‘220 
Mr. Tennant on the Action of 
140 grains; and 60 grains of the gold were found to have been 
dissolved. Upon the addition of water about 50 grains of the 
gold were precipitated, in the form of a black powder. The 
gold which was thus precipitated was principally in its metallic 
state, the greater portion of it being insoluble in marine acid. 
The remaining gold, about 10 grains in weight, communicated 
to the alkaline solution, in which it was retained, a light yel- 
low colour. By dropping into this solution diluted vitriolic or 
nitrous acid, it became at first of a deeper yellow, but if viewed 
by the transmitted light, it soon appeared green, and afterwards 
blue. This alteration of the colour from yellow to blue arises 
from the gradual precipitation of the gold in its metallic form, 
which by the transmitted light is of a blue colour. Though 
the gold is precipitated from this solution in its metallic form, 
yet there seems to be no doubt that while it remains dissolved 
it is entirely in the state of calx. Its precipitation in the me- 
tallic state is occasioned by the nitre contained in the solution, 
which having lost part of its oxygen by heat, appears to be 
capable of attracting it from the calx of gold ; for I found that if 
the calx of gold is dissolved by being boiled in caustic alkali, 
and a sufficient quantity of nitre which has lost some of its air 
by heat is mixed with it, the gold is precipitated by an acid in 
its metallic state. * 
* As the precipitation of gold in its metallic form, by nitre which has lost some of 
its oxygen has not, I believe, been noticed, it may not be improper to mention some 
of those facts relating to it which seem most entitled to attention. Nitre which has 
been heated some time precipitates gold in its metallic state from a solution in aqua 
regia, if it is diluted with water. If a solution of gold in nitrous acid is dropped into 
pure water, the calx of gold is separated, which is of a yellow colour; but if the wa- 
ter contains a very small proportion of nitre which has lost some of its air by heat (as 
brie grain in six ounces), the gold is deprived of its oxygen, and becomes blue. The 
