30 An Acid Principle from the Lithic or Uric Acid. 
changes the colour of the purpurate of ammonia to yellowish 
-red, but produces no precipitate. 
Such is avery brief account of the purpurates, as far as [ have 
examined them. It may at first sight appear singular, that such 
an insoluble acid should form so many soluble compounds ; but 
when we reflect upon the subject, and consider what a very small 
~quantity of the purpurate of ammonia is retained in solution by 
water, and that this small quantity has been made the standard 
of comparison in the above experiments, our surprise is consi- 
derably lessened, and we feel no difficulty in conceiving, that if 
the purpurates were compared with the nitrates, for example, the 
former would be found by far the least soluble. 
From the very small quantities on which I have been obliged 
to operate, and from other circumstances, I can offer but little 
respecting the constitution of the purpurates. Those which IT 
have attempted to analyse appear to be anhydrous, and to be 
composed of two atoms of the acid, and one of the base; and if 
this be correct, the same composition may perhaps be referred 
to most, if not all the compounds above mentioned. The pur- 
puric acid, however, appears capable of forming subsalts and su- 
persalts, with most bases, many of which seem to be very little 
suluble. 
With respect to the characteristic properties of the purpurie: 
acid, I apprehend it may be readily distinguished from all other 
substances by the beautiful colours exhibited by its alkaline and 
earthy salts, independently of its other properties, which are like- 
wise peculiar. 
The purpuric acid and its compounds probably constitute the 
basis of many animal and vegetable colours. The well known 
pink sediment, which generally appears in the urine of those 
labouring under febrile affections, appears to owe its colour 
chiefly to the purpurate of ammonia, and perhaps occasionally to 
the purpurate of soda. Some of the purpurates, as for example 
that of lime, might be probably used as a paint. They might 
be also used for dyeing, especially wool and other animal produe- 
tions*. On this part of the subject, however, as I have little 
that is certain to offer, I do not deem it prudent to enter at 
present. 
* I may here observe, that the solution of lithic acid in nitric acid has 
the property of tingeing the skin and other animal substances in a very per- 
manent manner. Thecolour does not, in general, appear till the substance 
has been exposed to heat, or, what is more effective, to the light of the sun. 
In the latter case, particularly, a deep purple tint soon makes its appearance, 
and the substance tinged (more especially the skin) emits during the pro- 
cess a strong and peculiar smell, closely resembling that produced by the 
nitrate of silver, when applied to the skin, and exposed to similar circum- 
stances. 
VII. On 
