tion has since been denied by Stædeler, who obtained benzoic acid abundantly by Béchamp's method, and this he thinks must have been mistaken for urea. Urea is not formed in the kidneys; these glands appearing to act somewhat in the manner of filters, by means of which the urea is separated from the mass of the blood, where it exists already formed before reaching the kidneys.

Urea may be obtained artificially in a variety of ways, one of the most remarkable of which was discovered by Wöhler, who showed that a solution of ammonium cyanate, which is metameric with urea, when evaporated at a gentle heat, furnishes urea :

This conversion of the cyanate into urea occurs whether it be formed by the direct union of cyanic acid with ammonia, or by the double decomposition of a metallic cyanate, such as potassic cyanate, with a salt of ammonium. Neither cyanic acid nor ammonia can, however, be discovered in the compound obtained; but if a solution containing a mixture of nitrate of silver and urea be boiled for some time, it is partly resolved into cyanate of silver and ammonium nitrate.

Urea is likewise formed during the action of peroxide of lead and other oxidizing agents upon uric acid; by the action of alkalies upon alloxan, and upon kreatine; as well as by the action of sulphuretted hydrogen upon the double fulminate of copper and ammonium. It is also obtained as a product of the action of phosgene gas (COCl2) upon ammonia, and by the reaction of ammonia upon carbonic ether, at a temperature of 360° (182° C.) (1161; Natanson). Urea is metameric with carbamide, if not identical with it (1604).

Preparation.-1. Urea is readily procured by concentrating urine to the consistence of syrup, decanting from the deposited salts, and adding an equal bulk of pure colourless nitric acid, of sp. gr. 125. The mixture becomes nearly solid from the deposition of flaky crystals of urea nitrate, the quantity of which is increased by immersing the vessel in a frigorific mixture. The mother-liquor is then pressed out, and the crystals diffused through water, to which baric carbonate in powder has been added. Effervescence takes place from the expulsion of carbonic anhydride, baric nitrate is formed, whilst urea is set free, and is dissolved. If the mixture of baric nitrate and urea, obtained by evaporation, be treated with boiling alcohol, urea only is dissolved, and it is deposited on cooling or during evaporation in long, slender,

striated, colourless prisms. When an aqueous solution of urea containing not less than one-thirtieth of its weight of urea is mingled with an equal bulk of pure nitric acid, an abundant deposition of needles of urea nitrate occurs (EH ̧тЂHNÐ ̧). The nitric acid employed must be quite free from nitrous acid, since urea is completely decomposed by nitrous acid, into carbonic anhydride, nitrogen, and water :

CH_N_O+2HNO,=CO,+2N+3 HẠO.

2. Oxalic acid may be substituted for the nitric in the foregoing process; the urea oxalate thus formed may be decomposed by calcic carbonate, and the urea may then be separated from the calcic oxalate and carbonate, by water; on evaporating the aqueous solution, crystals of urea are obtained.

3. Urea, however, is more easily and abundantly obtained from potassic cyanate, procured by heating an intimate mixture of 56 parts of potassic ferrocyanide, which has been completely dried at 212°, with 28 parts of carefully-dried finely-powdered black oxide of manganese. The mixture is placed upon a plate of sheet-iron and heated to dull redness, when it slowly smoulders, care being taken to stir the mass from time to time. The residue of this operation when cold is to be digested in cold water, and 41 parts of ammonium sulphate are to be added to the solution; double decomposition then ensues, ammonium cyanate and potassic sulphate being formed: the ammonium cyanate may be dissolved by means of alcohol; it thus becomes converted into urea, which crystallizes as the solution cools. Red lead may be substituted for peroxide of manganese in the foregoing process: the proportions then being 15 parts of red lead, 8 of the dried ferrocyanide, and 3 of potassic carbonate. Eight parts of ammonium sulphate must be employed to decompose the solution of potassic cyanate obtained.

