GOLD. 
arisen from its explosion, in consequence 
of the friction of ground stoppers in bottles 
containing this substance, of which a small 
portion remained in the neck. 
Fulminating gold, when exposed by Ber- 
thollet to a very gentle heat in a copper 
tube, with the pneumatical apparatus of 
mercury, was deprived of its fulminating 
quality, and converted into an oxide at the 
same time that ammoniacal gas was disen- 
gaged. From this dangerous experiment it 
is ascertained, that fulminating gold consists 
of oxide of gold combined with ammonia. 
The same eminent philosopher caused ful- 
minating gold to explode in eoppqr vessels. 
Nitrogen gas was disengaged, a few drops 
of water appeared, and the gold was re- 
duced to the metallic form. In this expe- 
riment lie infers, that the ammonia was de- 
composed ; that the nitrogen, suddenly 
assuming the elastic state, caused the explo- 
sion, while the oxygen of the oxide united 
with the hydrogen of the alkaii, and formed 
the water. 
This satisfactory theory was still farther 
confirmed by the decomposition of fulmi- 
nating gold, which takes place in conse- 
quence of the action of the concentrated 
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 sol- 
vent, 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 silver, 
precipitate it of a deep and dull purple 
colour ; copper and iron throw it down in 
its metallic state ; bismuth, zinc, and mer- 
cury, likewise precipitate it. A piate of 
tin, immersed in a solution of gold, affords a 
purple powder, called the purple powder of 
Cassius, which is used to paint in enamel. 
There are various methods of managing this 
process. That described by Macquer con- 
sists in dissolving 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 suf- 
fered to be entirely dissolved before a 
second is added. This addition must be 
continued till the acid has acquired a yel- 
low 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 solutiou may be made, as expeditiously 
as the operator chooses, by the assistance 
of the heat of a sand bath. 
The solution of tin must then be largely 
diluted, as for example, with one hundred 
parts of distilled water ; and a small quan- 
tity of this may then be assayed, by sepa- 
rating it in tw'O parts, and diluting one of 
the parts still farther. Upon trial of both, 
by letting fall a drop of the solution of gold 
into each, it will be seen which affords the 
most beautiful purple precipitate. The 
whole of the solution of tin must accord- 
ingly be v altered, if necessary, by adding 
more water. Pour into this solution, 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 precipitate. By adding a small 
quantity of the solution of tin it will be 
seen whether the whole of the gold is pre- 
cipitated. The clear liquor must then be 
decanted, and the precipitate washed. It 
consists of metallic gold and oxide of tin, 
at a maximum in combination, and is the 
only known substance which has the pro- 
perty of communicating a purple colour to 
glass. This purple powder is perfectly 
soluble in ammonia. Nitric acid boiied on 
it brightens it to a tint approaching that of 
qnnabar. 
The difficulties attending the preparation 
of this article appear to depend on the 
state of the tin. If the solution of this 
metal be made with beat 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 fails 
down, varies in colour according as this 
term is approached : these are the chief cir- 
cumstances ; but there is no doubt that a 
complete examination of the process would 
indicate others worthy of notice. 
Ether, naphtha, and the essential oils, take 
gold from its solvent, and form liquors 
which have been called potable gold. The 
gold which is precipitated by evaporation 
of these fluids, or by tiie addition of sulphate 
of iron to the solution of gold, is of the ut- 
most purity. 
In the dry way, gold resists the action of 
neutral salts, more especially nitre, which 
deflagrates with the imperfect metals. 
