UREA (C2114N202) is an organic matter which derives its name from its having been originally discovered in the urine, of which it forms the most important and character istic iwrredient, It was until recently regarded as an organic base or alkaloid; but dur ing the last few years it has been placed amongst the amides, a group of neutral, and for the most part crystallizable compounds, of the ammonia type (Sec TYPES, CHEMICAL), in which one of the three equivalents of Hydrogen is replaced by the radical of an organic acid. For example, if R represent the radical, II N represents the corresponding 11 amide; and the character of the type is not affected by doubling the entire number of R2 molecules, or changing it into II, Now, if in this formula we substitute (ear bonic oxide or carbonyl, as it is now often termed) for 1122, we obtain 112 N2,wIlich is identical with C,114N202, and possesses the advantage of showing the probable grouping of the elements in urea. Pure urea, which has been allowed to crystallize slowly, occurs in white, glistening, streaked, four-sided prisms; but when the crystallization is rapid or disturbed, it separates in small white silky needles. It is devoid of smell, has a coolish, bitter taste, like that of saltpeter (which it closely resembles in iis external form) and is very slightly deliquescent. It is readily soluble in water and alcohol, but only slightly. in ether. When heated to about 246', it fuses, evolves ammonia, and becomes com pletely decomposed. A solution of pure urea in distilled water may be kept for a long time, and may even be boiled without undergoing decomposition; but if heated in a close tube to about each equivalent combines with tour of water, and is converted into carbonate of ammonia, according to the formula— Urea. Carbonate of Ammonia.
= 2114N0,C,04 A similar change takes place slowly at ordinary temperatures in the urea contained in the urine, the mucus of the bladder acting as a ferment, and thus rendering urine ammoniacal after it has been kept for a few days.
The following are some of the most important of the compounds of urea: Nitrate of urea. (C2114N202. N0,,I10) and oxalate of ttrea12(CJI4N209), are readily crystallizable salts, formed by the direct addition of the respective acids to a moderately strong solution of urea. As nitrate of urea requires eight parts of cold water. and is still more insoluble if an excess of free nitric acid is present, and the oxalate is more insoluble than the nitrate, while urea dissolves in its own weight of water, these salts may be employed to test for and approximately determine the quantity of urea.
Among the compounds of area with metallic oxides, those which it forms with Ihe oxides of mercury are especially interesting, and have been completely examined by Liebig. A result of his researches is his celebrated method, which is now in universal use. of determined the amount of urea volumetrically.
Urea occurs as the main and characteristic ingredient of the urine of man and mam mals, most abundant in that of carnivorous, and least so in that of vegetable feeders. The average quantity excreted by the human subject is mentioned in the arti cle URINE. R is also a constituent of the fluids of the eye, of the sweat, and (in minute quantity, of the blood, and of the liquor amnii (of the fetus). There can be no doubt that it is a final product of the regressive metamorphosis of the living tissues, or of their disintegration into simpler compounds, by means of which the final elimination of the worn-out structures is facilitated. Thus, we find that urea may be obtained by oxidiz ing agents from uric acid. creatine, allantoine, etc., in the laboratory, and there is every reason to suppose that similar changes may occur in the system. 'Whether, when an excess of food i3 taken, a portion of it may he formed in the blood into urea, and then at once separated without ever having entered into the structure of the higher tissues, is a point which is scarcely decided, Until the discovery by Liebig and Wohler of the artificial formation of urea, its only source was the urine, from which, after evoporation, the nitrate was separated. purified by animal charcoal, and the urea liberated by the addition of carbonate of haryta, and finally extracted by from which it was allowed to crystallize. It is now known that there are many different ways of forming it, as (1) by the action of chlorocarbonie oxide or phosgene gas on dry ammonia; or (2) by heating a mixture of car bonic ether and an alcoholic; solution of ammonia to about 356° in a closed tube (both of which methods show that urea is truly the amide of carbonic acid); but the best practical method is essentially the original one of mixing water solutions of eyanate of potash and sulphate of ammonia, and evaporating the solution, which leaves a residue consisting of urea and sulphate of potash, the former of which may be extracted by alcohol.