Properties.-Pure urea is colourless; it crystallizes in slender striated prisms, which are slightly deliquescent. Its solution has a cooling bitterish taste, and is neutral to test papers. It is very soluble in water and in hot alcohol, but very sparingly so in ether. It forms no definite compound with water, but it deprives many salts, such as sodic sulphate, of their water of crystallization when triturated with them, producing a soft semi-fluid magma. Urea melts at about 248° (120° C.), but at a temperature a little above this it is decomposed, yielding ammonia, with cyanate and carbonate of ammonium, cyanuric acid being left in the retort; this, if the temperature be raised further, in its turn undergoes

decomposition, and a white amorphous residue, consisting of melanuric acid, is obtained :

2 5 3

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Previously to the completion of this change, a substance termed biuret (Є,H,N,,,H,O) is formed (1606). By a still further elevation of temperature melanuric acid is decomposed and mellon is formed.

A solution of urea, if pure, may be kept at ordinary temperatures without alteration, and it may even be boiled without undergoing decomposition; but if heated in a sealed tube to about 284° (140° C.), each molecule of urea combines with 2 atoms of water, and is converted into ammonium carbonate:

¤H ̧Ñ‚Ð + 2 H ̧Ð = (H ̧N),¤ ̧.
NᎾ + 2

The same change takes place slowly at ordinary temperatures in the presence of the mucus of the bladder, the ammoniacal odour acquired by urine after keeping it for a few days being due to this alteration. A similar decomposition into carbonic anhydride and ammonia occurs when urea is fused with caustic potash, or treated with concentrated sulphuric acid.

Chlorine, when transmitted into an aqueous solution of urea, resolves the latter into carbonic anhydride and nitrogen, whilst hydrochloric acid is formed:

ЄH ̧Ñ‚Ð + H2+ 3 Cl2 = €0, + N2+ 6 HCl.

4 2

Urea does not possess well-marked basic properties, but it combines with certain acids, and yields crystallizable compounds, which preserve a strongly acid reaction. It absorbs hydrochloric acid gas, forming a deliquescent substance (EH,N,, HCl) which fuses at a gentle heat to a yellowish oil, and solidifies to a crystalline radiated mass as it cools; water decomposes it immediately. The most important salts of urea are those which it forms with nitric and oxalic acids. In combining with the monobasic oxyacids, urea unites with 1 atom of acid, no separation of water occurring. Urea nitrate (EH,N,O, HN→) is such a compound which crystallizes in rhombic prisms, or in brilliant leaflets which are sparingly soluble in water, especially if it contain excess of nitric acid. It is sparingly soluble in alcohol. Urea oxalate [2 ЄH ̧Ñ‚Ð, H ̧¤,0] crystallizes in long, thin, transparent, €HN Ꮎ, Ꮎ sparingly soluble prisms. It is almost insoluble in a saturated aqueous solution of oxalic acid.

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780

COMPOUNDS OF UREA WITH SALTS.

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(1602) Compounds of Urea with Salts, and with Metallic Oxides. -Urea forms definite compounds with certain metallic oxides, such as those of silver and of mercury. Its compound with silver oxide (3 Ag2, 2 ¤H ̧ч) is produced by digesting at a gentle heat freshly-precipitated oxide of silver in an excess of a solution of urea; a grey or yellowish crystalline powder is thus formed. Three such compounds may be obtained with mercuric oxide, viz., ЄHNO, Hg; 2 ¤H,N,O, 3 Hg; and ¤H ̧Ñ‚Ð, 2 Hg. The first of these compounds (EH,N,O, Hg) is formed, if successive portions of mercuric oxide suspended in water be mixed with a warm solution of urea; at first the oxide is completely dissolved, but the compound in question is gradually deposited as a white powder, which becomes yellowish on drying. The compound (2 ¤H ̧Ñ‚Ð, 3 HgÐ) is obtained by adding to a solution of urea rendered alkaline by potash, a solution of corrosive sublimate, taking care to maintain an excess of alkali; a white gelatinous precipitate is formed, which when well washed is converted by boiling water into a pale yellow granular powder, having the composition above indicated; and the compound (ЄH ̧Ñ‚Ð, 2 HgÐ) is obtained by precipitating a solution of mercuric nitrate by adding it to an alkaline solution of urea. (Liebig, Chem. Gaz., xii. 41.)

