THE METHOD OF POSITIVE RAYS 529 



on the third bombardment is invisible in all the substances I have tried 

 except monazite sand, where it is given off in exceedingly large quanti- 

 ties as long as the bombardment continues. It is remarkable that mon- 

 azite sand, which contains so many elements, gives no trace of the three 

 line when bombarded. 



I have also obtained the X 3 line and also the helium line when the 

 tube A was replaced by one containing a Wehnelt cathode; with this 

 the current of cathode rays through the tube was much larger than 

 with the other cathode, though the velocity of the rays was smaller. 

 The Wehnelt cathode gives the line without placing pieces of metal in 

 the tube, so that in this case nothing is bombarded by the cathode rays 

 but the glass walls of the tube; the strip of metal forming the cathode 

 is, however, bombarded by the positive rays. 



The three line when present at all continues even though the bom- 

 bardment is very prolonged. In some cases the bombardment has been 

 prolonged for twenty hours, and at the end of that time the line seemed 

 almost as bright as at the beginning ; indeed I could not feel certain that 

 there was any difference. This might lead one to suspect that X 3 was 

 manufactured from the lead or other metal by the bombardment rather 

 than stored up in it, and this view might be regarded as receiving some 

 support from the fact that very little of the X 3 is liberated by heating. 

 The following experiment is an illustration of this. I took a piece of 

 lead, and instead of bombarding it with cathode rays I placed it in a 

 quartz tube connected with vessel A, and heated the tube to a bright 

 red-heat for several hours. Large quantities of C0 3 and hydrogen were 

 driven off by this process ; this was absorbed by charcoal, and the resid- 

 ual gases, which had accumulated in A, were admitted into the vessel 

 B ; the X 3 line and helium line could just be detected, and that was all. 

 I then gave the lead a second heating, raising this time the temperature 

 until the quartz was on the point of softening. The lead was boiling 

 vigorously; the heating was kept up for about three hours. In this 

 time about three quarters of the lead had boiled away. I then let the 

 gases which had been given off at the second heating into the vessel B, 

 and took another photograph ; no trace of the line due to X 3 or helium 

 could be detected. The fraction of the lead which had not been boiled 

 away was now placed in A and bombarded by cathode rays. It now 

 gave the three line quite distinctly ; the helium line was visible, but 

 faint. By the bombardment with the cathode rays the lead was only 

 just melted, so that the average temperature was much less than when 

 it was heated in the quartz tube. This rather suggests that the X 3 

 might be due to a kind of dissociation of the metal by the cathode rays, 

 and not to a liberation of a store of that substance. Another experi- 

 ment shows, however, that for lead, at any rate, this view is not tenable. 

 I took some lead which had just been deposited from a solution of lead 



