400 Prof. W. H. Bragg on tlie Consequences of 



same weight as the air atoms were placed along C. If the 

 distances of the planes from A were b, c, and d, and if we 

 might assume the ft rays to be spent exponentially with a 

 space coefficient X, the ionization between the planes B and 

 C would be \{e~ bk — e~ ck ) in all the cases just described, 

 I being the initial energy of the radiation as it comes through 

 the hole at A. There is no need to trouble about secondary 

 radiation from a plate at C containing light atoms only, even 

 though we know that atoms of carbon and oxygen can return 

 some ft rays: all such effects are already fully accounted for 

 in the formula. 



Consider now a stream of 7 rays passing normally upwards 

 through the lower plate bounding such an ionization-chamber. 

 The upper plate can be made of cardboard, or some material 

 having approximately the same average atomic weight as air. 



Let k be the absorption coefficient of the material of the 

 plate for y rays in the sense that rays of energy I lose an 

 amount of energy kldx in passing normally through a sheet 

 weighing dx grams per sq. cm.: k is then the weight absorp- 

 tion coefficient. The meaning of this is to be that the energy 

 klda: becomes energy of ft rays which at the start continue 

 the line of motion of the 7 rays. 



Let X be the similar coefficient of the plate for ft rays such 

 as these y rays produce. This means that when a layer of 

 the same material as the plate, weighing x grams per sq. cm. 

 is placed normally to a stream of ft rays of energy I, the 

 energy which gets through the plate and is spent in ionizing 

 the air on the other side is le~ Kx . It is worth observing that 

 if some other gas, say a heavy one like methyl iodide, were 

 substituted for the air, the gas would return more of the 

 radiation into the plate, so that more would be spent in the 

 plate and less in the gas : it might be said that the absorption 

 of a plate depended on the gas or other material above it. 



Let k' and X' be the corresponding coefficients for the 7 

 and ft rays in air. 



The ft rays originated in a layer of weight dx, which is at 

 such a distance down in the plate that a layer of weight x 

 lies above it, will have an energy kle kx dx, where I is the 

 energy which the 7 rays possess as they enter the ionization- 

 chamber. These ft rays at first move directly upwards 

 towards the chamber, and a certain fraction, viz. e~ kx , of 

 their energy is transmitted across the layer x into the 

 ionization-chamber. The whole energy emerging is therefore 



I kle (k ~ k)x dx; and if the plate is thick enough to stop all 

 ft rays we may put the thickness t equal to infinity. The 