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When a solution of mercuric nitrate is mixed with one of urea, the urea enters into combination with the salt in several proportions. If warm dilute solutions of urea and of the mercurial nitrate be mixed, a crystalline precipitate (4 Hg, NO, 2 CH,N,O) falls as a heavy granular powder, which consists of concentric groups of minute needles. But if a dilute solution of mercuric nitrate be added to a solution of urea so long as a precipitate is formed, and the whole be maintained at a temperature of from 104° to 122° F., the precipitate is principally composed of six-sided prisms, consisting of (3 Hg,N ̧Ð ̧, 2 ¤H ̧Ñ‚Ð). If a solution of nitrate of urea be poured into a somewhat dilute solution of the mercurial nitrate, slightly acidulated with nitric acid, until a permanent turbidity appears, on filtering the liquid and allowing it to stand, crystalline crusts of small shining rectangular tables are gradually formed (2 Hg✪, N ̧Ð ̧, 2 CH ̧Ñ‚Ð). Nitrate of silver forms two similar compounds (AgNO ЄH,N,O), and [2 AgNO3, CH,N,0], which crystallize readily when a mixture of a solution of urea and of nitrate of silver is evaporated in vacuo over sulphuric acid. Urea also combines with other salts, but the compounds which it forms with them are of little importance.

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LIEBIG'S MODE OF DETERMINING UREA IN URINE.

781

(1603) Liebig's method of determining Chlorides and Urea in Urine. The foregoing observations are due to Liebig, who has founded upon them a method of determining the quantity of chlorine and of urea in urine, which is valuable when such analyses are numerous, since it enables a large number of comparative experiments to be made in a short time.

1. For the Chlorine.-A solution of urea is not precipitated by a solution of corrosive sublimate, but it is immediately precipitated by mercuric nitrate. Solutions of the chlorides of the metals belonging to the groups of the alkalies and alkaline earths, when mixed with mercuric nitrate, become decomposed into corrosive sublimate, and a nitrate of the alkaline or earthy metal; hence, when a solution of chloride of sodium is mixed with one of urea, no precipitate is occasioned in this liquid on the addition of mercuric nitrate, until the whole of the sodic chloride has been decomposed by the mercurial salt; 2 NaCl + Hg 2 NO ̧ becoming 2 NaNO,+HgCl2. After this point has been reached, every fresh addition of the mercurial nitrate occasions a precipitate of the compound (4 Hg,N,,, 2 ЄH ̧Ñ ̧); and if a solution of the mercurial salt of known strength and free from excess of acid be prepared, it is easy to determine the quantity of chlorides present by measuring the volume of this solution which it is necessary to add to a given volume of urine before it begins to occasion a precipitate.

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2. For the Urea.-Having ascertained the number of divisions of the mercurial solution which are consumed before a precipitate begins to be formed, a measured volume of a standard solution of nitrate of silver, sufficient exactly to effect precipitation of the chlorine thus indicated, is added, and afterwards the urea is determined by means of the solution of mercuric nitrate. The following are the details of the operation :

1. Preparation of the Solution of Mercuric Nitrate No. 1, employed for determining the Chlorine.-Crystals of pure mercurous nitrate are to be dissolved in moderately strong nitric acid, and the solution is to be heated until a sample is no longer rendered turbid by sodic chloride, which indicates that the whole has been converted into a mercuric salt. The solution is next to be evaporated on a water-bath to a syrupy consistence, diluted with about ten times its bulk of water, and neutralized by the addition of precipitated yellow oxide of mercury so long as the oxide is dissolved; it is then set aside for twenty-four hours, and, if necessary, filtered. In order to graduate the solution, it is requisite to procure a saturated solution of pure sodic chloride, prepared by digesting a large excess of the salt in