of Ronfgen Rays in Transmission through Matter. 667 



paper, it will be gathered that a primary beam, after passing 

 through a thin sheet o£ iron, is deficient in those constituents 

 which are especially capable of exciting iron radiation. If, 

 then, the issuing beam is absorbed by a further sheet of iron 

 of the same thickness, we should expect the second sheet to 

 absorb much less. This is well shown in the first example 

 above, for the iron sheet which previously absorbed 60 per 

 cent, now only absorbs 52' 6 per cent. 



A similar line of reasoning explains why the beam is 

 apparently harder to nickel after passing through nickel. 



An examination of Table VI. shows clearly that nickel 

 absorbs those constituents which especially excite the charac- 

 teristic radiation of iron (p. g. those of the hardness of nickel 

 and copper radiations) to a much less extent than it absorbs 

 more penetrating radiations (e.g. those of the hardness of Zn, 

 As, Se, &c). But Table VII. column IV. clearly shows that 

 the beam after passing through iron is richer in those con- 

 stituents which can be specially absorbed by nickel, for these 

 are cut down to a lesser extent than the average. 



As a further test of this point, the secondary radiations 

 from iron and nickel respectively were measured before and 

 after cutting down the primary beam by iron. A reference 

 to the last column of the first example shows that while the 

 whole beam is reduced by 60 per cent., the constituents 

 specially capable of exciting secondary homogeneous radia- 

 tion in nickel are only reduced by 54*9 per cent.; it also well 

 illustrates the fact that iron selectively absorbs those con- 

 stituents specially exciting radiation in itself — this absorption 

 65 per cent, being above the average. 



The results given in example II. equally confirm these 

 views. 



The phenomena so far described can be readily duplicated 

 with abeam composed of suitable proportions of various homo- 

 geneous radiations. For instance, a beam composed of the 

 homogeneous radiations from iron, nickel, zinc, and arsenic 

 each equally contributing to the ionization produced, shows 

 the following properties when tested by the sheets used in 

 these experiments : — 



A sheet of nickel absorbs of the composite beam fi3"3 per 

 cent., but after transmission, the absorption by a similar sheet 

 of nickel falls to 52 per cent.; on the other hand, if the beam 

 had previously passed through iron (cutting oft' 76*2 per cent.). 

 the subsequent absorption by the same sheet of nickel rises 

 to 78*6 per cent. 



A sheet of aluminium absorbing 33' 7 per cent, of the 



2 X2 



