the Electrical Conduction produced by it. 161 



ions blown out, and a positively electrified gas is obtained. 

 As the potential-difference between the electrodes is increased, 

 the amount of electrification obtained depends on two opposing 

 actions. The velocity of the carriers is increased, and conse- 

 quently the ratio of the number of carriers removed is dimi- 

 nished. But if the gas is not saturated, with increase of 

 electromotive force the number of ions travelling between the 

 electrodes is increased, and for small voltages this increase 

 more than counterbalances the diminution due to increase of 

 velocity. The amount of electrification obtained will there- 

 fore increase at first with increase of voltage, reach a maxi- 

 mum, and then diminish ; for when the gas is saturated no 

 more ions can be supplied with increase of electromotive 

 force. This is exactly the result which Beattie obtained, and 

 which I also obtained in the case of the separation of the ions 

 of Rontgenized air. The fact that more positive than nega- 

 tive electrification is obtained is due to the greater velocity 

 with which the negative ion travels. (See § 14.) 



The properties of this electrified gas are similar to that which 

 has been found from Rontgen conduction. The opposite sign 

 of the electrification obtained by Beattie for uranium in- 

 duction, and by myself for Rontgen conduction *, is to be 

 expected on account of the different methods employed. For 

 obtaining electrification from Rontgenized air a rapid current 

 of air was directed close to the charged wire. In that case 

 the sign of the electrification obtained is opposite to that of 

 the wire, as it is the carriers of opposite sign to the wire which 

 are blown out before they reach the wire. In the case of 

 uranium the current of air filled the cross-section of the space 

 between the electrodes ; and it has been shown that under such 

 conditions electrification of the same sign as the uranium is to 

 be expected. 



§ 18. Discharging-power of Fine Gauzes. 



Air blown over the surface of uranium loses all trace of con- 

 ductivity after being forced through cotton-wool or through 

 any finely divided substance. In this respect it is quite 

 similar to Rontgenized air. The discharging-power of cotton- 

 wool and fine gauzes is at first sight surprising, for there is 

 considerable evidence that the ions themselves are of molecular 

 dimensions, and might therefore be expected to pass through 

 small orifices ; but a little consideration shows that the ions, 

 like the molecules, are continually in rapid motion, and, in 

 addition, have free charges, so that whenever they approach 

 within a certain distance of a solid body they tend to be 

 attracted towards it, and give up their charge or adhere to 



* Phil. Mag. April 1897. 

 Phil Mag. S. 5. Vol, 47. No. 284. Jan. 1899. M 



