Gaseous Mixtures hy Rontgen and Corpuscular Radiations. 843 



to which the deeper layers are exposed varies with the 

 gas. We wish, however, to compare the ionizations pro- 

 duced in different gases by beams of the same intensity. If 

 i be the ionization produced by a beam of unit intensity and 

 unit cross-sectional area per unit length of gas traversed 

 when all or a definite fraction of the secondary radiation is 

 absorbed, we may write the ionization produced by a beam 

 of intensity I and cross-sectional area A in a layer of 

 thickness dx as iAIdx, and in a length I of the gas as 



4' 



i 



Ida, 



provided the same fraction of secondary radiation is ab- 

 sorbed. We will call i the coefficient of ionization of that 

 particular radiation in that particular gas. As the experi- 

 mentally determined ionization may or may not include the 

 ionization due to all the secondary radiations from the gas 

 itself, the values obtained for the ionization coefficients must 

 depend on the particular circumstances of the experiment. 

 The particular secondary radiations which are absorbed or 

 which escape must be stated along with the ionization co- 

 efficient. 



The total ionization in a length I of gas 



=iA I e' Xx dx 



where \, the coefficient of absorption, has the usual 

 meaning. 



Thus, owing to the screening of the deeper layers of the 

 gas, the ionization produced in the chamber is that which 



l-e"* 1 

 would be produced in one of length — , if all the gas 



were exposed to a radiation of the intensity of the beam as 

 it is at incidence. Calling this the effective length of the 

 ionization-chamber, the relative coefficients of ionization are 

 obtained by comparing the total ionizations of the gases in 

 chambers of equal effective lengths. 



It is necessary, also, to eliminate the effects of the secon- 

 dary radiations from the ends of the ionization-chamber. 

 There is, however, not only an increase of ionization due to 

 secondary radiations from the solid ends into the gas experi- 

 mented upon, but a loss of ionization owing to radiation 



