366 Professor Arthur Smithells [March 12, 



WEEKLY EVENING MEETING, 

 Friday, March 12, 1897. 



Sib Frederick Abel, Bart. K.C.B. D.C.L. LL.D. F.E.S. 

 Vice-President, in the Chair. 



Professor Arthur Smithells, B.Sc. F.I.C. 



The Source of Light in Flames. 



When hydrogen burns in oxygen the gases unite to form steam, and 

 a flame of simple structure is obtained. The light is of very feeble 

 intensity, so feeble when the hydrogen is highly purified and when 

 both gases are free from dust, that the flame is scarcely visible in a 

 room from which all other light is excluded.* 



To what is the light of this flame due ? It is not sufficient to 

 say that it is the result of chemical action attended by the evolution 

 of much heat. Light is of an undulatory nature, and the undulations 

 arise during an oscillatory process associated with matter. We 

 desire to know with what particular kind of atoms or molecules the 

 light of a hydrogen flame is associated. It may be said that when 

 hydrogen combines with oxygen the heat that is produced is 

 necessarily contained, as it were, in the steam, and that therefore it 

 is the steam that glows. This raises the question as to what evidence 

 we have, apart from flames, of the possibility of making gases glow 

 by the simple process of heating them. The evidence is nearly all 

 negative. None of the common gases, including those contained in 

 the best known flames, have been made to glow when heated by a 

 2)urely baking or roasting process to the highest obtainable tempera- 

 ture. The passage of an electric discharge through the gases is not 

 to be regarded as merely a heating process. 



Aii^ong the gases that can be made to glow, the most conspicuous 

 is iodine. The vapour of this substance shows a distinct red glow at 

 a temperature below that at which glass is visibly red.l [Experi- 

 ment shown.] It is possible that some chemical action, namely, 

 dissociation and recombination, may be in progress in the iodine 

 vapour, and that the emission of light may be due to this. A similar 

 glow, however, has been obtained with bromine, and, to a less extent, 

 with chlorine,^ at temperatures which exclude the likelihood of 

 dissociation. 



* Stas, CEuvres, tome iii. p, 228. 



t Salet, ' Analyse Spectrale,' p. 173 ; see also Phil. Mag, [v] 37, p. 245 (1894). 



X Evershed, Phil. Mag. [v] 39, p. 460 (1895). 



