/MAfty /Pays 



/l&SORPTION /N M Or SECONDARY ft/ID/ AT/ON (CHfiffACTEfl/3TK OfSj 



curve PQ (fig. 4). This proportionality continues until the 

 primary radiation is just as penetrating as the secondary 



