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strange and anomalous, tliat I was induced to make further researches into 

 tliis subject, for the purpose of ascertaining what other substances (if any) 

 could be added to this list of nuclei rendered, and whether any of them could 

 be nuclear for other metals during their slow precipitation, so that from an 

 enquiry thus extended some general principle might be discovered regulating 

 and explaining such deposits. 



The results of these I now beg to lay before the Society ; they have been 

 so singular and unexpected, and have taken a direction so different to that 

 contemplated for them, that I have had to change the title of this paper as 

 first adopted and worked to, to the one now assigned to it. 



I found, in the first place, on repeating the experiments adverted to, with 

 certain modifications, that in the case of wolfram, the tendency of gold to 

 deposit thereon might properly be referred to the well known reducing power 

 of the soluble proto-salts of iron upon salts of gold, since proto-oxide of iron 

 forms a considerable portion of this mineral, and is actually dissolved away 

 from it, to a small extent, by dilute acids at common temperatures ; at least, I 

 found it so in the case of a specimen of wolfram I had from the Museum, 



The case, therefore, so far as these results of Mr. Wilkinson's are concerned, 

 is reduced to one in which there only remains to consider the metallic sulphides 

 and arsenides — a set of minerals both chemically and mineralogically related 

 to each other. 



Now, in respect to these sulpliides, it is distinctly stated in Mr. Newberry's 

 paper, 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 regularly ;" having corroborated the correctness of this 

 statement, and also proved that the arsenides are similarly affected, it occurred 

 to me that, though hitherto quite unexpected, this decomposition was very 

 intimately connected with the first deposit of gold ujDon these sulphides ; that 

 in reality the commencement of metallic deposit was efiected, not by the inter- 

 action of organic matter as supposed, but by that of the several nuclei 

 themselves with the salt of gold. 



I therefore agitated a little finely-powdei'ed galena (siilpliide of lead) 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, the gold solution had become quite colourless, and on 

 testing it, not a trace of this metal could be found ; it had evidently been 

 absorbed as it were by the galena, and, in fact, a careful inspection of the 

 mineral showed it to be feebly gilded. 



Small cubes of galena simply immersed for a few hours in strong solution 

 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. 



