CHEMICAL PRINCIPLES. 
355 
acids of gold and of tin is precipitated for the same reason O.\itlesofj;old. 
as the oxides of bismuth and of antimony are precipitated by 
water. Still, however, this operation is not so simple as that 
the mere action of the water can be supposed to cause the 
production of the purple. For a mixture of the spirit of 
Libavius with muriate of gold cannot be precipitated by 
water ; and if caustic alkali be added, this will precipitate a 
combination pf oxide of gold and oxide of tin. This combi- 
nation is not, however, the purple of Ctissius. The preci- 
pitate is of a deep brownish blue, and preserves iliis colour 
when dried, and even when heated to a red heat. If precipi- 
tated with heat, it changes its colour, and becomes of a reddish 
hue, and contains a mixture of oxide of tin, at the maximum, 
with a metallic alloy of gold and lin. Hence it appears, 
that the purple is not composed of oxidum auricum and oxi- 
dum stanneum, the gold being here combined with a less por- 
tion of oxigen than in that oxide- 
In order to understand the nature of the purple of Cassius, 
it will be necessary to attend to the circumstances under w'hich 
it is produced. These are — 1 . The mutual and very strong 
affinity existing between gold and tin. 2 . The partial reduc- 
tion of the oxide of the murias auricus, by means of the mu- 
rias stannosus. And 3. The addition of a quantity of water 
sufficient to diminish the affinity of the muriatic acid to the 
oxides with which it is combined. Respecting the first of these 
circumstances, we have seen that it is on the mutual affinity of 
their radicals that the affinity of their oxides depends, and 
that this follows from those electro-chemical principles 
which were explained at the beginning of this memoir. We 
have seen, that the black precipitate produced in the murias 
auricus by the murias stannosus, is a metallic alloy of gold 
and of tin 5 and it seems very probable, that this alloy con- 
tains those two metals combined in the same proportion as they 
are in the purple. By way of shewing the strength of their 
mutual affinity, I shall mention the following experiments. 
A portion of the black metallic alloy, mixed with sulphur, 
and melted over the fire, did not suffer any decomposi- 
tion 
