Different Tiroes of 7 Rays from Radioactive Substances. 535 



when the effect o£ the soft rays was eliminated by aluminium 

 absorbers, this ratio dropped to about 5, whereas from 

 analogy with the hard 7 rays from radium B and radium C 

 it would be expected to drop to 2'26. Both in the case of 

 the very soft rays from radium D, as well as in the case of 

 the very hard rays from radium B and radium C, ionization 

 has been found proportional to absorption; there is no reason, 

 therefore, to believe that it will be different for the rays of 

 radium D which are intermediate in penetrating power. 

 It is probable that these rays also are absorbed five times more 

 in S0 2 than in air ; this means that in this case we are 

 dealing with a special type of rays whose absorption in 

 matter does not obey the density law, and at the same time 

 gives a lower constant ratio than do soft 7 rays and X rays 

 belonging- to the "K" series. Owing to the extremely 

 small effect of these rays (about one division per minute in 

 the experimental arrangement) it was impossible to verify 

 this point by actually measuring their absorption in any 

 gas. 



Part II. 



Relative Energy of the Different Types of 7 Rays. 



As already mentioned, the relative energy of different 

 types of 7 rays can be determined by comparing the total 

 number of ions produced by each type of rays in air or other 

 gas. The idea on which this determination is based is the 

 following : supposing two types of 7 rays present, differing 

 in penetrating power; let us consider a small layer of air of 

 thickness dx close to the source ; if n x is the number of ions 

 produced by one of the types, say the softer one, in a layer 

 of unit thickness, then the number of ions produced in the 

 layer dx will be n\dx j in another such layer placed at the 

 distance x from the source there will be n^e~^ x dx ions pro- 

 duced, where /x L is the absorption coefficient in air for this 

 soft type. Integrating 



f n l e~^dx = ^ l = ~ l 

 Jo /*i 



we obtain N 1? the total number of ions produced by these 

 rays in air. In the same way N 2 , the total number of ions 

 produced by the harder type of rays present, is given by the 

 integral 



Jo H* 



where n 2 is the number of ions produced in a layer of air of 



