Radiation from Gases subject to X-Rays. 697 



(3) For a given primary radiation the intensity of secondary 

 radiation is proportional to the density o£ the gas from 

 which it proceeds — the temperature and pressure being 

 practically constant. 



(4) The secondary radiation is not due to recombination of 

 the ions, 



Sagnac concluded from his experiments that the secondary 

 rays from air are more easily absorbed than the primary rays, 

 but his paper does not give an idea of the amount by which 

 they differ. As the secondary radiation is relatively weak, a 

 beam of considerable cross-section must be studied, so that 

 the rays do not pass normally through the absorbing plate. 

 In the experiments described above, the plates of aluminium 

 placed in the path of the secondary beam produced greater 

 proportional diminution of the resultant ionization than was 

 produced on the ionization caused by the primary, when this 

 beam was intercepted by similar plates. The difference, 

 however, was small, and not more than might be accounted 

 for by the greater thickness through which most of the rays 

 had to pass by striking the plate obliquely. 



The fact of the ionization produced in the electroscope by 

 the radiation from different gases being so accurately pro- 

 portional to the density of the gas from which it proceeds is 

 significant in this connexion, for the ionization depends upon 

 the intensity of the radiation and upon its nature. The 

 difference in the absorbability of the radiations is easily 

 proved insufficient to account for the difference in intensity 

 of ionization ; also it is extremely improbable that differences 

 in the intensity together with differences of the absorbability 

 would produce so accurate a law. It is much more probable 

 that differing intensities of secondary radiation alone account 

 for the differing intensities of ionization, and that the secondary 

 radiation has the same penetrability from whatever gas it pro- 

 ceeds. This again leads to the probability that the secondary 

 radiations from different gases are of the same nature as the 

 primary rather than that the primary is similarly transformed 

 by the different gases. 



As the primary and secondary radiations only differ appre- 

 ciably in intensity, we may reasonably conclude that the 

 radiation proceeding from gases subject to X-rays is due to 

 scattering of the primary radiation. 



As this scattering is proportional to the mass of the atom. 

 we may conclude that the number of scattering particles 

 is proportional to the atomic weight. This gives further 



