425 



GOLD. 



GOLD. 



426 



We shall now describe the more important compounds of gold. 

 Oxygen and gold unite, but not by direct action ; it has indeed been 

 stated that gold may be oxidised by the electric spark in atmospheric 

 air, but this is denied by Berzelius. 



Oxide, or Protoxide, of Gold (AuO) is prepared by adding a solution 

 of potash to one of protochloride of gold ; a green powder is separated, 

 which is the protoxide in question ; it must be washed and dried at a 

 temperature not exceeding 100 Fahr. ; if the heat exceed this it is 

 converted into metallic gold and peroxide ; indeed, this change is stated 

 by some authors to occur at almost any temperature ; it is, at any rate, 

 an extremely unstable compound. 



Peroxide, or Teroxide, of Gold (Au0 3 ) is best obtained, according to 

 Pelletier, by decomposing solution of perchloride of gold by digesting 

 it with a slight excess of magnesia ; the peroxide of gold precipitates 

 in combination with the magnesia ; after being washed the precipitate 

 is treated with dilute nitric acid, which dissolves the magnesia and a 

 little of the peroxide of gold, but leaves the greater part unacted upon ; 

 it is a hydrate, of a bright reddish-yellow colour ; but when concen- 

 trated nitric acid is used instead of dilute, the oxide is anhydrous, and 

 nearly black. This oxide is decomposed by exposure to daylight, its 

 oxygen being expelled. This oxide is with difficulty soluble in any 

 acid ; thus, although sulphuric acid dissolves a small portion, it is pre- 

 cipitated by water. It appears indeed to possess rather the powers of 

 an acid than a base, and has been called auric acid, and it combines 

 with potash, soda, and baryta, to form salts, which have been termed 

 aurates. 



Neither nitrogen nor hydrogen combines with gold. 



Chlorine and Gold unite to form two compounds. The perchloride 

 is most readily obtained; it may be formed by treating gold with 

 nascent chlorine, derived from the mutual decomposition of nitric and 

 hydrochloric acids, called a'iua reyia. 



Perchloride, or Terchloride, of Gold (AuClj) gives a yellow-coloured 

 solution, which becomes nearly red by evaporation, owing to concen- 

 tration and the expulsion of any excess of acid. Its taste is acrid and 

 bitter ; with excess of acid this salt crystallises in long needle-form 

 crystals of a bright yellow colour, which are unalterable in a dry 

 atmosphere, but deliquesce in a moist one ; on the contrary, when a 

 neutral solution is evaporated until chlorine commences to be evolved, 

 a deep ruby-coloured crystalline mass of perchloride of gold is obtained. 

 Gold is precipitated in the metallic state from the perchloride even by 

 the action of light ; hydrogen, charcoal, phosphorus, and many metals, 

 produce a similar effect ; so also do the protosulphate of iron, &c. The 

 skin is stained of a purple colour by this solution. 



Protochloride of Gold (AuCl) is obtained by heating the perchloride 

 to a temperature of about 350 Fahr. in a porcelain vessel. If it be 

 too strongly heated, gold is deposited ; it is better therefore to heat it 

 rather less, and to treat the residue with water, which dissolves the 

 perchloride and leaves the protochloride, which is a colourless saline 

 mass, unalterable in the air, but in contact with water gradually changes 

 into metallic gold and perchloride : boiling water decomposes it in- 

 stantly. 



Terbromide of Gold (AuBr,) is procured by dissolving the metal in a 

 mixture of hydrobromic and nitric acids. The solution yields by 

 evaporation a deep red-coloured saline mass ; sometimes it yields crystals. 

 This salt has so intense a colour that one part of it tinges 5000 parts of 

 water. 



.-iiuret of Gold (AuS,) is prepared by passing hydrosulphuric acid 

 gas into a solution of perchloride of gold ; it is a black powder, which, 

 when heated, readily separates into sulphur and gold. This sulphuret 

 is soluble in the alkalies potash and soda, and prepared in a different 

 mode it is employed in gilding porcelain vessels. 



Photphuret nf Gold may be obtained either by directly heating gold- 

 leaf and phosphorus in a tube deprived of air, or by passing phosphur 

 retted hydrogen gas into a solution of chloride of gold. As obtained 

 by the first process it is a gray substance of a metallic lustre ; when 

 n;d by the second it is a brownish powder. When heated in the 

 air it is decomposed. Its composition has not been determined. 



Tiriodide of Gold (AuI 3 ). Iodine and gold do not act upon each 

 other even when heated together ; but when a solution of iodide of 

 potassium is mixed with one of perchloride of gold, yellowish brown 

 iodide of gold is precipitated, which is insoluble in cold water, dissolved 

 by the alkaline solutions, and decomposed by heat. 



Having described the principal binary compounds which result from 

 the union of gold with non-metallic elements, we shall mention the 

 more important compounds which it forms with the metals. 



A lh,i/i of Gold. Most metals are susceptible of combining with gold ; 

 but it is to be observed that nothing is known of the compounds which 

 it !>,rta* with the metals of the alkalies and earths, as pot 



II, &C. 



.</ Gold. This alloy is obtained by heating gold-leaf and 

 arsenic ; with the application of a gentle heat the vaporised arsenic 

 combines with the gold; it is a very brittle, gray, metallic compound ; 

 it is readily decomposed by calcination, and the whole of the arsenic is 

 expelled; ,fo of arsenic Li sufficient to destroy the malleability of gold 

 without altering its appearance ; jj,, renders it gray and brittle. 



Tellurium and Gold, but mixed also with a considerable portion of 

 lead, occur in combination, constituting three varieties known aa 

 graphic tellurium, yellow tellurium, and black tellurium. 



