In its elementary composition it stands near to guanine, only containing one equivalent of nitrogen and hydrogen less than that base. It also presents great analogies to it in its compounds. Its elementary composition is identical with that of hypoxanthine, the base discovered by Scherer in the spleen, and since found by him in various organs. The two substances likewise agree in certain other characters; hypoxanthine, however, is almost insoluble in water and hydrochloric acida difference from carnine, which cannot be accounted for by simple difference of purity of the preparations. Hypoxanthine dissolves in warm nitric acid with evolution of gas, and on cooling yields a product of decomposition. The solution in nitric acid, on evaporation to dryness, leaves a yellow residue, assuming a red colour on the addition of caustic potash. Carnine, on the other hand, dissolves in nitric acid without evolution of gas, and the solution by evaporation on the water-bath, leaves a colourless mass, which does not become yellow unless very strongly heated; on addition of potash this residue assumes a red colour. Xanthine and guanine have very similar reactions. From its elementary composition, xanthine (CHN ̧O) might be considered as a compound of uric acid with carnine, 2C10H1N1O1 = C10H,NO + C1H1N ̧O2; 10 Xanthine. 10 4 6 4 10 41 but its chemical affinities do not accord with this view. By mixing a solution of hydrochlorate of carnine with urate of soda, not xanthine, but an isomeric compound-urate of carnine is obtained, which, under the influence of acids, separates into its original constituents. Literature. Wollaston, Philos. Transact.,' 1810, p. 223.-Cystine discovered, constituting a calculus, 1810. Baudrimont and Malaguti, 'Journ. d. Pharm.,' vol. xxiv, p. 633.-Proved the presence of sulphur, which had been overlooked by Prout and Lassaigne. Thaulow, 'Ann. d. Pharm.,' vol. xxvii, p. 197.-Best elementary analysis. Marchand, Journ. d. Pract. Chem.,' vol. xvi, p. 255. For the rest of the casuistic see Gmelin's Handb. d. Org. Chem.,' 4th edit., vol. ii, p. 133. Occurrence. Cystine is the principal constituent of a rare description of calculus; small calculi are sometimes made up of pure cystine, and then appear yellow, transparent, wax-like, of a crystalline texture. They cut like dry walnut, may be powdered, and are gritty between the teeth, tasteless, and neutral. When the calculi are not made up of pure cystine, they generally contain an admixture of earthy phosphates, and then are greenish-bluc, dirty greenish-grey, or fawn-brown. Cystine gravel has occasionally been obtained from dogs.1 Latterly cystine has been discovered by Scherer2 to be present in the liver of typhus patients. Another observers has once obtained it from the kidneys of oxen, but could not meet with it again in the same substance. Two calculi of cystine were found in the calyces of the kidneys of an old woman by Professor Virchow. Lastly, it is met with in the urine in solution and as a deposit. Where it occurs dissolved it is probably by means of some alkali, either the alkali by means of which it was excreted from the kidneys, or an alkali formed in the urinary passages, ammonia which has lost some carbonic acid, as is usual in decomposed urine. But this is merely a supposition, supported, however, by the observation of Julius Müller, who found cystine dissolved in the alkaline urine of a boy six and a half years old. Here the alkaline condition seemed to be due to the presence of a large cystine calculus in the bladder. After the operation of lithotomy, the urine became acid. Two months later, the urine became again alkaline, and gave a deposit which, when cleared of earthy phosphates by means of acetic acid, showed itself to be cystine again. The urine, on being filtered and treated with acetic acid, after the lapse of twenty-four hours yielded another precipitate of cystine. This case shows that the conditions under which cystine occurs in an insoluble or in a dissolved condition require further investigation. Decompositions. When subjected to dry distillation, cystine yields prussic acid, carbonate of ammonia, a fluid, thick and stinking oil, and leaves a spongy charcoal. Heated in the open air, it develops sulphurous acid, recognisable by the smell, but does not fuse, or melt, or blister. When fused with caustic potash, it gives off an inflammable gas, which burns with the flame of sulphocarbon and under production of sulphurous acid. (This gas is most probably sulphuretted hydrogen, as is made probable by the following reaction.) When cystine is boiled in caustic potash solution, in which some hydrated oxyde of lead has previously been dissolved, a large precipitate of sulphuret of lead is obtained (Liebig). The solution of cystine in excess of nitric acid, on evaporation by boiling, 1 Heller, Heller's Archiv.' vol. vi. p. 458. 2 3 4 Archiv,' loc. cit., p. 230. leaves at first a white, not transparent mass, which becomes brown and black, and contains sulphuric acid. Cystine crystallizes in six-sided plates of the arrangements represented in fig. 1, plate VI. These crystals are very characteristic, and therefore a valuable means of diagnosis. Cystine is insoluble in water, but soluble in dilute strong acids, from which solution it is precipitated by carbonate of ammonia. The solution on evaporation yields a salt, mostly crystallizing in groups of needles. The phosphate, sulphate, hydrochlorate, and nitrate are the compounds best known. Cystine is easily soluble in a watery solution of ammonia, potash, soda, and lime; also in the bicarbonates of potash and soda, but not in bicarbonate of ammonia. The solution in ammonia on spontaneous evaporation leaves the cystine pure in crystals. The other solutions leave granular crystals, the composition of which is not ascertained. From the alkaline solutions, cystine is precipitated, after a few seconds, by acetic, tartaric, and citric acids, in the form of a fine white powder; these solutions are, however, not precipitated by either sulphuric, hydrochloric, or nitric acid. Cystine is soluble in a watery solution of oxalic acid. |