GOLD. 



property of detonating with a very consi- 

 derable noise, when gently heated. It is 

 known by the name of fulminating gold. 

 The presence of ammonia is necessary to 



an aqua-regia previously made, by adding 

 sal ammoniac to nitric acid ; or by preci- 

 pitating the gold from pure aqua-regia, 

 by means of sal ammonia, instead of the 

 ammonia alone. The fulminating gold 

 weighs one-fourth more than the gold 

 made use of. A considerable degree of 

 precaution is necessary in preparing this 

 substance. It ought not to be dried but in 

 the open air, at a distance from a fire, be- 

 cause a very gentle heat may cause it to 

 explode. Several fatal accidents have 

 arisen from its explosion, in consequence 

 of the friction of ground stoppers in bot- 

 tles containing this substance, of which a 

 small portion remained in the neck. 



Fulminating gold, when exposed by 

 Berthollet to a very gentle heat in a cop- 

 per tube, with the pneumatical apparatus 

 of mercury, was deprived of its fulminat- 

 ing quality, and converted into an oxide, 

 at the same time that ammoniacal gas was 

 disengaged. From this dangerous expe- 

 riment it is ascertained, that fulminating 

 gold consists of oxide of gold combined 

 \vith ammonia. The same eminent philo- 

 sopher caused fulminating gold to explode 

 in copper vessels. Nitrogen gas was dis- 

 engaged, a few drops of water appeared, 

 and the gold was reduced to the metallic 

 form. In this experiment he infers, that 

 the ammonia was decomposed ; that the 

 nitrogen, suddenly assuming the elastic 

 state, caused the explosion, while the 

 oxygen of the oxide united with the hy- 

 drogen of the alkali, and formed the water. 

 This satisfactory theory was still far- 

 ther confirmed by the decomposition of 

 fulminating gold, which takes place in 

 consequence of the action of the concen- 

 trated sulphuric acid, of melted sulphur, 

 fat oils, and ether ; all of which deprived 

 it of its fulminating quality, by combining 

 with its ammonia. 



Sulphurets precipitate gold from its 

 solven^ the alkali uniting with the acid, 

 and the gold falling down combined with 

 the sulphur ; of which, however, it may 

 be deprived by moderate heat. 



Most metallic substances precipitate 

 gold from aqua-regia : lead, iron, and sil- 

 ver, precipitate it of a deep and dull pur- 

 le colour ; copper and iron throw it 

 own in its metallic state ; bismuth, zinc, 

 ajid mercury, likewise precipitate it. A 

 of tin a immersed in a solution of 



gold, affords a purple powder, called tbc 

 purple powder of Cassius, which is used 

 to paint in enamel. There are various 

 methods of managing this process. That 

 described by Macquer consists in dissolv- 

 ing tin by very small portions at a time, 

 without heat, in an aqua-regia composed 

 of two parts of nitric and one of muriatic 

 acid, previously weakened with water 

 equal in weight to both the acids. The 

 first small portion of tin must be suffered 

 to be entirely dissolved before a second is 

 added. This addition must be continued 

 till the acid has acquired a yellow colour, 

 and scarcely acts at all upon the tin last 

 added. 



On the other hand, the purest gold must 

 be dissolved in an aqua-regia composed 

 of three parts of nitric and one of muriatic 

 acid. This solution may be made, as ex- 

 peditiously as the operator chooses, by 

 the assistance of the heat of a sand bath. 

 The solution of tin must then be large- 

 ly diluted, as, for example, with one hun- 

 dred parts of distilled water ; and a small 

 quantity of this may then be assayed, by 

 separating it in two parts, and diluting 

 one of the parts still farther. Upon trial 

 of both, by letting fall a drop of the solu- 

 tion of gold into each, it will be seen 

 which affords the most beautiful purple 

 precipitate. The whole of the solution of 

 tin must accordingly be altered, if neces- 

 sary, by adding more water. Pour into 

 this aolution, in a large glass or earthen 

 vessel, nearly half as much of the solution 

 of gold as it contains of solution of tin, 

 stirring the mixture with a glass stick. In 

 a short time the liquor will become of a 

 beautiful red colour, which will gradually 

 disappear on the subsidence of the pre- 

 cipitate. By adding a small quantity of 

 the solution of tin, it will be seen whether 

 the whole of the gold is precipitated. The 

 clear liqour must then be decanted, and 

 the precipitate washed. It consists of me- 

 tallic gold and oxide of tin, at a maximum 

 in combination, and is the only known 

 substance which has the property of com- 

 municating a purple colour to glass. This 

 purple powder is perfectly soluble in am- 

 monia. Nitric acid boiled on it brightens 

 it to a tint approaching that of cinnabar. 

 The difficulties attending the prepara- 

 tion of this article appear to depend on 

 the state of the tin. If the solution of 

 this metal be made with heat and rapidity, 

 it becomes too much oxyded to adhere to 

 the acid, or to precipitate the gold ; and 

 the combination of the two metals, which 

 falls down, varies in colour according as 

 this term is approached: these are the 

 chief circumstances ; but there i BO 



