crystallization may be completed or accelerated by exposing the mixture to the influence of cold-by putting it into ice or a freezing mixture. Nitrate of urea crystallizes in the rhombic system. If a pure crystallization be obtained, the rhombic prisms may be seen to perfection. Neubauer says that on letting pure nitric acid combine with urea under the microscope, flat rhombic octahedra are formed at first, which, by apposition of crystalline matter, become converted into rhombic plates. This metamorphosis, however, most improbable in itself for crystallographical reasons, has not been observed by me, or by any other writer on the subject as far as I know. The rhombic prisms are either flat, single, and primary, or combinations of several prisms; like those of pure urea, becoming hexagonal, and ultimately thin plates. (See Plate I.) The nitrate obtained from urine directly, almost always crystallizes in large plates, of which many lie upon each other, mostly with their principal crystallographical axes parallel to each other. This parallelism is observed also in crystallizations of pure urea: the plates only show the prismatic character a little more. The crystals obtained from urine are pressed between bibulous paper, until the paper is no longer stained. They may then be washed on a filter with some water containing a little nitric acid, in which they are not very soluble, and may then be pressed a second time. They are then almost white, and of a silky gloss, or satin-like lustre, resembling mother-of-pearl. Nitrate of urea is not changed by the influence of the air. It is soluble in water. Heated on platinum foil it explodes, when the temperature has been raised quickly to a high point; but if only heated to 281° F. 140° C. it decomposes, carbonic acid, suboxyde of nitrogen, urea, and nitrate of ammonia being produced. + 8 Urea and oxalic acid. U+0. (2C2H ̧N2O2, CH2O Gerhardt.)-Oxalic acid has a stronger affinity for urea than nitric acid, so that when it is added to a solution of the nitrate, oxalate of urea will be formed, which not being very soluble in water containing nitric acid, is precipitated. Oxalate of urea crystallizes in rhombic prisms and rhombic plates,2 some varieties of which are very much like the prismatic plates of nitrate of urea. But frequently the oxalate has 'Loc. cit., p. 7. He refers to Funke's Atlas,' taf. ii, fig. 5; but there only prisms, and not a single octahedron, are to be seen. * Vide Dr. Beale, The Microscope,' p. 264, fig. 214. O. Funke, 'Atlas,' taf. ii, fig. 6. Bowman, Med. Chemistry,' p. 5, fig. 1. more tendency to produce crystals in which the axes are of a more equal length; these crystals, though smaller in outline, have then more body. (See Plate I.) To the naked eye a precipitate of the oxalate of urea appears as a white crystalline mass of plates. This salt is soluble in 23 parts of water of a temperature of 59° F. (15° C.), but is soluble in a much smaller quantity of boiling water. Urea and nitrate of mercury.'-On adding to a solution of urca a solution of nitrate of mercury, a white flocculent precipitate is immediately produced, which contains urea, oxyde of mercury, and nitric acid. According to the proportion in which both solutions are mixed, and the amount of free acid contained in the solution of mercury, one of three compounds or a mixture of three compounds is produced, which are distinguished from each other by containing different quantities of the protoxyde of mercury. These three different combinations have the following characters in common: On combustion with oxyde of copper they develop a mixture of gases, in which nitrogen and carbonic acid are present in the proportion of three volumes of the one to two volumes of the other. This is the same proportion as in the nitrate of urea. On removing the oxyde of mercury by sulphuretted hydrogen, there remains in the fluid after filtration from the precipitate, pure nitrate of urea, which crystallizes to the last drop. These combinations, therefore, only differ from each other by a varying amount of protoxyde of mercury; they are entirely soluble in hydrocyanic acid and hot nitric acid. In the latter solution, potash produces a white precipitate. If the dry precipitate of one of them is heated for a length of time in a current of warm air, a decomposition takes place; it assumes a yellowish colour, and the solution in nitric acid gives now a yellowish precipitate with potash. The formulæ of these three combinations are A. NO, U+4HgO C. NO, U+3HgO From these formulæ, and the manner in which they are decomposed by sulphuretted hydrogen, sulphuret of mercury being precipitated and nitrate of urea going into solution, we may consider these bodies to be combinations of one Justus Liebig, Ueber eine neue Methode zur Bestimmung von Kochsalz und Harnstoff im Harn,' Heidelb., 1853. Annal. d. Chem. und Pharm.,' lxxxv, p. 294. equivalent of nitrate of urea, with four, two, and three equivalents of the protoxyde of mercury respectively. The first of these combinations is produced in the quantitative analysis of urea in the urine. Diagnosis of Urea in Urine and other Animal Fluids. To demonstrate the presence of urea urine, the following modified method of Liebig should be adopted: A quantity of urine should be mixed with about half its volume of a solution of baryta (consisting of two volumes of saturated solution of baryta or baryta water, and one volume of a saturated solution of nitrate of baryta), or a quantity of that solution sufficient to precipitate the phosphoric and sulphuric acids. The fluid is then filtered from the precipitate, neutralized with nitric acid, and evaporated to dryness on the water bath; the residue is extracted with alcohol; the alcoholic extract is again evaporated, and exhausted a second time with absolute alcohol. This last solution contains the urea very pure, so that it crystallizes out in colourless needles. This process may be employed for producing urea directly from urine without precipitating it by either nitric or oxalic acid, and may serve as a mode of finding the quantity of urea in any fluid, particularly when