Luminosity of Gases. 131 



only metallic gold. There can be no doubt, indeed, that gold 

 trichloride is decomposed completely at a temperature far 

 below the average one of the flam? in which it yields a 

 spectrum. It has, however, been observed by Debray ( Compt. 

 Bend. lxix. p. 984, 1869) that gold chloride may be obtained 

 at 300° by passing a current of chlorine through a tube con- 

 taining the metal, from which it is evident that the salt may 

 exist at abnormally high temperatures provided an excess of 

 chlorine be present. To test this question further, I per- 

 formed the following experiment. 



By means of an electric current a pure gold wire was 

 raised to bright redness in a tube partially exhausted of air. 

 Chlorine was then allowed to enter the tube in considerable 

 quantity. When this was done, a slight sublimate was imme- 

 diately formed on the sides of the tube opposite to the glowing 

 wire, and this sublimate on examination proved to be gold 

 trichloride. From this it is apparent that in presence of 

 abundance of chlorine, gold chloride may be formed at a red 

 heat, and so emit its characteristic spectrum. The case is not 

 an exception, but rather an example of the generalization 

 that dissociable bodies become stable in presence of excess of 

 one of the products of dissociation. 



It is now easy to understand why gold chloride introduced 

 into a flame on a platinum wire gives a spectrum. The great 

 bulk of the salt is decomposed so as to give an atmosphere of 

 chlorine in which a small portion of the salt volatilizes with- 

 out decomposition. As has been already noted, much metallic 

 gold remains on the wire. 



I have shown that in using the separator the gold-chloride 

 spectrum may be maintained by introducing abundance of 

 hydrochloric acid or chloroform (the interconal gases con- 

 tained no free chlorine), and that though the coloration may 

 be for the most part quenched by making the flame hotter, it 

 is most persistent at the edges. From these facts it appears 

 that, so far as the formation and stability of gold chloride are 

 concerned, an atmosphere of hydrochloric acid and oxygen is 

 potentially one of chlorine, the stable arrangement of the 

 system at a high temperature changing as follows * : — 



30 2 , 12HCl,4Au becomes 6H 2 0,4AuCl 3 . 



It is clear, therefore, that provided we have a sufficient 

 quantity of hydrochloric acid or free chlorine, gold chloride 

 may exist in a flame at a temperature sufficiently high to 

 produce its spectrum. 



* It is stated in Gmelin's ' Handbook,' vi. p. 215, on the authority of 

 Proust, that gold is soluble in hot hydrochloric acid in presence of air. 



K 2 



