157 



treated with bromine water. After two hours the solution was filtered, and 

 evaporated to a bulk of 20 minims ; this gave a good reaction of gold to the 

 " chloride of tin " test. 



4th. 100 grammes of the hematite, with precipitated gold at the rate of 

 J dwt. per ton, treated as before, but this time well washed, at the expiration 

 of the two hours, and the washings evaporated along with the first filtrate, 

 gave a fainter, but still decided, reaction of gold to the same test. 



5th. Iodine, as tincture, substituted for bromine in experiments 3 and 4, 

 gave similar results ; the only variation made..was, that as a precautionary 

 measure allowing for its feebler, or rather slower, action, I gave contact for 

 twelve hours. 



To compare the results of the common amalgamating process with the 

 foregoing, I have made some careful experiments • and I find that it is not 

 certain, with the same expenditure of labour, to get reliable indications of gold, 

 when present in less quantity than 2 dwts. per ton, operating upon about 

 100 grammes of material, which is about the quantity I usually take. 



In summing up the results of these experiments, it appears then, that for 

 qualitative examinations for gold, or for quantitative determinations in certain 

 cases, iodine and bromine are each superior to mercury. It also appears that 

 a proportion of gold equal to J dwt. per ton, upon a bulk of 100 grammes 

 (about 4 ozs) of ferruginous matters, can be easily and rapidly detected. 



Of course, by operating upon larger quantities, gold could be discovered 

 by this process, were it present in far less quantities, but this is sufficiently 

 near for the majority of cases. 



These processes are especially adapted for the separation of gold from 

 sulphides, as the preliminary roasting is extremely favourable to them, not so 

 much chemically as mechanically, I think ; the loss in the substitution of 

 oxygen for sulphur, amounting to 25 per cent., by weight, while the volume 

 remains constant (or nearly so) ; hence there is a corresponding porosity in the 

 product, by which it is certain every atom of it is thrown open to contact 

 with the solution of these agents. 



This mechanical accessibility obviously cannot be taken advantage of by 

 mercury. 



With sulphides these processes are practically exhaustive, while, at 

 the same time, the simultaneous extraction of other matters is avoided, or, at 

 any rate, is so trifling, that the proper tests for gold can be safely applied 

 directly to the concentrated solution. 



Regarding the choice between iodine and bromine, I would prefer the 

 former, when mere traces of gold are supposed to be present ; or if the ore is in 

 a finely divided state, as is generally the case when the matrix is iron pyrites. 



In the roasting of such pyrites it is necessary to raise the temperature 

 towards the end to a full-red heat, in order to decompose the ferruginous 

 sulphates, since if these remained much iron would get into the solution. 



In the case of much carbonate of lime being present, it is proper to gently 

 reignite the roasted mineral, etc., with carbonate of ammonia, or much lime 

 might get into the iodine or bromine solution. 



On the other hand a very high temperature is to be avoided, for, from my 

 own experience, I find a considerable quantity of fine gold can escape detec- 

 tion in this way, by the partial vitrification of the more fusible of the 

 silicates. 



The identification of gold by the combustion of its salts with filter-paper, 

 as suggested in this paper, seems to promise a rapid method of estimating it, 

 comparatively, by the aid of a series of prepared test-papers, representing gold 

 in different degrees of dilution. 