the apparatus and testfluids of the Centigrade analysis should happen not to be at hand. The presence of albumen in urine requires a modification of this process. It is not advisable to remove the albumen by boiling, with the addition perhaps of some drops of acetic acid, in cases where accuracy is desired. For albumen, on passing from its dissolved into an insoluble condition, incloses into its substance a certain amount of urea, which cannot afterwards be separated from it without great loss of time and trouble. This fact must always be borne in mind in analysing fluids containing a small amount or only a trace of urea; as in that case the whole of the urea may adhere to the coagula of albumen, and the fluid may appear to have contained no urea at all; or urine may in this way be made to appear to contain a smaller amount of urea than it actually does. To avoid erroneous conclusions, therefore, albuminous urine, after precipitation with the solution of baryta, should be extracted with absolute alcohol, which, having a stronger affinity for urea than albumen has, takes Loc. cit., p. 23. the former into solution in the same moment as it renders the albumen insoluble. The alcoholic extract may then be evaporated to the bulk of the urine employed, and the amount of urea determined by means of the mercurial test-fluid; or it may be evaporated to dryness, the residue extracted a second time, when, on a second evaporation, the whole of the urea will be obtained in a crystalline state ready for weighing. It is mainly owing to the oversight of this fact that urea has not been found either in blood or in the juice of flesh by many observers, since it is always present in both. Liebig's Method of ascertaining the Absolute Quantity of Urea in Urine.! This mode of ascertaining the quantity of urea dissolved in urine is equal in accuracy to any hitherto in use, but has the great advantage over them of requiring much less time for its execution, and no particular ability on the part of the operator-two circumstances which pre-eminently qualify it as a means for the diagnosis of certain pathological conditions of the system. It is based upon the property of urea to be precipitated by the addition of nitrate of mercury, in combination with one equivalent of nitric acid and four equivalents of the oxyde of mercury, three equivalents of nitric acid remaining free in solution. On gradually adding to a dilute solution of urea a dilute solution of nitrate of mercury, and neutralizing the free acid of the mixture from time to time, by the addition of some baryta water or a dilute solution of carbonate of soda, a flocculent, bulky, snowy-white precipitate is obtained, which is insoluble in water. If the alternate addition of the nitrate of mercury and carbonate of soda be continued as long as a precipitate is formed, there will be a point at which the mixture, or the spot where the drop of the solution of the carbonate falls into the mixture, will assume a yellow colour, owing to the formation either of oxyde of mercury, or basic nitrate of mercury, or carbonate of mercury. If the fluid is now filtered, it does not any longer contain any appreciable quantity of urea; the whole of the urea has been precipitated. The precipitate is composed of one equivalent of urea and four equivalents of oxyde of mercury. By mixing solutions of urea and nitrate of mercury, both of known strength, we can easily convince ourselves that precipitation Liebig, loc. cit., p. 19. of the yellow oxyde or carbonate by the addition of carbonate of soda does not take place until we have added, for ten parts of urea in the solution of urea, a volume of the solution of mercury, in which there are contained seventy-seven parts of the oxyde of mercury. This amounts to four equivalents of the oxyde for one equivalent of urea. If we continue adding a solution of nitrate of mercury to a solution of urea so long as a precipitate is produced, the mixture will remain white on the addition of carbonate of soda. But if the original mixture be allowed to stand for several hours, the precipitate after the lapse of that time will have changed its properties, and have become crystalline. One may now easily recognise the six-sided plates of the combination of urea with one atom of nitric acid and three atoms of the oxyde of mercury; and the clear fluid which stands over the precipitate, and which, on the admixture of an alkali, gave a white precipitate, is now precipitated yellow by the same alkalies. In the acid fluid the combination, containing four atoms of the oxyde of mercury and one of nitric acid, is reduced to a combination containing less oxyde, because a part of the oxyde is redissolved by the free acid of the fluid. In order to recognise whether an amount of the solution of the nitrate of mercury sufficient to produce the combination of urea with four atoms of the oxyde of mercury has been added, the neutralization with carbonate of soda after the addition to the solution of urea of the mercurial solution becomes necessary. If a drop of the mixture, when added to a drop of a solution of carbonate of soda on a watch-glass, or on a flat piece of glass, remains white, we may be quite sure that there still is uncombined urea in the mixture. When, however, on the two drops mixing, a yellow pellicle is produced, then we have added a sufficient amount, or rather a little more than sufficient, of the mercurial solution to precipitate the whole of the urea. It requires only a small excess of the salt of mercury to indicate that the quantity sufficient to precipitate all the urea has been added. It is therefore evident, that if we know the amount of mercury contained in the solution of it, we may, from the quantity of this solution used for precipitating urea in the manner described, determine the quantity of urea contained in solution; or if, for precipitating a known quantity of urea, say 100 milligrammes, we have used a certain volume of the solution of mercury, this same volume of the same solution will indicate the same quantity of urea in fluids containing an unknown amount of urea. From the volume of the mer |