THE GROWTH OF OUR KNOWLEDGE OF HELIUM. 275 



appear that 667 represents a line of some compound which 

 is simultaneously dissociated. 



The unequal behaviour of the lines has been further 

 noted in another experiment, in which the products of 

 distillation of broggerite were observed in a vacuum tube 

 and photographed at various stages. After the first heating 

 D 3 and 447 1 were seen bright, before any lines other than 

 those of carbon and hydrogen made their appearance. 

 With continued heating 667, 5016, and 492 also appeared, 

 although there was no notable increase of brightness in the 

 yellow line ; still further heating introduced additional lines, 

 5048 and 6347. 



These changes are represented graphically in the fol- 

 lowing diagram (Fig. 13). 



D 3 



447. 492.501. 5876. 634 667. 



504 

 Fig. 13. — Diagram showing order in which lines appear in spectrum of vacuum tube 



when broggerite is heated. 



It was recorded further that the yellow line was at times 

 dimmed, while the other lines were brightened. 



In my second note, communicated to the Royal Society 

 on the 8th May, I stated that I had never once seen the 

 lines recorded by Thalen in the blue, at A 4922 and 4715. 



It now seems possible that their absence from my 

 previous tubes was due to the fact that the heating of the 

 minerals was not sufficiently prolonged to bring out the 

 gases producing these lines. 



It is perhaps to the similar high complexity of the gas 

 obtained from cleveite that the curious behaviour of a tube 

 which Professor Ramsay was so good as to send me, must 

 be ascribed. When I received it from him the glorious 

 yellow effulgence of the capillary while the current was 

 passing was a sight to see. But after this had gone on 

 for some time, while the coincidence of the yellow line with 

 D 3 of the chromosphere was being inquired into, the lumi- 

 nosity of the tube was considerably reduced, and the colours 



