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dryness, dissolving the residue in water, and crystallizing in the usual manner.

Crystals of artificial urea are represented in fig. 5.

Urea crystallizes in four-sided prisms, which appear to be composed of a number of acicular crystals placed in apposition. Hollow spaces are usually present in the interior of the crystals in considerable number. These contain a fluid differing considerably in refractive power from the crystal itself. When the crystals are dried, these spaces are occupied with air. They are seen in almost all the crystals represented.

It is very curious that urea exerts a great influence upon the crystallization of chloride of sodium and muriate of ammonia. The former, which ordinarily crystallizes in cubes, in the presence of urea, assumes the form of octohedra; and the latter, whose ordinary form is an octohedron, that of a cube. Some octohedra of chloride of sodium are represented in Plate I, fig. 1.

The best test for urea consists in adding nitric acid to a highly concentrated solution of the fluid suspected to contain it, when crystals of the nitrate are formed.

PLATE III.

NITRATE OF UREA, C,H,N,0,H0, NO.

Crystals of nitrate of urea are easily prepared by adding nitric acid to a concentrated solution of urea, or to ordinary Urine evaporated to half its bulk, or less. The crystals of nitrate soon appear in the form of scales, which are composed of a number of rhomboidal plates, of the shape represented in Plate III. The character of the crystals varies slightly according to the amount of acid added, and the degree of concentration of the urea solution.

Not unfrequently, especially in cases of acute disease, in this country, the Urine contains so much urea when

PLATE III.

NITRATE OF UREA, C,H,N,O,,HO,NO.

Fig. 1. Crystals of nitrate of urea, formed by adding excess of nitric acid to concentrated Urine.

Fig. 2. Nitrate of urea, formed by adding a quantity of nitric acid, not sufficient to combine with the whole of the urea present.

Fig. 3. Nitrate of urea, obtained by adding a moderate quantity of nitric acid to slightly concentrated Urine in a test tube, and allowed to crystallize slowly.

Fig. 4. Obtained by adding a marked excess of nitric acid.

Fig. 5. Crystals of nitrate of urea, formed by adding only two drops of nitric acid to highly concentrated Urine.

Fig. 6. Crystals of pure nitrate of urea, obtained by dissolving some of the nitrate in water, and evaporating so that crystals may form.

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