DR. TROUT ON ANIMAL CHEMISTRY. 617 
monia when treated with a fixed alkali. It often forms a portion 
of lithic calculi, but seldom entire calculi; and when this is the 
case, it generally occurs before puberty. 
Where there is a strong tendency to the deposition of lithic 
acid in the urine, and the patient under these circumstances takes 
the carbonate of soda, the urine will often be found to contain the 
lithate of soda; and I have seen this substance occasionally con¬ 
stituting a considerable portion of a urinary calculus. Hence, 
for some years past, 1 have made it a rule to give the carbonate 
of potash when the lithic acid prevails in the urine. In explana¬ 
tion of these circumstances, it may be remarked, that the alka¬ 
lies pass the kidneys when taken in large doses, and combine 
with the lithic acid either in the act of being separated, or after 
it has been separated by the kidney. 
It remains, in the last place, to make a few remarks on the 
purpuric acid . I am induced to notice this substance here for 
two reasons; first, its great importance in a pathological point of 
view, as constituting the principal colouring matter of the various 
lithate of ammonia sediments; and secondly, because its exist¬ 
ence has been denied by the French chemists, and been doubted 
by some in this country. With pure lithic acid, and a little 
ordinary dexterity, I do not consider it as at all difficult to pre¬ 
pare, and am quite at a loss to account for the failures that have 
been said to have taken place on the subject. The first step con¬ 
sists in dissolving as much pure lithic acid in pure nitric acid, 
rather diluted, as it will take up. The solution when concen¬ 
trated and put by in a cool place, will soon deposit large trans¬ 
parent and colourless crystals, constituting the erythric acid of 
Brugnatelli. These crystals are to be dissolved in water, and 
while the solution is boiling hot, pure ammonia is to be dropped 
carefully into the solution till a full purple colour is produced. 
This (which I here exhibit to you) is the only delicate part of the 
operation ; for too little or too much ammonia injures, if not 
altogether spoils, the results. The solution on cooling deposits 
the purpurate of ammonia in the form of an imperfectly-crystalline 
dark purple powder. To obtain the pure acid from this, it is to 
be dissolved in a solution of pure potash, by the application of 
heat, which expels the ammonia. The solution, then gradually 
dropped into diluted sulphuric acid, is again decomposed, and 
the purpuric acid falls down in the state of a yellowish or grey 
powder. It may be still further purified by dissolving it in pure 
concentrated sulphuric acid, and dropping the solution carefully 
into cold distilled water, when the purpuric acid is deposited 
nearly white, and as pure as it probably can be obtained. 
The combinations of this acid with different bases (one or two 
VOL. IV. 4 P 
