liquor amnii by Wöhler,' but is certainly present in exceptional cases only, as I have shown in an essay to which the Faculty of Medicine of the University of Heidelberg awarded a prize medal.

I must not overlook some observations said to be made of the occurrence of urea in milk, in blister-serum, in the perspiration of persons with diseased kidneys, and in the copious evacuations from the intestines of such patients, produced by the action of elaterium.

If we had been favoured by the authors of these statements with the particulars of the process adopted for obtaining the urea, and of the proofs that the substances obtained were really urea and nothing else, there would be no difficulty either in admitting these observations, or rejecting them upon the basis of analytical criticism. But at present they are mere assertions, the correctness of which can in no way be controlled. The greatest chemists do not allow themselves to throw assertions upon their readers, without giving the whole of their analytical proofs. The more we must demand similar and even more stringent proofs of men, who, whatever their professional standing, must acknowledge themselves more liable to error than a Berzelius, a Graham, or a Liebig. One fact will show the necessity of this caution. It goes the round through all the handbooks and encyclopædias of all nations, that Marchand found urea in the blood of the cow or calf. The indiscriminate authors do not inquire how he found it. On referring to the original article,2 or Gmelin's 'Handbuch,' we find that Marchand concluded upon the presence of urea from the crystallization in octahedra of the chloride of sodium. We now know that that occurrence by no means justifies us in the assumption even of the merest trace of urea, and, consequently, the assertion that Marchand found urea in the blood of the calf is devoid of foundation. We could not have corrected this error, had Marchand been credited with his statement upon the mere value of his name as a chemist.

History.-Urea, in an impure state, was first obtained from urine in 1773, by Rouelle, jun., who called it extractum saponaceum urine. It was first obtained pure by Fourcroy and Vauquelin in 1799. Its chemical composition was doubtful, even after its decomposition to ammonia and cyanic acid had been proved. In consequence of this discovery, a successful attempt was made by Liebig to produce urea artificially; and we are now in possession of several methods 1 Ann. d. Chem. und Pharm.,' lviii, p. 98.

2 Erdmann's Journal,' 1838, p. 500.

whereby it may be produced in any quantity, and more pure than it can be obtained from urine, without much difficulty.

As pure urea is a necessary instrument for preparing the test-fluids by which the chlorides and urea of the urine are determined quantitatively, it may be of use to the reader to have the mode of artificially making urea described in this place.

Mix intimately 28 parts of dry ferrocyanide of potassium with 14 parts of manganese, and heat on an iron sheet plate over an open fire, until the mixture ignites and is slowly burned through. Extract the blackish-gray mass which remains with cold water; add to the filtered liquid 204 parts of dry sulphate of ammonia; let it stand, in order that the sulphate of soda may crystallize; separate the crystals from the solution containing urea; evaporate the latter to dryness, and extract with alcohol, which leaves the rest of the sulphates undissolved, and, on evaporation, gives perfectly pure and white urea.

Two new methods for the artificial preparation of urea, by Natanson and Regnault, have been described.1

Urea is formed by evaporating a mixture of cyanic acid and ammonia (H,N+HO+C2NO) = (C2H ̧N2O2). It is a product of the decomposition, by oxydizing agents, such as peroxide of lead, of uric acid; and, by another process, of creatine; and the regularity with which this transformation is effected has established the opinion that urea is contained in uric acid, as it were, in a preformed condition, or that uric acid is only a preparatory state of urea.

Physical properties. Of the physical properties of urea, its form of crystallization deserves our attention for a moment. The crystals of artificial urea, or of urea obtained from urine by the process of purification to be described, are known well enough as white silky needles, or white transparent prisms, square on section, striated, and of a silky lustre. One or two inclined planes form the ends of the prisms.2 Much more perfect crystals, however, than are obtained by any of these modes, are occasionally produced by the simple spontaneous evaporation of a drop of urine on an object-glass.

The urine of fever-patients, which has a high specific gravity, and contains a very small proportion of inorganic salts, is very suitable for this experiment. The rapid formation of the prisms and their combinations, and their solubility in water, glycerine, and preserving solutions, ensures the

The Lancet,' July 26, 1856, p. 144.

2 Vide Dr. L. Beale, The Micros. in Clin. Med.,' p. 264, fig. 215. Funke, Atlas d. phys. Chemie,' taf. ii, fig. 4. See also Plate I, fig. 1.

diagnosis. They polarize with a weak blue colour, which is much like the glare of the sea in moonlight.

Urea crystallizes in quadratic prisms, from a solution in water with a rectangular terminal plane, from a solution in alcohol with octahedral planes. The long four-sided prism becomes six-sided when two of its diagonal edges are replaced by two vertical planes of a secondary prism, and eight-sided when the other two edges are also replaced by the secondary prism. These relations must be borne in mind when examining crystallizing solutions under the microscope with a view to diagnose urea. On the object-glass, one of the planes, by coming in contact with the surface, is preeminently large, and its two adjoining planes are very much narrower, thus showing that the crystal was prevented from growing in the direction of the glass. A secondary plane may in this way become larger than its diagonal one, and one or more primary planes may be prevented from obtaining the size of their correlatives.

Urea is colourless, of a bitterish, cooling taste, like saltpetre, which has been mentioned as being in part the taste of urine. When dry, it is not changed by exposure to the air, and then attracts very little moisture. A pure solution in water, even when dilute, does not undergo any spontaneous chemical change. It has no reaction on vegetable pigments. It is soluble in its own weight of water of 15° C., in five parts of alcohol, of 0.816 specific gravity, at the ordinary temperature of the air, and in its own weight of boiling alcohol. It is almost insoluble in ether, and quite insoluble in oil of turpentine.

