Urea occurs as an essential component of the urine of man and animals, being more particularly abundant in the urinary excretion of the flesh-eating mammalia; nor is it altogether absent from the urine of birds and amphibia. According to Bischoff and Voit, urea is the result of tissue metamorphosis. The greater number of inquirers, however, hold an opposite opinion, and believe that it is derived from the albuminous constituents of the food, when these preponderate over the quantity required for the nutrition of the body. Dr Lionel Beale says his “own researches render it probable that all pabulum entering the system must, before its elements can he applied to the nutrition of the tissues, or removed by the organs of respiration or secretion, be first of all taken up by cells (chyle-corpuscles, white blood-corpuscles), and
become living or germinal matter, which, after passing through certain definite stages of existence, becomes serum of the blood, and the formed matter of the red blood-corpuscles.
The whole of the cuts illustrating “Urea,” “Urinary diseases,” and “Urine” are taken from Dr Beale’s work on ‘Kidney Diseases, Urinary Deposits, and Calculous Deposits,’ by that gentleman’s kind permission.
“The products resulting from the disintegration of this formed matter may be taken up by the germinal matter of the tissues, and at length become tissue, or by that of secreting cells, in which case it is removed in the form of the constituents of various excretions from the body altogether.” About an ounce and a quarter of urea is daily excreted by a healthy man, although of course there will be a variation in this amount principally depending upon the quantity of animal food and active exercise taken during the twenty-four hours. As might be anticipated a smaller quantity of urea is excreted by women than by men, since they are unable to indulge to the same extent in muscular exercise. A diminished quantity also results from breathing impure air, and from a diseased and unhealthy condition of the lungs or of the circulation, and also, as might be expected, from an insufficiency of food.
Prep. (Thénard.) Fresh urine, gently evaporated to the consistence of a syrup, is treated with its own volume of nitric acid of sp. gr. 1·19; the mixture is shaken and immersed in an ice bath, to solidify the crystals of nitrate of urea (p. 1689); these are washed with ice-cold water, drained, and pressed between sheets of blotting paper; they are next dissolved in water, and the solution is decomposed and precipitated with carbonate of potassium (or carbonate of barium); the whole is then gently evaporated nearly to dryness, and the residuum is exhausted with pure alcohol, which dissolves the urea, which crystallises out as the solution cools.
Urea, Factitious. Mix 28 parts of well-dried ferrocyanide of potassium with 14 of black oxide of manganese (both in fine powder), and heat them to dull redness on an iron plate. Lixiviate with cold water, add 221⁄2 parts of dry sulphate of ammonia, concentrate by evaporation with a heat not exceeding 212° F., decant the concentrated liquid, treat it with rectified spirit, and crystallise. This is intended as a cleanly substitute for the preceding.
Urea, Ni′trate of. Syn. Ureæ nitras, L. Prep. From urine, as described above; or it may be prepared by saturating artificial urea with nitric acid. Diuretic.—Dose, 2 to 5 gr. twice or thrice daily; in dropsy.
U′RIC ACID. C5H4N4O3. Syn. Lithic acid; Acidum lithicum, A. uricum, L. A substance discovered by Scheele, and peculiar to the urine of certain animals, and the excrement of serpents and several birds. The fæces of the boa constrictor consist of little else than urate of ammonium. It constitutes one of the commonest varieties of urinary calculi, and of the red gravel or sand which is voided in certain morbid states of the urine.
Guano derives its principal value as a manure from the presence of urate of ammonium. The gouty concretions of the joints, popularly known as chalk-stones, consist chiefly of urate of sodium.