264 ANNUAL OF SCIENTIFIC DISCOVERY. 



/ 



until it gives an alkaline reaction with turmeric paper. Turbidity imme- 

 diately ensues, when it is mixed with chloride of barium ; aurate of 

 baryta precipitates as a yellow powder. When the precipitate begins to 

 assume a whitish appearance, the addition of chloride of barium must be 

 discontinued, as all the gold oxide has gone down, and the alkali commenced 

 to act upon the baryta of the chloride. The aurate of baryta is then to be 

 washed until the waste- waters cease to be precipitated by sulphuric acid. 

 The aurate is then heated to boiling, with dilute nitric acid, in order to 

 eliminate the oxide of gold. By washing until the water no longer reddens 

 litmus paper the oxide becomes pure, and must be dried between the folds 

 of bibulous paper by exposure to air. Jour, de Pharm. 



ON THE KECOVERY OF GOLD AND SILVER FROM THE FLUIDS 

 EMPLOYED FOR ELECTRO-PLATING AND GILDING. 



The following method of recovering gold and silver from the fluids em- 

 ployed in electro -plating and gilding is described by Piof. Bolley, in the 

 " Centrablatt," (German magazine of science.) 



The cyanide of gold, dissolved in an excess of cyanide of potassium, 

 resists most means of separation ; even sulphuretted hydrogen produces 

 no precipitate in it. The complete separation of the gold cannot be effected 

 in the humid way ; and this has given rise to the propositions of Bottcher, 

 Hessenberg, Eisner and others, to evaporate the fluid, mix the dry resi- 

 due with an equal quantity of litharge, fuse the mixture at a strong red 

 heat, and dissolve the lead from the fused mass by hot dilute nitric acid ; 

 by this means the gold is left as a loose sponge. A more recent proposi- 

 tion is that of Wirnmer, by which the mass left by evaporating the fluid to 

 dryness on the water- bath is mixed with one and a half times its weight 

 of nitrate of potash, and thrown in small portions into a red-hot Hessian 

 crucible. The explosions must be waited for, and the process continued 

 until the entire mass runs smoothly. The first process has nothing 

 against it, except the necessity of a strong fire and the employment of 

 nitric acid ; the second, on the contrary, is very unpleasant and unsafe in 

 its performance. It is sufficiently well known that there is no substance 

 with which nitrate of potash detonates so violently when heated as with 

 cyanide of potassium. If the portions of the mixture employed be only a 

 little too large, very violent explosions are produced, which cannot take 

 place without loss. 



The following process may be adopted in small operations with a plat- 

 inum crucible over a spirit-lamp. The dried mass of salts is mixed with 

 an equal quantity of powdered muriate of ammonia, and gently heated. 

 The ammoniacal salts decompose the cyanides of the metals, forming 

 cyanide of ammonium, which is decomposed and volatilized, whilst the 

 acid of the ammoniacal salt or the halogen combined with the ammonium 

 unites with the metal which had been combined with cyanogen. In the 

 present case, muriate of ammonia forms chloride of potassium, chloride of 



