598 Professor Sir J. J. Thomson [Jan. 17, 



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 

 bombardment 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 sus- 

 pect that X3 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 X3 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 con- 

 nected with vessel A, and heated the tube to a bright red-heat for 

 several hours. Large quantities of CO2 and hydrogen were driven off 

 by this process ; this was absorbed by charcoal, and the residual 

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

 the X3 line and hehum line could just be detected, and that was all, 

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

 ture 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 

 X3 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 X3 might be due to a kind of dissociation of the 

 metal by the cathode rays, and not to a hberation of a store of that 

 substance. Another experiment 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 acetate by putting a piece of 

 zinc into the solution, and forming the well-known lead-tree. When 

 I bombarded this freshly precipitated lead, I could get no trace of the 

 X3 line ; the helium line, too, was absent. I then tried another ex- 

 periment. I took a piece of lead and divided it into two parts. The 

 first of these I bombarded by the cathode rays : it gave the X3 line 

 quite distinctly. The other part I dissolved in boiling nitric acid, 

 getting lead nitrate. The nitrate was heated and converted into 

 oxide, and this was bombarded by the cathode rays : it did not give 

 the X3 line, showing that the X3 is not produced by the bombard- 

 ment, but is something stored up in the lead, which can be detached 



