42 8 Dr. Prout’s description of an acid principle , &c. 
same composition may perhaps be referred to most, if not all 
the compounds above mentioned. The purpuric acid, however, 
appears capable of forming subsalts and supersalts, with 
most bases, many of which seem to be very little soluble. 
With respect to the characteristic properties of the pur- 
puric 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 likewise 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 purpu- 
rates, as for example that of lime, might be probably used 
as a paint. They might be also used for dying, especially 
wool and other animal productions.* 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 pro - 
perty of tinging the skin and other animal substances in a very permanent manner. 
The colour 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 process a strong and peculiar smell, closely resembling that 
produced by the nitrate of silver, when applied to the skin, and exposed to similar 
circumstances. 
