158 PROCEEDINGS OF SECTION B. 



In respect to sulphides, it is distinctly stated by Mr. Newbery, 

 that in even weak solutions of terchloride of gold (the salt used in 

 his experiments), they decompose but so slowly as not to " interfere 

 with the deposit taking place i^egularly." Having corroborated 

 this statement, and also proved that the arsenides are similarly 

 affected, it occurred to me, that in reality, the commencement of 

 metallic deposit was affected, not by the intei'action of organic 

 matter as supposed, but by that of the sevei^al nuclei themselves, 

 with the salt of gold. 



I therefoi'e agitated a little finely-powdered galena with a weak 

 solution of terchloride of gold, omitting the addition of organic 

 matter, and taking every precaution against its presence 

 accidentally, when I found, after a little while, that the gold solution 

 had become colourless, and on testing it, not a trace of this metal 

 could be found ; a careful inspection of the galena showed it to 

 be feebly gilded. 



Small cubes of galena simply immersed for a few hours in weak 

 solutions of the gold salt, without organic matter, were so 

 thoroughly gilded over the greater part of their surfaces, that in 

 certain positions they could not be distinguished by the eye from 

 gold. 



Chloi'ide of gold was also found to be reduced by contact with 

 the following sulphides, — including those mentioned by Mr. 

 AVilkinson : — sulphides of iron (proto and bi-sulphide), sulphides 

 of copper (ferros-sulphide and sub-sulphide), and the sulphides of 

 zinc, tin, molybdenvim, lead, mercury, silver, antimony, bismuth, 

 arsenic, platinum and gold ; and among the arsenides, mispickel, 

 and arsenide of silver. Cubical iron pyrites is rather slow in its 

 action upon this solution of gold, while sulphide of antimony 

 scarcely affects it at all at first, but after some hours contact with 

 it, reduction goes on rajoidly, perhaps by aid of some voltaic 

 action. All these effects were produced at common temperatures 

 (with the exception of that with sulphide of bismuth), and other 

 experiments with iron and copper pyrites prove that similar 

 effects are produced when all light is excluded, so there is no 

 reason to suppose that light has been concerned in any of these 

 re-actions. 



In the case of some of the highly-coloured minerals, such as 

 cinnabar and arsenide of silver for instance, it is necessary to 

 operate upon their streak, in order to obtain a visible deposit 

 upon them. 



A portion of the metal of the sulphide operated upon was 

 uniformly found in the solution afterwards, and also a little 

 sulphuric acid ; the mode, tlierefore, in which these effects were 

 produced, was evidently by the oxidation of both the constituents 

 of the nucleus employed, at the expense of the chloride, or rather 

 the hydrochlorate of oxide of gold, supposing as seems probable, 

 an equivalent of water combined with it as administered. 



