26 Description of an Acid Principle 
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. Jodine 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 ef- 
fected ; and if to this solution a little ammonia be added, and the 
whole evaporated to dryness, a perceptible quantity of the beauti- 
ful purple compound of ammonia and the new acid principle will 
be obtained. I am not aware that any other substance is capa- 
ble of producing this change, though the circumstance is by no 
means improbable. 
To prevent circumlocution, | shall in future call this principle 
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 speci- 
fic gravity is considerably above that of water, though, from the 
minute state of division in which it exists, it usually takes a con- 
siderable time to subside in that fluid. When suffered to sepa- 
rate slowly from a large quantity of water, or any other fluid ca- 
pable of holding it in solution, it sometimes assumes fhe form of 
thin pearly scales. 
The purpuric acid is very little soluble in water. One tenth of 
a grain boiled for a considerable time in 1000 grains of water 
was not entirely dissolved. ‘The water assumed a purple tint, 
which it retained after it was cold, though it became very slightly 
turbid on cooling *. The purpuric acid is insoluble in alcohol and 
ether. In all the mineral acids, wheu concentrated and in excess, 
and in solutions of the different alkalies, it dissolves readily ; but 
it is insoluble, or nearly so, in dilute sulphuric, muriatic, and 
phosphoric acids, and also in svlutions of the oxalic, citric, and 
tartaric acids. Concentrated nitric acid readily dissolves it with 
effervescence ; and if the acid be in excess, and heat be applied, 
* T ain not quite sure whether the purple tint here mentioned depends 
upon the actual solution of a minute portion of the purpuric acid; and, con- 
sequently, whether it naturally forms a purple solution, or whether the co- 
lour be owing to the formation of a little ammonia from the decomposition 
of a minute proportion of the acid, which, combining with the remainder of 
the acid, forms the purpurate of ammonia. I incline to the latter opinion. 
a portion 
