Experiments on Positive Hays. 233 



as energy is concerned the positive particle has ample store 

 for liberating a considerable number of negative particles. 

 We have indeed direct evidence that it actually does so ; for 

 Fiichtbauer {Pliys. Zeit. vii. p. 153) found that when positive 

 rays, produced in a tube with a potential difference of about 

 20,000 volts between the anode and cathode, fell on a plate 

 of aluminium, the number of cathode particles ejected from 

 the plate was about four times that of the positive particles 

 falling upon it. We should therefore expect the number of 

 cathode particles starting from the cathode to be considerably 

 greater than the number of positive particles arriving at it : 

 this would imply that the greater part of the current through 

 the tube is carried by the cathode particles. So that though 

 the ionization produced by one cathode particle may be 

 less than that due to one positive one, the excess of cathode 

 particles Over positive may make the total number of ions 

 produced by the cathode particles comparable with that 

 produced by the positive. In consequence, however, of the 

 much greater speed of the negative corpuscles, their density 

 will be less than that of the positive, so that there is at any 

 time in the dark space an excess of positive electricity. 



The positive rays have been shown by Fiichtbauer (loc. 

 eit.) and Austin (Phys. Review, xxii. p. 312) to be reflected 

 to some extent when they strike against a plate of metal ; 

 i. £., some of the rays rebound from the cathode without 

 giving up their charge of electricity; after rebounding, they 

 will be again attracted to the cathode by the electric field 

 and have a second opportunity of communicating their charge 

 to it. This rebound of the positive rays will increase the 

 density of the positive electrification near the cathode, while it 

 wiil diminish the current carried by the positive rays below 

 the value calculated on the assumption that all the positive 

 rays give up their charge to the cathode the first time tin y 

 strike against it. The density of the positive electrification 

 near the cathode will therefore be greater than the value 

 deduced on the assumption that the current carried by the 

 positive rays is equal to the density of* the positive electricity 

 multiplied by the velocity which a positive ion would acquire 

 under the electric field in its journey from the place where it 

 originated to the cathode, This I think may be the expla- 

 nation of an important point to which Mr. F. W. Aston has 

 called attention : this point is that, even if we suppose the 

 whole of the current through the tube to be carried by the 

 positive electricity (we have seen that probably only a 

 fraction is), then the density of the positive electricity 



