Induced-Radioactivity in Air. 357 



investigating the action of water was for the time abandoned 

 in favour of the following, which gave quite regular effects. 



The air in the large lank, previously described, was made 

 to circulate through water. To effect this two tubes were 

 fastened into the tank, through one of these the air was 

 sucked by a water-pump into a closed vessel, from which it 

 found its way back into the tank through the other tube ; 

 the air got thoroughly mixed up with water during the 

 process of pumping. A plug of glass wool was placed in the 

 return-tube to stop water-spray. 



The air which had thus been forced through water was 

 found to be a very much better conductor of electricity than 

 air in the normal state, the increase in the conductivity is sur- 

 prisingly large ; thus, after the circulation of the air through 

 water had continued for about two hours, the saturation- 

 current was more than twenty times the value before the 

 circulation commenced. The air, when once it has been 

 modified in this way, retains its new properties for a very 

 long time : thus, if the tank is kept closed so that the air 

 cannot diffuse out, it takes several days after the stoppage 

 of the circulation for the conductivity of the air to fall to its 

 normal value. The following numbers give an idea of the 

 rate at which the modified gas returns to its normal condition. 

 At 5.20 p.m. on Saturday afternoon the current through the 

 tank with the central wire positively charged was 600, with 

 the central wire negatively charged it was 330 ; at 10 a.m. 

 on Monday, i. e., after more than 40 hours, the current with 

 the central wire + w T as 143, with the central wire — 115. 

 The saturation-current through air in the normal state was 

 30 whichever way the w r ire was electrified. 



The properties of the modified air cannot be explained by 

 the negative electrification which Lord Kelvin has shown to 

 be present in air which has bubbled through distilled w 7 ater,. 

 nor could they be explained by supposing that the bubbling 

 of the air through the water filled it once for all with a 

 supply of positive as well as of negative ions. In the air 

 modified by passing through water there must be a continuous 

 production of ions. A gas which contains a mixture of 

 positive and negative ions, but in which no fresh ions are 

 being produced, though it will conduct electricity, will 

 exhibit peculiarities which will distinguish it from a gas in 

 which spontaneous ionization is taking place ; the current 

 through a gas in which no fresh ionization is taking place 

 will increase with the electromotive force acting on the gas ; 

 each increase in the electromotive force producing an increase 

 in the current. There will be no approach to the state of 