Antimony and Gold. According to Hatchett, gold loses its ductility 

 by combining with ^-^ of its weight of antimony. It is prepared 

 by fusing the metals together ; it is of a pale colour and fine grained ; 

 when long calcined in an open crucible the antimony is entirely 

 expelled. 



Mamjanese and Gold. This alloy is of a pale colour ; breaks readily 

 under the hammer, and exhibits a spongy coarse-grained fracture. 



Zinc and Gold yield an alloy of a pale greenish colour like brass, and 

 totally devoid of ductility. 



Tin and Gold. Colour very pale whitish-yellow; bends readily when 

 the bar is not more than one-eighth of an iuch thick ; but when passed 

 between rollers breaks longitudinally into several pieces. The grain of 

 the fracture is fine, inclining to an earthy appearance, and is of a pale 

 yellowish-gray colour. 



Iron and Gold form an alloy of a pale yellowish-gray colour ; it is 

 very ductile, and may be rolled from the thickness of three-quarters of 

 an inch to that of a guinea. 



AVoM and Gold. This alloy is of a fine brass colour ; it immediately 

 breaks under the hammer, with a coarse-grained earthy fracture. 



Cobalt and Gold. A pale yellow alloy mixed with gray ; it is brittle, 

 and has a fine-grained earthy fracture. 



Copper and Gold combine in all proportions, without altering the 

 colour of the gold; th density is diminished, but the hardness is 

 increased. The alloy employed for ordinary jewellery contains 23'6 

 per cent, of copper ; this tarnishes by use, owing to the oxidLsement of 

 the copper ; the original colour is restored by treating with ammonia. 

 Copper is used to alloy the gold of coin, in order that it may be ren- 

 dered sufficiently hard to stand the wear to which it is exposed ; the 

 amount of copper is 1 in 12. Its specific gravity is 17'157. Of this 

 alloy, 20 troy pounds are coined into 934 sovereigns and one half- 

 sovereign. 



Sinmuth and Gold form a very brittle alloy. It is sufficient to alloy 

 gold with ^ of bismuth to render it brittle. A compound of 8 

 bismuth and 92 gold is of a pale yellow colour and brittle. 



Sitter and Gold combine well ; the resulting alloys are very ductile. 

 A small quantity of silver renders gold pale ; 5 parts to 100 are suffi- 

 cient for this effect. This compound was used by the ancients, and 

 called by them eleclrum ; we have already mentioned the existence of 

 several native alloys of these metals, and that they are always combined 

 in definite proportions. 



Lead and Gold. This alloy is very brittle, even when the lead forms 

 only ^ of the alloy ; even the fumes of lead destroy the ductility of 

 gold. 



Mercury and Gold combine with great facility, and yield a white 

 alloy, usually called an anutlf/am, much used in gilding. [GlLDiNo.] 

 On account of the readiness with which these metals unite, mercury is 

 used, as already noticed, for separating gold in the process called ' 

 amalgamation. 



J'/iittnum and Gold combine in all proportions ; these alloys are 

 fusible. Platinum readily destroys the colour of the gold ; the presence 

 of only 0'02 of platinum is recognised by the extraction of colour. 



Salts of Gold, in which the oxide is a base, are obtained with great 

 difficulty ; indeed, when the peroxide is dissolved in nitric, acetic, or 

 sulphuric acid, they require to be concentrated, the . oxide does not 

 saturate the acids, and the solutions are all decomposed by water. 



Salts of gold in which the peroxide acts as an acid do not, except 

 one of them, possess any remarkable properties ; this peroxide of gold 

 is soluble in potash and soda, but no very definite or crystallised com- 

 pounds of them have been formed. Ammoniuret of gold, sometimes 

 called aurate of ammonia, is fornjed when ammonia is added to a solu- 

 tion of perchloride of gold ; water is decomposed, and peroxide of gold 

 precipitated in combination with a portion of the ammonia. This 

 substance is of a yellowish-brown colour ; it is to be collected in a 

 filter, washed with a little water, and dried at a temperature below 

 212". When this substance is heated it explodes violently, the gold is 

 reduced to the metallic state, water is formed by the union of the oxy- 

 gen of the oxide of gold with the hydrogen of the ammonia, and azotic 

 gas is given out. It probably consists of two equivalents of ammonia 

 and one equivalent of peroxide of gold ; it may, however, possibly be a 

 nitride of gold. 



Some of the> most permanent salts of gold are the double chlorides : 

 thus the potaasio-chloride of gold crystallises in small hexagonal tables ; 

 it is, however, quickly efflorescent in the air. The sodio-chloride of 

 gold crystallises hi long quadrilateral prisms which are not altered by 

 exposure to the air. When heated it melts iff its water of crystalli- 

 sation, and afterwards loses a little chlorine. It appears to be the most 

 stable of all the salts of gold. It consists of 14'US parts of chloride of 

 sodium, 76-32 of perchloride of gold, and 9 of water. The chloride of 

 ammonium, the chloride of lithium, barium, &c., form crystalline 

 double salts with the chloride of gold. 



There is a compound containing gold which has been long used for 

 giving a red colour to glass, under the name of the purple po<cd' r nf 

 Catsiut. According to Berzelius, it is composed of 28'35 of gold, 64 of 

 peroxide of tin, and 7'65 of water. Other chemists have assigned a 

 different constitution to this substance. It is of a fine purple colour, 

 and may be prepared by adding a solution of perchloride of gold to a 

 mixed solution of protochloride and perchloride of tin, or simply by 

 putting tin-foil into a solution of gold. 



