36 Changes on treating Uric with Nitrous Acid. 
Desirous of assigning some reason for the change of colour im 
the red liquor in consequence of heat, many arguments induced 
me to suspect that it must be owing to the influence of an acid, 
In fact, the acids produce a similar effect; and if at first they are 
unable to do it, the assistance of heat renders them immediately 
capable of effecting it. Besides, the crystallized points which 
we found detached from the colouring matter have all the ap- 
pearance of a salt. It is certain that, if all that remains on the 
filter be burnt, the water which dashes the carbonaceous resi- 
duum has alkaline characters. 1 have also observed that potash 
revives the colour of the first reddish liquor, but scarcely sepa- 
rates any of the coloured flakes: lime produces a similar effect. 
I have likewise seen that the coloured precipitates were insolubie 
in water, but very soluble in very dilute sulphuric acid. It is 
singular that this solution is effected with a species of efferves- 
cence, which appears to me to be increased in proportion as the 
brilliant points are more numerous. Hitherto however I have 
obtained them in too small quantities, and too impure, to subject 
them to that particular examination which they merit. At pre~- 
sent, indeed, it appears to me that the above phenomena may 
receive a satisfactory explanation, supposing that the action of 
heat on the red liquor determines the formation of an acid, or 
puts it in astate to alter the colour in the above-mentioned man- 
ner; and that this acid may be scattered in very minute mole- 
cules, by uniting of which to a base, they may likewise give 
origin to those very small crystals. The solution, indeed, changed 
by heat, has a nauseous sweetish as well as acid taste, and does 
not sensibly redden the blue tincture of vegetables: but this may 
be attributed to the weakness of the supposed acid, in which the 
extraneous residuary matter may be more than sufficient to neu- 
tralize it. In obtaining the red colour by the aetion of fire, it 
was observed that, in finally drying, a yellow acid liquor destroyed 
the red colour previously formed as soon as it touched it. Neither 
are the metallic solutions inactive on this red liquor. Some 
make the colour yellow without affording a. precipitate, as for- 
instance, copper; others yield precipitates of the most beautiful 
colours, and separate all the colouring matter. Thus the solutions 
of silver, mercury and lead, vield sufficiently agreeable violet co- 
lours of different intensity, which fix themselves tenaciously on 
the paper in which they are collected. It is to be hoped that 
painting may derive something from such colours, as they serve 
to make sympathetic ink and other chemical sports. But it 
must be observed that they are changed as much by acids as by 
alkalies. In fact, the substance on “which depends the faculty 
of dissolving uric in nitrous acid, and of becoming red with heat, 
has all the characters of an acid, ’ 
Part 
