1016 Profs. 0. W. Richardson and C. B. Bazzoni on the 



dimensions and so insulated that they could easily be raised 

 to a white heat or even melted down with the electron 

 discharge from the cathode. The diffusion of ions from the 

 discharge chamber to the detection chamber was prevented 

 by a parallel plate condenser set in the slit-like tube con- 

 necting the two chambers. No windows or partitions were 

 used. In order to render this method of protection against 

 diffusion successful, particular relations of the potentials of 

 the various electrodes were necessary. After the particular 

 arrangement was reached through trial no difficulties from 

 stray ions were encountered. 



The radiations were stimulated by impact of electrons of 

 known velocities on small targets of the element under 

 investigation located similarly to A 1 of fig. 1 of our former 

 paper (loc. cit. p. 2$8) . It was soon obvious that the amount 

 of radiation to be dealt with, as judged by its photoelectric 

 activity, was very much smaller from the solid targets than 

 when gases were used. It therefore became desirable before 

 attempting any magnetic analysis of the photoelectric elec- 

 trons to explore the conditions under which any radiation 

 is generated, using some more sensitive method. We 

 accordingly connected the box in the receiving chamber 

 to the electrometer and looked for discontinuities in the 

 negative current from it, due to the photoelectric effect of 

 radiation falling on it, as the potential driving the electron 

 current in the discharge chamber was gradually increased. 

 It is, of course, an assumption that these increases are really 

 due to radiation generated at the bombarded target, but we 

 believe we have made subsidiary tests on this point which 

 exclude every possible alternative. It is a further assump- 

 tion which can only be defended by analogy with the known 

 laws of production of X-rays of higher frequency, that these 

 photoelectrically active rays are really X-rays of frequency 

 corresponding, on the quantum relation, to the voltage of 

 the exciting discharge. It is our hope to be able to 

 supply the proof or disproof of this by the magnetic or 

 by an electrostatic method, but it is not possible to do 

 this until a preliminary exploration of the conditions under 

 which some sort of radiation is generated has been made. 

 In the helium work the electrostatic method (Joe. cit. p. 287) 

 was discarded as hopeless, but with the radiation from solids 

 the secondary effects which we then had to contend with 

 may not be so serious. 



The success of the measurements depends, amongst other 

 things, on obtaining and maintaining a vacuum as nearly 

 perfect as possible, since the radiations set up from gas 



