URIC or LITITIC ACID derives its names from its being a constituent of urine and of urinary calculi respectively. In a state of purity, it occurs in the form of a loose white powder or scales consisting of minute crystals, devoid of smell or taste, only very slightly soluble in water (1 part requiring about 15,000 parts of cold, and 1800 of boiling water), and quite insoluble in alcohol and ether. This acid is, however, soluble without decomposition in strong sulphuric acid, and it may be thrown down from this solution by the addition of water. It is also soluble in the carbonates, borates, phosphates, lactates, and acetates of the alkalies, extracting from these salts a part of their base, with which it forms acid urates. Litmus paper is reddened by its moist -crystals, or by a hot watery solution. This acid is not volatile, and by dry distilla tion is decomposed into carbonate of ammonia, urea, cyanuric acid, hydrocyanic acid, .etc. On heating a mixture of uric acid and water, with gradual additions of peroxide of lead, which' is a strong oxidizing agent, the uric acid becomes oxidized, takes up water, and becomes converted into urea, allantoine, and oxalic acid, as is shown by the Uric Acid. Oxygen. Water. • Ci0H4N406 + 20 + 3H0 = Urea. Allantoine. Oxalic Acid.
• + 040e, 1 of the lead being left in combination with the oxalic acid. This is a most important result d in its physiological bearing, since it shows how uric acid is a probable stage towar the formation of urea, and as explaining a probable source of the oxalic acid, which is often present in the urine in cases of deficient respiration and aeration; and this view is confirmed by the experiment-of introducing uric acid into the stomach or veins o an animal. when the presence of oxalate of lime, and the augmentation of urea, are 'very soon perceived in the urine.
brie acid is a very weak bibasic acid, forming with bases two series of salts, the neutral and the acid, of which the former are the more soluble. Among the most important of these urates are—(1) neutral urate of potash, which occurs in the form of a white granular crystalline powder, difficult of solution in water (requiring 44 parts of .cold, and 35 of boiling water), but readily soluble in an excess of potash, from which solution carbonic acid throws down the acid orate of potash in the form of a transparent jelly, which sometimes falls in the form of a powder; (2) two urates of soda, each of which is lass soluble than the corresponding potash salt; (3) acid urate of ammonia; (4) Acid urate of magnesia; (5) two urates of lime; and (6) urate of Willa, which is more soluble than any other urate; and hence lithia water is an important therapeutic agent in converting uric acid and the more insoluble urates into a soluble salt in the living :body.
Uric acid is widely distributed throughout the animal organism. It occurs not only in the urine of man and carnivorous animals, but is the chief constituent (either free or in combination) of many calculi occurring in the kidneys or bladder, and of numerous urinary sediments. The urinary secretion of birds and reptiles consists almost entirely •of urates, which are also found in the excrements of caterpillars, butterflies, beetles, etc.,
and of many mollusks. Moreover, in very minute quantities, it occurs as a urate in healthy blood, in which fluid it has been found in excess in gout and Bright's disease, and is a constituent of the aqueous extract of the spleen, liver, lungs, pancreas, and brain. The occurring about the smaller joints, and in the lobes of the ear •of gouty patients, consist mainly of urate of soda.
The best and most ready mode of obtaining pure uric acid is from the semi solid .urine of serpents, which consists almost entirely of 'rates. The mass is boiled with potash, which expels any ammonia that is present, and a stream of carbonic acid is then_ passed through the strained potash solution, which throws down acid urate of potash. This precipitate is dissolved in water, and decomposed by hydrochloric acid, which throws down the uric acid iu minute crystals. The form in which this acid crystallizes. is liable to great variations. Sometimes we have flat tablets resembling whetstones, or sections with a double knife through biconvex lenses; at other times, more or less per fect rhombic tablets, and sometimes hexagonal plates, barrel-shaped prisms, and toothed crystals. If there is any doubt as to the nature of the crystals, they should be dissolved (under the microscope) in a little potash solution, and a drop of hydrochloric acid then added, when sufficiently characteristic forms are sure to appear.
The marvelous researches of WW1les and Liebig on the products of decomposition of uric acid by nitric acid, constitute an epoch in organic chemistry. The most impor tant products, some of which are obtained directly, and others ipdirectly, are: (1.), alloxan, (which under the influence of various reagents, yields alloxanic acid, mexoxalic acid, mykomelinic acid, and dialuric acid); (2) parabanic acid, (which yields oraluric acid); (3) alloxantine, (which yields alloxan, anitcrna acid, 'amine& thionuric acid, aralumc acid, and murexide); (4) yields niurexan, or caramel, and alloxan); and (5) urea, which breaks up into carbonic acid and ammonia. Moreover, by boiling oxaluric acid with water it is decomposed into oxalic acid and urea. Of these substances, we shall only notice murexide, which has two special claims to notice. It is used as a pigment in dyeing. being probably the old Tyrian purple (see MunsixinE), and it affords a ready test for a mere trace of uric acid. In using it for the latter purpose, the substance supposed to contain uric acid is heated on a slip of glass with a drop of nitric acid, and carefully evaporated to dryness. If uric acid is present, the residue has a red color, which is converted by vapor of ammonia into a tine crimson, and the addition of a drop f a solution of potash develops a splendid purple tint, which disappears on the application of heat.
Uric acid must be regarded, like urea, as a product of the disintegration of the tissues; and it is far from improbable that all the urea which is secreted may have pre-existed in the form of uric acid—a view which is supported by the fact, that whatever (like exces sive exercise or water-drinking) increases the amount of urea, decreases that of uric acid, and vice versa. See UunsE,