Luminosity of Gases. 133 



we ascribe it to the fact that, if momentarily dissociated, there 

 is instantly a reunion. Whilst, therefore, on the one hand, 

 the high temperature at which the gold-chloride spectrum is 

 developed compels the separation of the gold and chlorine, 

 the large excess of chlorine on the other hand, by the action 

 of mass, compels a recombination. Though this in a sense is 

 tantamount to saying that the gold-chloride molecules remain 

 undissociated, the dynamical view of dissociation obliges us 

 to picture a constant interchange between atoms of gold and 

 atoms of chlorine. 



We are obliged to conclude, therefore, that the experiments 

 recorded above are quite in harmony with the view advocated 

 by Pringsheim. 



In a previous part of this paper I have offered some criti- 

 cisms of the experiments on which Pringsheim based his 

 conclusions. Notwithstanding this I have never considered 

 his view to be .disproved : on the contrary, it seems prima 

 facie to be a reasonable explanation of many phenomena 

 occurring in flames. The facts I have now brought forward 

 are in harmony with it, but I believe the evideuce is still far 

 from complete, and that further experiments are necessary 

 to establish the doctrine that the light- emission from coloured 

 flames is a direct consequence of chemical processes. 



I have to express my grateful acknowledgments to Sir G. 

 Gr. Stokes, Bart., for the interest he has taken in the work 

 recorded in this paper and for his invaluable and ever ready 

 counsel. I have received much assistance from Mr. Frank- 

 land Dent, B.Sc, who especially has devoted the most patient 

 labour to the drawing of the spectra. 



The chief conclusions arrived at in this part of the paper 

 are : — 



I. When cupric chloride is introduced into a flame, three 

 substances are formed : metallic copper, cuprous chloride, 

 and cuprous oxide. The first of these give a bright yellow 

 flame and a continuous spectrum ; the second a bright blue 

 tint and brilliant spectrum of bands and lines; the last a green 

 tint and spectrum of not very strongly developed banas. 

 Under certain circumstances cupric chloride may exist in a 

 flame, when it gives a feeble ruddy tint and a continuous 

 spectrum. 



II. Gold chloride gives a flame- spectrum only in presence 

 of an excess of chlorine or of hydrochloric acid and oxygen. 



III. In the above cases the development of a spectrum is 

 concomitant with chemical changes affecting the substance 

 concerned ; a fact in harmony with the view as to the origin 

 of flame-spectra advocated by Pringsheim. 



