GOLD. —Compounds of Gold. 109
a crucible, in a suitable furnace ; tlie magnesia, salammoniac, lime, and tartar are then added, separ-ately and by degrees, in the form of powder; thewhole is stirred for about thirty minutes to thoroughlymix the ingredients, and the zinc is then thrown onthe surface, having first been ground into very smallgrains; the stirring is continued until the fusion iscomplete. The crucible is now covered, and thefusion continued for about thirty-five minutes, whenit is uncovered and skimmed with care, and thecontents are run into a mould of moist sand ormetal. The material which results may be castat such a temperature that any ornamental form maybe given to it. It is very fine-grained, and is alsodamascene, malleable, and capable of taking a verybrilliant polish. When tarnished by oxidation, itsbrilliancy can be restored by a little acidulated water.If tin be employed instead of zinc, the alloy will bestill more brilliant.
Compounds of Gold. —Gold is known to formcompounds with oxygen, sulphur, and the haloids—•chlorine, bromine, iodine, cyanogen, &c., but thenumber of its compounds is few in comparison withmost other metals.
Oxides. —These are two in number, namely:—(1.)The suboxide or protoxide (Au 2 0), which may beprecipitated as a dark green powder by adding adilute solution of potash to a solution of the proto-chloride of gold. If this precipitate be treated withammonia, fulminating gold is formed ; an extremelydangerous substance, exploding with fearful violence,in consequence of which its composition has not, asyet, been accurately determined. Heated to about250° C. (482° Fahr.), this oxide is decomposed intooxygen and metallic gold.
(2.) Peroxide, or anhydrous auric acid (Au 2 0 3 ),may be prepared in several different ways. Themethod usually given is analogous to the above forpreparing the protoxide, and consists in decomposinga solution of the tcrchloride of gold by means ofmagnesia or oxide of zinc: insoluble aurates ofmagnesia or zinc are formed, which on being treatedwith cold dilute nitric acid yield hydrated auricacid, from which anhydrous auric acid may beobtained by heating at 100° C. (212° Fahr.), inorder to expel the water. M. E. Feemy gives asomewhat complicated method for preparing thisoxide in a state of purity, which is too long to begiven here in detail, and for which the reader musttherefore be referred to the “Traite de Chemie” ofMM. Pelouze and Fremy, tome iii. p. 1209. Itconsists in the preparation of aurate of potash bymeans of terchloi'ide of gold and potash, and fromthis aurate the auric acid is subsequently extractedby sulphuric acid. It is a brown or yellowpowder (the colour varying with the strength ofacid used), and is readily reduced by light or atemperature of 245° C. (473° Fahr.) to metallic goldand oxygen. Carbon, boiling alcohol, and nearlyall the organic acids (especially oxalic) have thesame power of reduction. It is soluble in thehaloid acids, forming the salts referred to below. Asis the case with the protosalt, when treated with [
ammonia, fulminating gold is produced in a manneranalogous to that in the case of silver. Auric acidforms salts termed aurates. Thus we have theaurate of potassium (KAu0 2 ,3II 2 0), a solution ofwhich may be used in electro-gilding.
Sulphide. —When sulphuretted hydrogen gas ispassed through a cold solution of terchloride of gold,a bisulphide (Au 4 S 4 ) is produced, which however isunimportant. If instead of a cold solution it beboiling, the aurous sulphide (Au 2 S) is formed. Itis a dark brown or black powder, which combineswith the protosulphide of potassium or sodium,forming double salts. The former of these is em-ployed in the arts for the purpose of gilding china,and goes by the name of Burgos lustre.
Chlorides. —Gold and chlorine form two distinctcompounds, namely, the protochloride (AuCl) andthe terchloride (AuC1 3 ).
The protochlmide is formed by gently heating theterchloride to a temperature of about 200° C. (392°Fahr.), when a portion of the chlorine is expelled,leaving a pale yellow salt, which is decomposed onexposure to light.
Terchloride. —This, the most important compoundof gold, is obtained when gold is dissolved in aquaregia, and if this solution be evaporated to dry-ness at a temperature not exceeding 250° Fahr.(taking care to have an excess of hydrochloric acid),the salt will be obtained as a red deliquescentdifficultly crystallizable mass. It is decomposed byeither light or heat into free chlorine and proto-chloride of gold, and if the heat be too great this isfurther decomposed into chlorine and metallic gold.It is a remarkable fact that although this salt isdecomposed at 200° C. into Cl and AuCl, and ata still higher temperature into Au and Cl; yetAuCl s may be sublimed and then obtained incrystals of a considerable size, even at temperatureshigher than that at which it is usually decomposed.The perchloride is very soluble in water and alcohol,also in ether, and this latter solution was formerlyused in medicine under the name of potable gold ; itwas also used for the gilding of steel. In com-bination with hydrochloric acid it forms a yellowcrystalline salt, which is very soluble in water ; andfrom this solution, by evaporation, the long yellowcrystals may be obtained. If treated with causticpotash, aurate of potassium and chloride of potas-sium are produced; and as with the oxides, theaddition of ammonia causes the formation of ayellow precipitate of fulminating gold. An immensenumber of substances have the power of depositinggold from its solution in aqua regia, among whichmay be mentioned carbonic oxide, phosphorus,nitric dioxide, many animal and vegetable sub-stances, most metals, sulphurous and phosphorousacids, salts of protoxide of mercury and of pro-toxide of iron, &c. Some of these afford a con-venient method for the preparation of pure gold,as has been described on p. 68. The gold is inmany cases deposited in minute particles, whichgive a red or purple colour to the solution, andFaraday made some interesting discoveries with