540 Further Experiments on Delta Rays. 



they have been found to possess is not dee, as was suggested 

 in a former pnper, to secondary causes. 



5-11. Experiments with a new form of apparatus have 

 proved to the satisfaction of the author that there is no 

 difference in the velocities of delta rays emitted from different 

 materials. The conclusion formerly announced that these 

 Velocities are also independent of the velocity of the exciting- 

 alpha ray receives much more complete confirmation. 

 Evidence is quoted against the view that the delta rays from 

 all materials appear to be the same, because they come from 

 a layer of air on the surface of those materials and not from 

 the materials themselves. 



The same experiments show that the delta rays are very 

 heterogeneous in velocity ; most of them appear to have 

 Velocities less than 3 volts, but a very few may have 

 velocities as high as 10 or 20 volts. It is possible, but not 

 probable, that this heterogeneity is imposed on originally 

 homogeneous rays by passage through the surface layers of 

 the material from which they come. 



11-13. The results of some measurements on the currents 

 through air and hydrogen at low pressures and under low 

 P*D/s are quoted. They exhibit remarkable features of 

 which no explanation is offered. 



If the conclusions of this paper are accepted, the ivesti- 

 gation enters on a new phase. If the delta rays are emitted 

 with velocities which depend neither on the nature of the 

 material from which they come nor on the velocity of the 

 rays which excite them, on what do those velocities depend ? 

 It seems that either we must imagine that they depend on 

 some structure common to all materials, or on the nature of 

 the helium atom of which all alpha rays consist. Some 

 decision between the alternatives may be obtained by 

 examining the process of ionization by agents other than 

 alpha rays. Several recent investigators have spoken of the 

 " delta rays liberated by Rontgen rays/'' but there appears 

 to be no experimental evidence whether the electrons ejected 

 from atoms by these agents possess finite velocities at all 

 similar to those possessed by the electrons liberated by alpha 

 rays. Endeavours will now be made to obtain such experi- 

 mental evidence. 



The University of Leeds, 

 May 1912. 



Note added Aug. 22.' — Experiments which will be described 

 in the next number of the Phil. Mag. show that Rontgen rays 

 liberated delta rays precisely similar to those liberated by 

 alpha rays. 



