Memoir on various Comlinalio7is of Gold. 409 



and chemists were ignorant whether it forms several oxides, of 

 the manner of preparing them, of the quantity of oxygen that 

 enters into them, and of the various circumstances that accompany 

 the precipitations of this metal. To solve these questions is tlie 

 principal oljject of this memoir, which may be serviceable, as 

 preparations of gold appear to be coming into use in medicine. 



Dissolution of Gold. 

 The best solvents of gold are nitro-muriatic acid and oxy- 

 muriatic acid. M. Proust has observed that muriatic acid will, 

 in time, also dissolve it, if heat be employed, especially if it be 

 previously well divided, as by precipitation by sulphate of iron 

 at a minimum. Nitric acid has but little action on this metal : 

 nitro-muriatic acid is generally employed : but I prefer the oxy- 

 muriatic acid, because it gives a more pure solution, and with 

 less redundance of acid. 1 have remarked that when the solu- 

 tion has been exposed to heat, to render it less acid, that its 

 golden yellow colour has passed to a deep tint of a brownish 

 red, and that it retains this_ colour, though proportionably 

 weakened, when diluted with water. Tlie nitro-muriatic solu- 

 tion, when deprived of the excess of acid by evaporating to dry- 

 ness, presents the same result ; which proves that the orange 

 yellow colour of solutions of gold is owing to excess of acid. 

 By saturating them with potash or soda they become red, as 

 wlien exposed to heat. 



Precipitation of Gold hj Hydrogen. 

 If a stream of hydrogen gas be made to pass for some time 

 through a diluted solution of muriate of gold, it assumes a fine 

 purple-red colour like that of wine, without forming any preci- 

 pitate. I left some of this for a long time in a glass vessel 

 hermetically closed. No sensible change took place in the co- 

 lour, but an extremely light precipitate was formed at the bot- 

 tom of the vessel, which at first appeared purple, but when put 

 into water exhibited that blue tinge peculiar to gold held in sus- 

 pension in a metallic state. The liquid exposed to heat be- 

 came again yellow ; metallic traces formed on the surface, and 

 the capsule took a violet colour, having all the metallic brilliancy. 

 The action of muriatic acid did not affect this appearance ; but 

 nitro-muriatic acid destroyed it speedily, and had then all tiie 

 characters of a solution of gold. 



Precipitation of Gold ly sulphuretted Hydrogen. 

 A current of sulphuretted hydrogen gas passed through a so- 

 lution of gold immediately renders it turbid, and a black preci- 

 pitate is (mickly formed, which is perfectly homogeneous, and 

 ^ ^ J exhibits 



