﻿242 Mr. L. V. King on Absorption 



also of the position of P in its interior. This result is em- 

 ployed by Fourier as a verification of the cosine law *. 



§ 4. In deducing the formula just given, we have employed 

 the simple exponential law of absorption 



i=v— . 



This by itself involves a violation of the principle of con- 

 servation of energy. In the case of gases, we know that 

 absorption of light is accompanied by lateral scattering- 

 according to the well-known Rayleigh law. In the case of 

 incandescent solids the problem is very much more com- 

 plicated. Each of the radiating elements, besides being 

 subject to a primary disturbing cause, is also subject to the 

 aggregate radiation from all the other elements in the sur- 

 face S. This constitutes what Schuster f has called "Radia- 

 tion in a Foggy Atmosphere/' The complete expression of 

 this problem can be given in terms of an integral equation in 

 three dimensions. This, together with some applications to 

 the intensity of skylight, the writer hopes to communicate in 

 a future paper. 



XX. Absorption Problems in Radioactivity. By Louis Vessot 

 King, B.A. (Cantab.), Lecturer in Physics, McGill Uni- 

 versity % . 



[Plate VII.] 



§ 1. npHE intensity of (3- and 7-rays in their passage 

 JL through matter decreases approximately according 

 to the exponential law 



i=V-- ...... (i) 



I being the intensity at an arbitrary origin #=0 as measured 

 by the ionizing power of the rays, and k the coefficient of 

 absorption. Under these conditions it can easily be shown 

 that the intensity at a distance r from a point-source of rays 

 in a medium whose coefficient of absorption is k is given by 



i=£«— , (->) 



s being the intensity at unit distance when there is no 

 absorption. 



* Fourier, he. cit. p, 31, § 47. 



t Schuster, Astrophvsical Journal, Jan. 1905, p. I. 



% Communicated by Prof. H. T, Barnes, F.E.S. 



