294 



The Hon. R. J. Strutt 



4piil 24, 



charge. Although the recovery of insulation is very rapid, it is not 

 absolutely instantaneous. I have here an experiment bearing on this 

 point. This metal box has a window of aluminium, through which 

 the Eontgen rays can pass, so that the air in the box is exposed to 

 them. This air is blown out through a considerable length of tubing 

 on to the electroscope, and you see that it is able to discharge it. So 

 that it is evident that some of the conducting power is retained 

 during the time that the air takes to pass through about 2 feet of 

 tubing, a considerable fraction of a second. 



There is another way in which air can be made to lose its insulat- 

 ing power, and that is by exposing it to the action of the mysterious 

 rays given out by radio-active bodies, notably by radium salts. 



I charge the electroscope again, and you sec that when I bring 

 near it this sample of radium salt, the leaves fall together, as under 

 the influence of Eontgen rays. 



We may now consider more in detail the behaviour of gases 

 made conducting by these methods. 



When the electric current passes through a metal or an electro- 

 lyte, the relation between the current and the electromotive force 

 applied is the simplest possible — the current is proportional to the 

 electromotive force. The conductor is said to obey Ohm's law. But 

 with a gas under the influence of Eontgen or Becquerel rays it is far 

 otherwise. The current increases at first in proportion to the E.M.F., 

 but when the E.M.F. is increased beyond a certain point, the current 

 no longer increases correspondingly. Finally, when the E.M.F. is 

 very large, a maximum value for the current is reached, and further 

 increase in the E.M.F. is without influence upon it. The value of 

 this limiting current, and the E.M.F. necessary to produce it, will of 

 course depend on the strength of the rays. It is evidently of interest 

 to compare the maximum or saturation current with difl'erent gases. 

 Such comparisons have been carried out, and I will give you in a 

 table some of the results. 



You will observe that, under the action of Becquerel rays, the 

 saturation current is nearly propt)rtional to the density, while, under 



