Rays emitted \>y Substances exposed to 7 Rays. 633 



We have seen that i£ the average velocity of ejection of 

 an electron from a gaseous molecule ionized by /3 or 7 rays 

 is the same, the ratio of the primary ionization in any particular 

 gas ionized by j3 rays to the primary ionization when the gas 

 is ionized by 7 rays is constant, and this constant is equal to 

 unity if the primary ionization in any gas is expressed in 

 terms of that in air at the same pressure. If the average 

 velocity of ejection of an electron from a molecule in a 

 substance ionized by /3 or 7 rays is the same, the ratio of the 

 values found for the quantities M x and M 2 for any particular 

 substance gives, whatever these values may represent, the 

 correct ratio of the quantities M, and M 2 according to the 

 definition. For, since the radiation from a substance is 

 measured by the ionization it produces in air, and the ionizing 

 power of the radiation is the same for both ionizing agents, 

 the ratio of the number of electrons radiated from the sub- 

 stance to the ionization they produce, is the same with both 

 ionizing agents for any particular substance, and the ratio 

 of the quantities Mj and M 2 therefore in this case independent 

 of the ionizing power of the electrons. Also, the number of 

 electrons that do not get free from the layer of the substance 

 must be in both cases approximately the same fraction of 

 the number of electrons that do get free from the substance, 

 and therefore the ratio of M, and M 2 for any particular 

 substance also independent of the absorbable radiation pro- 

 duced. Now, the ratios of the values found for M x and M 2 

 are approximately equal to unity ; and we therefore conclude 

 that the average velocity of the electrons from molecules 

 ionized by /3 or 7 rays is approximately the same, and is not 

 influenced by the state of aggregation of the molecules. 



It will be useful to collect the formulas expressing the 

 relation between the various quantities involved in the 

 secondary radiation from a layer of a substance exposed to 

 /3 or 7 rays. 



McClelland has shown that the relative value p of the 

 radiation from a layer of a substance exposed to the ft rays 

 of radium is given by 



1 



1>= - 



K 



2 



It has been shown in this paper that the relative value Ri of 

 the radiation with y rays is given by 



/x(l-«) 2-K-h2 v / T^ 



