414 Memoir on various Combinations of Gold. 



of tills number. These differences may have proceeded from 

 differences in the degree of purity of the precipitate. 



Triple Salts of Gold. 



The precipitation by alkalies affords different results according 

 to the excess of acid in the solutiow. Even when we employ a 

 solution as little acid as possible, alkalies I'onn precipitates in it 

 cold J but even assisted by heat the precipitation is not com- 

 plete. If the solution be very acid, the precipitate cannot be 

 obtained cold, uor even hot, but in small quantity. This is 

 ejisily explained. All alkalies possess the property of forming, 

 v/ith the oxide, very sohible triple salts. As a proof of this : — 

 If we take a solution slightly acid, from which, consequently, 

 we may by alkalies precipitate oxide, cold, and add to it a sut- 

 ficient quantity of muriate of potash, or ot soda, or of barytes, 

 or strontian, or lime, the precipitates uill no longer be obtain- 

 able. Even ammonia, which precipitates gold in the fulminat- 

 ing powder, does not form it in a solution to which a sufficient 

 quantity has been added of muriate of ammonia. 



These very soluble triple salts, though not deliquescent, cry- 

 stallize with difficulty ; nor does the form of the alkaline salts 

 seem sensibly altered by the muriate of gold combined with 

 them. 



Though any of the alkalies will precipitate the oxide of gold, 

 with barytes the precipitate is most abundant. 



Precipitation of Gold hy Muriate of Tin at the Minimum. 



Precipitates by muriate of tin differ much according to cir- 

 cumstances. When very concentrated solutions of muriate of 

 gold and muriate of tin at the minimum are mixed, in whatever 

 proportion, the precipitate is gold in a metallic state ; but when 

 the proportion of tin is excessive the precipitate is blackish : and 

 on the contrary, when these solutions are mixed, diluted with 

 water, the precipitates are purple, but inclining more to violet 

 when the quantity of muriate of tin is more considerable — a cir- 

 cumstance which also causes in the precipitate a greater pro- 

 portion of oxide of tin. These different shades are rendered 

 still more sensible by applying them on porcelain : the preci- 

 pitates formed Avhen muriate of gold predominates, give a shade 

 more or less ro v •, those from mixtures in which muriate of tin 

 predominates give a colour bordering on violet. 



These precipitates are of a verv variable nature. M. Proust, 

 hy treating that which he tried with very weak nitro-muriatic 

 acid, concluded that it was composed of oxide of tin at the 

 minimum 76, and gold in a metallic state 24. By employing 



the 



