GOLD. 
sened with water: when perfectly dry, it is not alter 
ed. This is not peculiar to the action of combustibles: 
on metallic salts, it holds also, as we shall see afterwards, 
with respect to the metals. But it is by no means easy 
to see what makes water so indispensably necessary. It 
is not, as is commonly supposed, in order to secure the 
fluidity of the mixture : for Mrs Fulhame has shewn 
that ether, though a liquid, has no effect in reducing 
gold unless water be present. She accounts very inge 
niously for the phenomena, by supposing that the wa 
ter is decomposed. The combustible combines with its 
oxygen, while its hydrogen combines with the oxygen 
of the gold, and reproduces water. This theory accounts 
very well for the phenomena; but it would require some 
direct proof to establish it completely. 
The greater number of the metals, when plunged in 
to the solution of muriat of gold, occasion a precipitate 
either of gold in the metallic state, or of its purple ox 
ide; while at the same time a portion of the precipita 
ting metal is oxidated and dissolved by the acid. Zinc, 
iron, bismuth, copper, mercury, precipitate it in the 
metallic state. Lead, silver, and tin, precipitate it in 
the state of purple oxide. The sulphat of iron preci 
pitates it in the metallic state, and is at the same time 
converted into the oxy-sulphat by the oxygen which it 
has absorbed : whereas the oxy-sulphat of iron produ 
ces no effect at all. The same thing happens with tin. 
The muriat of that metal precipitates the gold in the 
state of purple oxide, combined with oxide of tin ; a 
precipitate well known by the name of precipitate of 
Cassius, and used to give a red colour to porcelain and 
glass: But the oxy-muriat of tin, which is already at 
Ffa 
455 
Chap. III. 
And by 
metals,