﻿384 • ■ Mr. J. A. Crowther on the Coefficient of 



powders the area was that of the tray or mica sheet.) This 

 method had the advantage of giving the ratio X/p directly. 



Fig-. 2. — Absorption-Curves. 



For if 



then 



Mass per unit Area 



x is the thickness of the layer, 

 co its mass, and 

 a its area, 

 co = apx, 

 co 



X = 



and 



i/i. 



6 p 'a 



An accurate knowledge of the density of the particular 

 specimen of the element used was thus rendered unnecessary. 

 For convenience in comparison, thickness in fig. 2 is repre- 

 sented by mass per unit area. 



