Experiments on Delta Rays. 535 



The unit of P.D is 1*035 volts, and the quantities termed 



" current " are the values of . °— — for curves obtained, like 



those marked 3 in figs. 2 and 3, by substracting the values 

 of the current with the magnet on from those with the magnet 

 off; thus they are corrected for the presence of residual air. 

 It will be seen that it is quite impossible to say precisely for 

 what P.D. saturation is attained. The curves when D is 

 covered with soot appear to require a higher potential for 

 saturation than the others, but I am sure that the apparent 

 saturation in the latter case is due to the influence of reflexion 

 which has already been noted ; it will be observed that in one 

 case the current for V = 20 is smaller than for V=10. The 

 uncertainty attaching to any single measurement of current 

 does not exceed '005, but it was found that, owing to the 

 causes just mentioned, readings could not be reproduced with 

 this accuracy when the apparatus had been taken down and 

 put up again. 



The influence of reflexion at the electrode D in the first 

 half of the table and that of the Volta effect in the second 

 half make it unlikely that, even if the distribution of speed 

 among the delta rays from different substances were the same, 

 all the numbers in the same row of the table would be the 

 same. But it will be seen that there is a very general agree- 

 ment between the figures for different metals, and such 

 divergencies as there are in one half of the table are not 

 reproduced in the other. The figures for gold in the first 

 half are less than those for aluminium, in the second half 

 they are greater, I have no hesitation in ascribing these 

 divergencies to the influence of reflexion of the Volta effect 

 and of unavoidable small changes in the apparatus, and I 

 have no hope of producing more satisfactory evidence that 

 there is no material difference in the quality of the delta rays 

 emitted by different substances. It is true that the differences 

 between the various materials is greater here than in the 

 experiments described in the first two papers, which were not 

 regarded as conclusive as to the identity of the rays, but the 

 sources of error are now definitely known, whereas before it 

 was uncertain whether the measurements gave any informa- 

 tion at all as to the speed of the delta rays, 



11. Accepting this conclusion some further questions 

 require consideration. 



Bumstead * has suggested that the delta rays are emitted 

 from an air film on the surface of the materials and not from 

 the materials themselves, so that it is only natural that they 



* Bumstead, PhiJL Mag. Dec. 1911, p. 907. 



2N2 



