364 Prof. J. J. Thomson : Experiments on 



Thus, for example, washing the wire with water and drying- 

 it by heating with a bunsen does not destroy its activity, nor 

 does heating it in a bunsen to a red heat seem to have much 

 effect upon it ; an amalgamated copper wire was made active 

 and then heated until the mercury was given off, even after 

 this treatment it retained some activity. 



Theory of the preceding Phenomena. 



These experiments show, I think, that induced radio- 

 activity caused by negative electrification is not necessarily 

 due to the deposition of a radio-active substance. This 

 hypothesis does not seem admissible in the case of the pre- 

 ceding experiments ; for when the air is put in the modified 

 state by means of the water-pump, only a limited supply of 

 air is used, the volume of which, as we have seen, is too small 

 to give rise to radio-activity when the air is in its normal 

 condition, hence if in these experiments the effects produced 

 by negative electrification are due to the deposition of a radio- 

 active substance, such a substance must have come from the 

 water. In the experiments with the Gouy sprayer, however, 

 the amount of water used w T as very small : to see whether there 

 was any radio-active substance in it which could produce the 

 observed effects, the water in the sprayer was evaporated to 

 dryness on a metal plate ; the plate, however, after this treat- 

 ment did not show any ionizing power. Again, the amount 

 of air passed through the sprayer was not large enough to pro- 

 duce a supply of radio-active substance large enough to 

 produce the observed effects, for a larger volume of air than 

 that passed through the sprayer was drawn past a negatively- 

 electrified wire without imparting to it any ionizing power. 

 The experiments have led me to the conclusion that the 

 ionizing power imparted to the wire in the preceding experi- 

 ments arises in the following way : — In consequence of the 

 negative electrification of the wire positive ions move up to 

 it when it is placed in the modified gas ; some of these ions 

 do not discharge to the wire, but stick close to it, forming a 

 coating of positive electricity around it. Between this coat- 

 ing and the wire there will be a strong electric field tending 

 to draw negative electricity from the wire. Now there are 

 many phenomena which lead us to the conclusion that a wire, 

 even at ordinary temperatures, contains rapidly-moving 

 negatively-electrified corpuscles which, under ordinary cir- 

 cumstances, remain in the wire because their kinetic energy 

 is not sufficient to carry them beyond the attraction of the 

 metal. When, however, there is a layer of positive electricity 

 just outside the metal, the attraction of this on the negative 



