2. _Lithic_, or _Uric[192] Acid_. As this principle is not found in the blood, but is constantly present in healthy urine, it follows that it must be generated by the action of the kidneys. M. Majendie[193] has lately endeavoured to prove that its secretion depends upon the _Azote_ received in alimentary substances, and for the following reasons, _viz._ 1. Azote is a component part of _Lithic Acid_[194]—2. Those persons who use a large portion of animal food, and fermented liquors, are liable to calculous disorders—3. When animals are confined to food which contains no Azote, no Lithic acid is formed—but of this anon.—Berzelius and other animal chemists have supposed that this acid exists in urine in a free state; but Dr. Prout, whose arguments appear very satisfactory and decisive, is of opinion that it is always in combination with ammonia (_Lithate of Ammonia_), from which however it is very easily separated by the addition of any acid, even the carbonic, in the form of a red powder. It moreover appears to be susceptible of several important modifications, with which it behoves the pathologist to be acquainted; the profession is greatly indebted to the ingenuity and industry of Dr. Prout for some very essential additions to our knowledge, respecting the habitudes of _Lithic acid_ with different bodies. _Erythric Acid._ When nitric acid diluted with about an equal bulk of water, is poured upon pure lithic acid, and a moderate heat is applied, an effervescence takes place, and the lithic acid is dissolved; if we then concentrate this solution by a gentle evaporation, we obtain transparent colourless crystals, which have been found to constitute a peculiar acid, to which M. Brugnatelli has given the name of _Erythric acid_. _Purpuric Acid._ Dr. Prout has discovered that if into a strong solution of the above crystals in water, whilst boiling hot, we carefully drop some pure ammonia, the solution acquires a beautiful purple[195] colour, and crystals of _purpurate of ammonia_ speedily begin to form and subside. If these crystals are treated by means of potass and sulphuric acid, pure _Purpuric acid_ is obtained in the form of a yellowish, or cream-coloured powder.