39? 
duce any effeét upon filver and 
platina. 
With this intention, I put fome 
thin pieces of gold into the tube, 
together with nitre, and expofed 
them toa ftrong red heat for two 
or three hours. After the tube 
was taken from the fire, the part of 
the nitre which remained, confift-’ 
ing of cauftic alkali and of nitre 
partially decompofed, weighed 140 
grains; and fixty grains of the gold 
were found to ‘have been diflolv- 
ed. Upon the addition of water, 
about fifty grains of the gold 
were precipitated, in the form of 
a black.powder. The gold which 
was thus precipitated was princi- 
pally in its metallic ftate, the greater 
portion of it being infoluble in 
marine acid. The remaining gold, 
about ten grains in weight, commu- 
nicated to the alkaline folution, in 
which it was retained, a light yel- 
low colour. By dropping into this 
folution diluted vitriolic or nitrous 
acid, it became at firft of a deeper 
yellow ; but, if viewed by the tranf- 
mitted light, it foon appeared green, 
and afterwards blue. This altera- 
tion of the colour, from yellow to 
blue, arifes from the gradual preci- 
pitation of the gold in its metallic 
ANNUAL REGISTER, 
' light, is of a blue colour. 
1797+ 
form, which, by the tranfmitted 
Though 
the gold is precipitated from this 
folution in its metallic form, yet 
there feems to be no doubt that, 
while it remains diffolved, it is en- 
tirely in the ftate of calx. Its pre- 
cipitation, in the metallic ftate, is 
occafioned by the nitre contained 
in the folution, which, having loft 
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 diffolved, by 
being boiled in cauftic alkali, and 
a fufficient quantity of nitre, which 
has loft fome of its air by heat, is 
mixed with it, the gold is precipi- 
tated by an acid in its metallic 
ftate *. 
Having found-that nitre would 
diffolve gold, I tried whether it. 
would produce any effect upon 
platina. 
It has been formerly obferved, 
that the grains of platina, in the 
impure ftate in which it is originally 
found, might, by being long heated 
in a crucible with nitre, be reduced 
to powder. Lewis, from his own 
experiments, and thofe of Margraaf, 
thought that the iron only which 
is contained in the grains of platina, 
* As the precipitation of gold in its metallic form, by nitre which has loft fome 
of its oxygen, has not, I believe, been noticed, it may not be improper to mention 
fome of thofe facts relating to it which feem moft entitled to attention, Nitre 
which has been heated fome time, precipitates gold in its metallic ftate, from a fo~ 
Jution in aqua regia, if it is dilured with water. Ifa folution of gold in nitrous 
acid is dropped into pure water, the calx of gold is feparated, which is of a yeliow 
colour; but, if the water contains a very {mall portion of nitre which has loft 
fome of its air by heat (as one grain in fix ounces) the gold is deprived of its 
oxygen, and becomes blue. The alkali of the nitre does not aiffift in producing 
this effect. Nitrous acid alone, which does not contain its full proportion of 
oxygen, occafions the fame precipitation, unlefs it is very ftrong: and, if a mix- 
ture of fuch ftrong nitrous acid, and of a folution of gold in nitrous acid, is dropped _ 
into water, the gold is deprived of its oxygen, and is precipitated of a blue colour. 
Two caufes contribute to produce this effect upon the addition of water. The 
adhefion of the calx of gold to nitrous acid is by that means weakened; and the 
oxygen is attached more ftrongly to the imperfeét nitrous acid, in confequence of 
their attraction for water when they are united. 
was 
