Rontgen Radiation from Gases and Vapours. 655 



secondary radiation by comparing the rates of leak o£ two 

 small gold-leaf electroscopes, the one exposed to the secondary 

 rays to be measured, the other to a small pencil of the primary 

 rays. 



For the purpose of comparing the secondary radiation from 

 a number of gases and vapours, however, it seemed to be 

 more desirable that the comparison should be made, not with 

 a small beam of the primary rays, but directly with the 

 secondary radiation from a standard substance, air. In this 

 way it became possible to ensure that the radiation from any 

 gas, and that from the standard gas air, were measured under 

 exactly the same conditions. Moreover, if, as seemed probable 

 (and as was subsequently proved to be the case), the secondary 

 radiation from a given volume of air was proportional to the 

 pressure, it would be easy, by adjusting the pressure of the 

 standard air, to make the radiation from it equal to that from 

 the given gas or vapour. In this way the method could be 

 transformed into a " null " method, thus affording not only 

 much greater convenience in measurement (leaving only one 

 electroscope to be read instead of two), but also a much higher 

 degree of accuracy and sensibility. 



Preliminary experiments having shown that the assumption 

 as to the proportionality between the pressure of, and the 

 secondary radiation from air was correct, this method was 

 finally adopted. 



Fig. 1. 



*—ToFvmp 



The construction and disposition of the apparatus will be 

 evident from the diagram. A, A (fig. 1) are two brass boxes 



2X2 



