the Electrical Conduction produced by it. 139 



carbonic acid than in air, and greater in air than in hydrogen. 

 As the distance between the plates is increased, these values 

 tend to approximate equality. If, however, the rate of leak is 

 taken between two plates some distance from the radiating 

 surface (e. g. the plates A and B in fig. 3) , the ratio of the 

 rates of leak for different gases will depend on the distance 

 of the plate A from the surface of the uranium. If the 

 plate A is several centimetres distant from the uranium, the 

 rate of leak will be greater with hydrogen than with air, and 

 greater in air than in carbonic acid — the exact reverse of the 

 other case. These considerations will also apply to what is 

 meant by the conductivity of a gas for uranium radiation. 



In a previous paper * I found the coefficient of absorption 

 of a gas for Rontgen rays to be roughly proportional to the 

 conductivity of the gas. The conductivity in this case was 

 measured by the rate of leak between two plates close together 

 and not far from the Crookes tube. The absorption in the 

 air between the bulb and the testing apparatus was small. 

 If it were possible to completely absorb the Rontgen radia- 

 tion in a gas and measure the resulting conductivity, the total 

 current should be independent of the gas in which the radia- 

 tion was absorbed. This result follows at once if the ab- 

 sorption is proportional to the ionization produced for all 

 gases. The results for uranium and Rontgen radiation are 

 thus very similar in this respect. 



§ 12. Variation of the Current between two Plates with the 

 Distance behveen them. 



The experimental arrangement adopted was similar to that 

 in fig. 3 with the plate A removed. Two horizontal polished 

 zinc plates 10 cm. in diameter were placed inside a bell-jar. 

 The lower plate was fixed and covered with a uniform layer 

 of uranium oxide, and the upper plate was movable, by 

 means of a screw, parallel to the lower plate. The bell-jar 

 was air-tight, and was connected with an air-pump. The 

 lower plate was connected to one pole of a battery of 200 

 volts, the other pole of which was earthed, and the insulated 

 top plate was connected with the electrometer. The exterior 

 surface of the glass was covered with tinfoil connected to 

 earth. 



The following table gives the results of the variation of the 

 rate of leak with distance for air at pressures of 752, 376, 

 and 187 mm. The results have been corrected for change of 



* Phil. Mag. April 1897. 



