﻿996 Prof. C. G. Barkla and Mr. V. Collier on tie 



gases. The work of Benoist with heterogeneous beams 

 showed that, and since his time hundreds of experiments 

 have only shown that X-ray phenomena are independent of 

 the physical state of the substance concerned. It is, how- 

 ever, necessary in many investigations to know the magnitude 

 of the absorption of homogeneous X-radiations by substances 

 which are usually in the gaseous state, and which were not 

 dealt with by Barkla and Sadler. 



Experiments were made on the absorption by air at various 

 pressures. The absorption was found to be proportional to 

 the pressure within small errors of experiment. The ab- 

 sorption of the more penetrating rays was too small to be 

 determined experimentally by the method used, but the 

 absorption of the fluorescent radiations (series K) from 

 copper and zinc were found as below. Knowing the ab- 

 sorption of the homogeneous radiations in aluminium and 

 the absorption of these two particular radiations in air, by 

 the law of proportionality of absorption (after elimination 

 of that portion due to scattering) the absorption in air of 

 many radiations was calculated *. Thus, using the notation 



. P (X'—s r ) in air . . . 



as berore, ^r r^ — irr 1S constant. 



' (X,— s) in Al 



In determining the absorption coefficients for any other 



gas, the ionization in an electroscope was observed first with 



air in the absorbing chamber in the path of the beam and 



subsequently with the particular gas in that absorbing 



chamber. Then 



I —I p- (X g- X J x 



where \ a and \ are the coefficients of absorption in air and 

 in the gas, x the length of the path through the absorbing- 

 gas, 1^ and I a the intensities as measured by the ionization 

 after traversing the gas and air respectively. The only 

 unknown was X g . 



In the experiments on vapours, the absorption by a mixture 

 of air and vapour at known partial pressures was determined 

 and a correction was subsequently made for the absorption 

 by the air present. The results are given below (Table III.). 

 The values given in italics were obtained by interpolation. 



From these results it may be observed that the laws of 

 absorption are the same for gases as for solids : — 



The absorptions in two gases A and B bear an approxi- 

 mately constant ratio one to the other through a range of 

 absorbability not including a spectral line of either A or B, 

 and not approaching one of these spectral lines on the more 

 penetrating side. 



* Sadler has experimentally determined X in air for As radiation. The 

 agreement with the value given is very close. 