Decompositions.-Urea may be decomposed by the influence of heat, acids, alkalies, salts, putrid animal matter, and yeast. When exposed to a temperature of 120° C. (248° F.) it fuses, but the temperature being raised a few degrees, ammonia and carbonate of ammonia are evolved, leaving ammeline, an amorphous white matter, then cyanate of ammonia, cyanuric acid and its derivatives, until at last the residue chars, and on raising the temperature to red heat, burns without leaving any residue.

Nitrous acid, and nitric acid coloured red by the presence of the former, decompose urea into carbonic acid, nitrogen, and water.

C2HNO2+2NO2 = 2CO2+4N+4HO.

The same decomposition is produced by a solution in nitric acid of the nitride of the suboxyde of mercury. This decomposition is employed in the quantitative analysis of Millon.

Urea, when fused with potash, or treated with concentrated sulphuric acid, is transformed into carbonic acid and ammonia. C2H ̧N2O2+2HO = 2CO2+2NH3.

The quantitative analysis of Heintz and of Ragsky is based upon this influence of sulphuric acid upon urea.

The same decomposition of urea may be produced by the influence of heat in the presence of water. A solution of urea, when enclosed in a glass tube, by the assistance of the blowpipe, and kept for several hours in an oil bath or well-regulated air bath, at a temperature of 140° C. (288° F.), will decompose in this manner; and if a sufficient amount of hydrate of baryta be present in the tube, the decomposition at a temperature of from 210° C. (410° F.) to 240° C. (468° F.) will be accelerated by the alkali, and the carbonic acid evolved will be immediately fixed by the baryta. From the quantity of carbonate of baryta so produced, we may ascertain the quantity of urea present in the fluid before the experiment. This is the process employed in the method of Bunsen.

The decomposition of urea into carbonic acid and ammonia is further the result of true fermentation, induced by ferments, such as yeast, or decomposing mucus of the urinary bladder (vide alkaline urine, p. 11), or any other putrefying animal matter, such as albumen.

A solution of urea, when heated with caustic lime, or magnesia, to a temperature of above 50° C. (122° F.), will evolve ammonia. Below 50° C. (122° F.), lime and magnesia exert no influence upon urea in solution, and may therefore be employed with safety for the quantitative analysis of ammonia in urine.

A solution of urea, when mixed with liquor sodæ chlorinatæ, evolves the whole of the urea in the form of nitrogen, carbonic acid, and water. As the carbonic acid is immediately absorbed, nitrogen only is left, the bulk of which, on measuring, is a ready means of determining the amount of urea of which it was a part. (Method of E. W. Davey.)

A mixture of urea and nitrate of silver in solution is on evaporation transformed into nitrate of ammonia and crystalline cyanate of silver. This experiment is the reverse of the process by which urea is produced artificially. Its formula is thus : C2H ̧N2O1⁄2 + AgO, NO,= NHO, NO, + AgO, CyO.

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Combinations of Urea.

Urea enters into combination with several bases, acids, and salts. Of these compounds, those are of great importance which, by being insoluble in water and watery

solutions, enable us to transform dissolved urea into a precipitate, and thereby determine its quantity.

+

Urea and protoxyde of mercury.—a. With two equivalents of protoxyde of mercury. U+2HgO.-On adding to a warm solution of urea, oxyde of mercury diffused in water, we observe the first portions of the oxyde to be perfectly dissolved; an excess of the oxyde of mercury is in the fluid gradually changed into a white or yellowish-white powder; the filtrate from the latter, after the elapse of twenty-four hours, deposits thin hard crusts on the walls of the vessel. These crusts and the powder have the above composition. The preparation frequently contains some cyanate of mercury. +

b. With three equivalents of protoxyde of mercury. U +3HgO.-On adding to a solution of urea, caustic potash, and then a solution of bichloride of mercury, with a renewed addition of potash ley, so that the fluid is always kept alkaline, a thick, gelatinous, snowy-white precipitate is obtained, which, when perfectly washed out and in its moist condition transferred into boiling water, transforms into a sandy or granular powder of a yellow or yellowishwhite colour. After drying the powder is reddish-yellow. When heated while moist it frequently explodes, with evolution of light, water, carbonate of ammonia, and metallic mercury. The powder is soluble without effervescence in hydrocyanic and hydrochloric acid; in the latter solution alkalies produce a whitish-yellow precipitate.

c. With four equivalents of protoxyde of mercury. U+ 4 HgO.-If, instead of a solution of bichloride of mercury, a solution of nitrate of protoxyde of mercury is precipitated by an alkaline solution of urea, a white and less voluminous precipitate is obtained, which in boiling water shrinks to a sandy powder.

Urea and chloride of sodium. U+NaCl +2HO.—This salt crystallizes in clino-rhombic prisms of great lustre, when a mixture of solutions of urea and chloride of sodium is evaporated. The same salt is obtained in large coloured crystals on evaporation of human urine. (Strecker.)

+

Urea and nitric acid. U+NO5.-If we mix a concentrated solution of urea, or urine concentrated by evaporation, with an excess of colourless nitric acid, the mixture will immediately crystallize into an almost solid mass of white shining scales or plates (yellow from urine) of nitrate of urea. This Liebig, Ann. d. Chem. und Pharm.,' lxxx, p. 123; lxxxi, p. 128; lxxxii, p. 232; lxxxv, p. 189.

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