GOLD AND SILVER REFINING 719 



great difficulty and expense, if the proportion of gold be too small to admit of the 

 employment of muriatic acid. 



By cupellation with lead, gold may be deprived of any antimony united with it. 



Tin gives gold a remarkable hardness and brittleness ; a piece of gold, exposed for 

 some time over a bath of red-hot tin, becomes brittle. The same thing happens more 

 readily over antimony, from the volatility of this metal. A two-thousandth part of 

 antimony, bismuth, or lead destroys the ductility of gold. The tin may be got rid of 

 by throwing some corrosive sublimate or nitre into a crucible, containing the melted 

 alloy. By the first agent, perchloride of tin is volatilised; by the second, stannate 

 of potash forms, which is carried off in the resulting alkaline scoriae. 



Gold treated by the process of amalgamation contains commonly nothing but a little 

 silver. The silver is dissolved out by nitric acid, which leaves the gold untouched; 

 but to make this parting with success and economy on the great scale, several pre- 

 cautions must be observed. 



If the gold do not contain fully two-thirds of its weight of silver, this metal, being 

 thoroughly enveloped by the gold, is partially screened from the action of the acid. 

 Whenever, therefore, it is known by a trial on a small scale, that the silver is much 

 below this proportion, we must bring the alloy of gold and silver to that standard by 

 adding the requisite quantity of the latter metal. This process is called quartation. 



This alloy is then granulated or laminated ; and from twice to thrice its weight of 

 sulphuric or nitric acid is to be boiled upon it ; and when it is judged that the solu- 

 tion has been pushed as far as possible by this first acid, it is decanted, and new acid 

 is poured on. Lastly, after having washed the gold, some sulphuric acid is to be boiled 

 over it, which carries off a two or three thousandth part of silver, which nitric acid 

 alone could not dissolve. Thus perfectly pure gold is obtained. 



The silver held in solution by the sulphuric or nitric acid is precipitated in the me- 

 tallic state by copper, or in the state of chloride by sea-salt. See SILVER. 



Gold has less affinity for oxygen than any metal. When alone, it cannot be 

 oxidised at any degree of heat with contact of air, although in combination with other 

 oxidised bodies, it may pass into a state of an oxide, and be even vitrified. The purple 

 smoke into which gold-leaf is converted by an electric discharge is not an oxide, for it 

 is equally formed when the discharge is made through it in hydrogen gas. There are 

 two oxides of gold ; the first or protoxide is a green powder, which may be obtained 

 by pouring, in the cold, a solution of potash into a solution of the metallic chloride. 

 It is not durable, but soon changes in the menstruum into metallic gold and peroxide. 

 Its constituents are 96*13 metal and 3*87 oxygen. The peroxide is best prepared 

 by adding magnesia to a solution of the metallic chloride ; washing the precipitate with, 

 water till this no longer takes a yellow tint from muriatic acid ; then digesting strong 

 nitric acid upon the residuum, which removes the magnesia, and leaves the peroxide 

 in the form of a black or dark brown powder, which seems to partake more of the 

 properties of a metallic acid than a base. It contains 10*77 per cent, of oxygen. For 

 the curious combination of gold and tin, called the purple precipitate of Cassius, see 

 PURPLE OF CASSIUS ; COLOURS. 



GOX.D AND SILVER REFIWING. Since the object of this book is to treat 

 more especially of the application of scientific processes to commercial undertakings, 

 it will not be out of place to give some account of the processes by which gold and 

 silver are refined, or rendered free from other metals. In the laboratory, where 

 chemical manipulation has reached a great way to perfection, the precious metals are 

 separated by nitric acid and other agents, but the processes are far too expensive and 

 tedious to admit of being used upon a large scale. 



For the purposes of rendering gold containing foreign metals sufficiently pure for 

 the operations of coining, the late Mr. Warrington described a process by which 

 fused gold is treated with black oxide of copper, with a view to oxidising those 

 metals which render gold too brittle for manufacture into coin. Mr. Warrington 

 proposed to add to fused gold, which is found to be alloyed with tin, antimony, and 

 arsenic, 10 per cent, of its weight of the black oxide of copper, which, not being 

 fusible, is capable of being stirred up with the fused mass of gold, just as sand may 

 be stirred up with mercury, but with this great advantage, that the oxide of copper 

 contains oxygen, with which it parts readily to oxidise any metal having a 

 greater affinity for oxygen than itself. The metals, once oxidised, become lighter 

 than the fused metal, and mixing mechanically, or combining chemically with the 

 black oxide of copper, float to the surface and are removed. In the execution of 

 Mr. Warrington's proposition, it is imperative to use crucibles free from reducing 

 agents, such as carbon, and it is found that half an hour is sufficient time to allow 

 the contact of the oxide of copper with the fused gold. 



It has been generally stated by those supposed to be acquainted with the subject, 

 that gold containing tin, antimony, and arsenic is so brittle as to render it wholly unfit 



