Dr. Prout’s description of an acid principle , &c. 421 
little care requisite to obtain the acid quite free from colour, 
it may not be deemed superfluous to state the precise method 
I usually followed for that purpose. The compound with 
ammonia, above-mentioned, was dissolved in a solution of 
caustic potash, and heat applied to the solution till the red 
colour entirely disappeared. This alkaline solution was then 
gradually dropped into dilute sulphuric acid, which uniting 
with the potash, left the acid principle in a state of purity. 
The acid principle is likewise produced from lithic acid by 
chlorine. Iodine has also the same remarkable property, 
though in a much less striking degree. When lithic acid is 
boiled with iodine for some time, a partial solution of the 
lithic acid is effected ; and if to this solution a little ammonia 
be added, and the whole evaporated to dryness, a perceptible 
quantity of the beautiful purple compound of ammonia and 
the new acid principle will be obtained. I am not aware that 
any other substance is capable of producing this change, 
though the circumstance is by no means improbable. 
To prevent circumlocution, I shall in future call this prin- 
ciple the purpuric acid, a name suggested by Dr. Wollaston, 
from its remarkable property of forming compounds with 
most bases of a red or purple colour. 
The purpuric acid, as obtained above, usually exists in the 
form of a very fine powder, of a slightly yellowish or cream 
colour; and when examined with a magnifier, especially in 
water, appears to possess a pearly lustre. It has no smell 
nor taste. Its specific gravity is considerably above that of 
water, though from the minute state of division in which it 
exists, it usually takes a considerable time to subside in that 
fluid. When suffered to separate slowly from a large quantity 
