﻿60 Mr. Norman Campbell on Delta Rays. 



in which it is excited, while the experiments described in 

 the last paper undertaken specially to test the point showed 

 that the radiation from all materials was the same within a 

 few per cent. 



13. One more conclusion asserted in the last paper must 

 be withdrawn. It was stated that the quantity of delta 

 rays emitted by a plate covered with polonium was more 

 than that emitted by any other electrode on which the alpha 

 rays from the polonium fell, and that, therefore, there must 

 be true delta rays emitted by the polonium itself *. It is 

 now seen that the second part of this statement is not a 

 necessary consequence of the first. For, when the alpha 

 rays from the polonium plate fall on any other electrode, 

 only those delta rays can escape which are ejected towards 

 the surface of that electrode ; but, since the active material 

 is in a very thin and probably discontinuous film on the 

 surface of the plate, it is possible that not only the rays 

 generated in the polonium film, but also some of those 

 generated in the plate on which it is supported can escape 

 under the action of an electric field tending to drag them 

 out. The apparently greater emission of delta rays from 

 the active plate itself may be merely due to the fact that in 

 this case the rays generated over more than a hemisphere 

 can reach the opposite electrode. 



14. In view of the uncertainty which appeared to attach 

 to all the results obtained by the method of investigation 

 hitherto employed, an attempt was made to devise some 

 more satisfactory means of investigating the properties of 

 the delta rays. It was thought that possibly information 

 might be yielded by the study of ionization by collision, 

 using alpha rays as a primary ionizing agent. A good 

 many observations have already been made in this direction, 

 but an account of the results will be left for a future paper. 

 It need be only recorded here that the evidence, so far as it 

 goes, is still distinctly against the view that there is any 

 difference in the speed of the rays generated in different 

 materials, and that it appears very improbable that the 

 speed of those rays can be, as some investigators have con- 

 cluded, nearly as great as that corresponding to the potential 

 difference of 25-30 volts which Townsend gives as the 

 minimum speed at which an electron possesses sufficient 

 energy to form an ion. The conclusions then agree so far 



* The same conclusion has been advanced by Hauser (he. cit.), but 

 all his measurements of quantity as well as quality are somewhat 

 doubtful in the light of the considerations put forward in this paper. 



