128 MESSRS. A. SMITIIKU.S, KTC., CONDUCTIVITY AND LUMINOSITY OK 1 '!.. \MK.s. 



We can say, therefore, that hydrochloric acid prevents the occurrence in the flame 

 of the substance on which the emission of light depends, and that this substance is 

 not merely the metal existing as an ion. 



Thus the question which originally impelled us to our experiments is answered. 

 Since it is answered in the negative, some other explanation of the luminosity of 

 salt vapours must be sought, and we fall back upon the alternatives which have 

 already been discussed by one of us on a previous occasion (' Phil. Mag.,' (V), 37, 

 245 (1894)). 



It seems clear that the coloration of a flame containing vaporised sodium chloride 

 is dependent upon the presence of the vapour of the metal in the non-ionic state, and 

 the explanation of this, most conformable to our experiments, is that a very small 

 proportion of the chloride is first converted into oxide by oxidising gases in the 

 flame and the oxide then reduced by reducing gases in the flame. The presence of 

 hydrochloric acid prevents the formation of oxide and hence prevents also the 

 liberation of the metal. Whether the metal vapour glows solely in consequence of 

 its high temperature, or because of vibrations imparted to its atoms during chemical 

 change, is a question which our experiments were not designed to answer. 





