302 Hon. R. J. Strutt on the Discharge of 



difficulty in passing, the blue luminosity fills the entire bulb 

 (fig. ie). 



The boundary of the blue ring at the stage represented in 

 fig. 4 e is exceedingly definite. 



It will be seen from this description that the electrodeless 

 discharge brings out in a very striking way the change from 

 the red to the blue glow in argon first noticed by Sir W. 

 Crookes. The red glow is obtained when the pressure of the 

 gas is high, or when the discharge is feeble. The blue glow 

 is obtained when the pressure is low, or the discharge very 

 intense, as it is in the outer zone of the ring. The bulb re- 

 mained faintly luminous with a phosphorescent glow after 

 the discharge had been turned off. This glow was observed 

 after a purely red discharge (fig. 46) as well as after a purely 

 blue one (fig. 4#). 



The electrodeless discharge in helium does not show any 

 special features. The luminosity is yellow, the D 3 line being 

 prominent. 



Conductivity of Helium under Rontgen Rays. 



It is now well-known that gases exposed to Rontgen rays 

 are enabled to conduct electricity. The explanation of this 

 effect is, beyond doubt ; that the rays manufacture out of the 

 gas carriers charged with electricity. The motion of these 

 charged carriers or ions under an electromotive force con- 

 stitutes the current. This kind of gaseous conduction is 

 essentially distinct from the conduction which constitutes 

 disruptive discharge. The former takes place under small 

 E.M.F.'s, the latter, under given circumstances, requires a 

 definite large E.M.F. to produce it. It has been found (see 

 J. J. Thomson, ' The Discharge of Electricity through Gases,' 

 p. 35) that with a given shape of electrodes, and a given 

 distance between them, the current does not increase with the 

 potential-difference beyond a certain point if the radiation is 

 constant. The current becomes " saturated." This occurs 

 when ions are used up in conveying the current as fast as 

 they are produced by the rays. 



There is reason to think that the ion conveys the same 

 electric charge, whatever the gas from which it is manufactured 

 (see J. J. Thomson, Phil. Mag. Dec. 1898, p. 543). This 

 being so, it is clear that the ratio of the saturation currents 

 through two gases gives the relative rates at which the rays 

 produce ions in them. Measurements of the relative satura- 

 tion currents for the common gases have been made (see 

 J. J. Thomson, i Discharge of Electricity through Gases/ 

 p. 47). 



