Rontgen Radiation from Gases and Vapours. 659 



after subtracting this amount, measured the true effect due to 

 the gas in the chamber A 7 , the spontaneous ionization having 

 been allowed for, before plotting the curve. 



The experiment was repeated using A' as the standard 

 chamber, and varying the pressure in A; while another series 

 was performed with carbon dioxide, replacing the air in one 

 of the gas- chambers. 



In every case the curve had the same form, and consisted 

 of a straight line cutting the axis at a point some distance 

 from the origin. This distance, as explained above, measures 

 the radiation from the solid portions of the gas-chamber. It 

 was found to be the same whether the gas in the chamber, 

 before exhausting, had been air or carbon dioxide; and it had 

 the same value, within the limits of experimental error, for 

 both of the gas-chambers. It amounted to 22 per cent, of the 

 total radiation from the standard gas-chamber A, when filled 

 with dry dust-free air at normal atmospheric pressure. 



The fact that the curve was a straight line showed that the 

 other portion of the radiation, namely, that due to the gas in 

 the chamber, was directly proportional to the pressure. The 

 secondary Hontgen radiation from air and carbon dioxide 

 was thus, within the limits of experimental error, proportional 

 to their pressure. 



During the course of these experiments the X-ray bulb 

 was worked at different hardnesses and with different degrees 

 of intensity ; but these variations were not found to affect 

 the results to any appreciable extent. 



The relative magnitudes of the spontaneous ionization in 

 the ionization-chambers, and the ionization due to the second- 

 ary radiation from the gas-chambers, varied with the nature 

 and intensity of the primary rays used in exciting the latter. 

 In general it amounted to about 10 per cent, of the total 

 ionization in the chamber. 



The two ionization-chambers were now connected to the 

 same electroscope, and the second electroscope removed. 



Although it had been thought desirable to investigate each 

 of the sources of ionization separately, the methods employed, 

 as we shall now proceed to explain, eliminated everything 

 except the secondary radiation from the gas. 



The secondary radiation from the gas in the gas-chambers 

 depends upon 



(i.) The intensity of the primary rays producing it ; 



(ii.) The nature of the gas in the chamber ; 



(iii.) The pressure of the gas in the chamber ; 



(iv.) A factor depending upon the size and shape of the 

 gas-chamber, but independent of the nature of the 

 gas and the intensity of the rays. 



