498 



Prof. A. Smithells on the 



This aureole was developed most plainly at the tips of the 

 electrodes and thinned off towards the middle of the spark. 

 As the purification proceeded, that is, as fresh supplies of 

 hydrogen were used, the Swan spectrum gradually faded 

 until eventually a state was reached where the spectrum was 

 not observed on first sparking, the discharge between the 

 carbon points being of a pure red colour. The Swan spec- 

 trum, however, appeared after continued sparking. The time 

 taken for the Swan spectrum to appear in this way was 

 longer as the operations continued. The phenomena in fact 

 pointed to the conclusion that something which facilitated 

 the production of the Swan spectrum was gradually being 

 eliminated as the purity of the hydrogen was increased. 



Another important observation also was made at this stage. 

 It was found that if the discharge were stopped when the 

 Swan spectrum had become well developed, and the whole 

 apparatus allowed to rest a few hours, on restarting the 

 discharge the Swan spectrum would have either wholly 

 disappeared or have become much reduced in intensity. This 

 disappearance or enfeeblement of the Swan spectrum was 

 only to be noticed when the sparking tube was attached to 

 the pump. If the tube was sealed off the fluctuation was 

 no longer to be noticed. This points unmistakably to the 

 conclusion that the appearance of the Swan spectrum was 

 contingent upon the accumulation in the sparking tube of 

 some gas which on standing diffused into the adjoining 

 chambers of the pump. 



I attach great importance to this observation since it seems 

 to connect in a positive way the appearance of the Swan 

 spectrum with the formation of a gaseous compound of 

 carbon. 



The ultimate stage of purification of materials and apparatus 

 reached in my experiments was such that the discharge between 

 the carbon points in the hydrogen could be passed for twenty 

 minutes before the Swan spectrum began to appear. 



If these experiments are accepted as conclusive evidence 

 connecting the Swan spectrum with a compound of carbon 

 there remains the further question as to what this compound 

 of carbon really is. A decision of this question is not to be 

 made at once. Acetylene and methane are both produced 

 by the discharge between carbon poles in hydrogen, and as 

 there is no spectrum known to be undoubtedly due to either 

 of these gases, my experiments might be held to support the 

 view of Angstrom and Thalen that the Swan spectrum is due 

 to acetylene or possibly to marsh-gas. Apart, however, from 

 • the other evidence already adduced in this paper to connect 



