Induced-Radioactivity in Air. 365 



corpuscles drags the latter from the wire ; as the corpuscles 

 move across the space between the coating and the wire they 

 acquire additional kinetic energy, and i£ the difference of 

 potential between the coating and the wire exceeds a certain 

 value they will emerge from the positive coating with suffi- 

 cient kinetic energy to enable them to ionize the molecules 

 of the gas with which they come into collision ; for this to 

 be the case the potential-difference between the positive coating 

 and the wire must exceed 2 volts, as Mr. H. A. Wilson has 

 shown that the energy required to ionize a molecule of a gas 

 is of the order of that given to a charge equal to that on a 

 corpuscle when it moves through a potential-difference of 

 about two volts. 



Thus, on this view, the ionizing power of the wire is due to 

 a kind of polarization, which produces an electric field which 

 makes the wire into a cathode emitting cathode-rays of feeble 

 penetrating power which ionize the gas in the neighbourhood 

 of the wire. 



If the wire, when in the conducting gas, had been positively 

 electrified the electric field due to the polarization would have 

 tended to force back the corpuscles into the wire rather than 

 pull them out ; there would therefore in this case be no 

 emission of cathode-rays and no ionization of the gas. 



The amount of polarization seems to depend upon the way 

 the gas in which the negatively-electrified wire is placed is 

 ionized. Thus we have seen that when the gas is made a 

 conductor by bubbling through water the effect on the nega- 

 tively electrified wire is much greater than when the gas is 

 made a conductor by Rontgen rays, although the conductivity 

 of the gas is greater in the latter case than in the former. 

 Again, 1 made the gas in the vessel a very good conductor by 

 keeping a Bunsen burner burning in the vessel, but in this 

 case I could not detect any ionizing power in a negatively- 

 electrified wire which had been kept in the vessel. Air which 

 has been passed over phosphorus is a conductor of electricity, 

 but 1 could not detect any ionizing power in a negatively- 

 electrified wire immersed in it. 



The principle by which we have explained the ionizing power 

 of the negatively electrified wire — the emission of cathode- 

 ravs from the wire under the influence of a coating of positive 

 electricity close to the surface of the wire — will also, I think, 

 explain the conductivity produced in air when it bubbles 

 through water. We may suppose that by this process very 

 minute drops of water get mixed with the air, these drops 

 must be exceedingly small, otherwise they would not be able 

 to pass through a plug of* glass-wool ; the very slow rate 



