Absorption and Scattering of X- Rays. 275 



curve for paraffin-wax ; it must also occur in the curve for 

 paper, but the " step " is only about half as big as in paraffin- 

 wax, and the fractional change is smaller still. There appears 

 to be indication of this step (see fig. 1), but it is within the 

 limits of experimental error in this case, and has therefore 

 not been shown in the curve. It would indeed be barely 

 appreciable in the figure.] 



The wave-lengths obtained from the known relation 

 between wave-length and absorption in aluminium are 

 approximately*: 



C -42xl0- 8 cm. 



"39 



Al -37 



Absorption, Scattering, Fluorescence. 



One of us has previously pointed out that an X-radiation 

 loses energy by two independent processes. One is the 

 process which we may briefly call scattering ; the other is 

 that which is associated with the emission of corpuscular 

 (electronic) radiations and their accompanying fluorescent 

 (characteristic) X-radiations. 



The former absorption varies slowly with the wave-length 

 — over certain regions it is approximately independent of 

 wave-length ; the latter varies very rapidly — over certain 

 ranges it is approximately as the (wave-length) 3 . 



The former — produced by equal masses of different ele- 

 ments — varies little with the atomic weight of the absorbing- 

 element; the latter considerably. 



* These values are somewhat lower than those previously given. There 

 is, however, some uncertainty as to the absolute values, — though not so 

 much as to relative values — owing to the fact that neither the cha- 

 racteristic radiations nor the primary radiations used in these expe- 

 riments were homogeneous. 



The values previously given as *56 X 10 cm. for nitrogen and *o X 10 ~ 8 

 for sulphur were for the constituents of longest wave-length (u, x lines), 

 on the assumption that J radiations were of constitution similar to the 

 K radiations. There are indications that the J radiations are more 

 homogeneous than K radiations, and that the wave-lengths of the corre- 

 sponding j8 lines which are approximately *5xl0~ and '44xl0~ 8 cm. 

 respectively, are more correctly the values to be assigned to nitrogen and 

 sulphur by the ionization method. 



There is thus a small discrepancy between the values obtained by the 

 ionization and absorption methods, but the variation of Xj with atomic 

 weight is about the same in both. In the ionization experiments K 

 characteristic radiations were used ; in the absorption experiments, 

 primary radiations. Equal average penetrating powers in these two 

 radiations do not correspond to exactly the same effective wave-lengths. 

 Greater accuracy will be obtained by the use of more homogeneous 

 radiations, or by the interference method. 



