Natures of various Electric Radiations. 439 



o£ absorption A, the fraction that emerges from the further 

 side of the plate is not e~ kd ; much of the radiation passes 

 obliquely through the plate and is absorbed to a greater 

 degree than that which passes normally. This has often been 

 pointed out, e.g., by N. R. Campbell (Phil. Mag. April 1905, 

 p. 541), who also gives some figures from which the proper 

 curve of absorption may be drawn. 1 am not aware, how- 

 ever, that it has been noticed that the form of the absorption 

 curve, which is far from an exponential curve for a thin 

 radiating layer, approximates much more closely to it for a 

 thick radiating layer* And it is interesting to find that the 

 experimental curves which are most nearly exponential are 

 those for which the layers of radioactive material were thick 

 compared to the penetration of the rays under investigation. 

 As examples, we may take those of uranium and actinium 

 already mentioned. On the other hand, the curve which 

 H. W." Schmidt {Ann. d. Pliys. Bd. xxi. 1906, p. 651) has 

 obtained for the /3 rays of RaC, the radioactive material 

 being deposited in a very thin layer on metal'foil, shows just 

 about the amount of departure from the exponential form 

 which is to be expected if the absorption is truly exponential, 

 and there is only one absorption coefficient, not two, as 

 Schmidt has suggested. 



The following figures give the proportional amount of the 

 original radiation which passes through a plate of thickness 

 ti/\, where A is the absorption coefficient : (1) for a thin 

 layer ; (2) for a thick layer. The figures are also given, for 

 the sake of comparison, for the case of a plane wave, or a 

 pencil of rays passing through the plate normally : — 



Radiation from Radiation from Plane wave (purely 



thin layer. thick layer. exponential). 



1-000 1-000 1-000 



•1 -723 -834 -905 



•2 -573 -702 -819 



•3 -467 -600 -742 



•4 -387 -510 -671 



•5 -323 -437 -607 



•6 -274 -378 -548 



•7 -235 -328 -498 



•8 -200 -283 -450 



•9 -171 -248 -405 



1-0 -145 -214 -368 



The absorption of a material used in a thin sheet naturally 

 appears greater than the absorption when the thickness of 

 material is increased, because the rays which are moving 

 obliquely are absorbed first. 



