remain on the filtering paper as a scarcely visible deposit, while the urine itself passes through perfectly bright. On gently drying the filtering paper, the deposit assumes a varnish-like aspect.

(156.) UREA.

Fig. 53.

a. Its detection. A little of the filtered urine, concentrated by careful evaporation in a watch-glass, and treated with a few drops of strong colourless nitric acid, yields either at once or very speedily a crystalline deposit of nitrate of urea. This deposit, when examined under the microscope, is seen to consist of delicate six-sided plates, superimposed upon one another so as usually to prevent more than three or four of the sides of any one crystal being recognisable, as shown in fig.53. The nitrate of urea may be formed in a watch-glass and then transferred to a slide, or it may be dissolved in water and recrystallised on a slide, or nitric acid may be added to some concentrated urine previously placed in the field of the microscope, and the actual process of crystallisation observed.

The

B. Preparation of urea. A couple of ounces or so of fresh and filtered urine are evaporated on a water-bath or gently heated sand-bath to a syrupy consistency, and a quantity of strong colourless nitric acid about equal in bulk to the concentrated urine added thereto, when the mixture, upon cooling, becomes semi-solid from the formation of nitrate of urea. crystalline mass is drained on a tile, or pressed between several folds of blotting paper, then dissolved in a little warm water, and the resulting solution treated with an excess of carbonate of barium. Upon concentrating the filtered liquid, nitrate of barium crystallises out first, while urea remains in the mother liquor, which is evaporated to dryness over a water-bath. From

this residue warm alcohol extracts the urea, and, on cooling or slow evaporation, deposits it in the form of long flattened prismatic crystals, as shown in fig. 54.

Fig. 54.

y. Properties of urea. Urea behaves in some respects like an organic base, being capable of uniting with certain acids, notably the nitric and oxalic acids, to form salts. It dissolves readily in water and alcohol, producing solutions which are neutral to test-paper. The formula of urea is CH,N2O, and that of nitrate of urea CH,N2O.HNO3• Urea is isomeric with cyanate of ammonium NH,CNO, which undergoes spontaneous conversion into it; and also isomeric, if not identical, with carbamide N2H4 (CO)". Heated with water under pressure, it is transformed into carbonate of ammonium, thus: CH,N2O+2H2O =(NH4)2CO3. The same change takes place spontaneously in putrefying urine, and is also brought about by acting on urea with strong potash or sulphuric acid, except that the resulting carbonate of ammonium is then broken up by the reagent employed. 1,000 parts of urine contain, on the average, about 15 of urea.

(157.) URIC ACID.

a. Its detection. This compound occurs but in small quantity in healthy human urine, 100 parts of which contain, on the average, not more than half a part of uric acid. In order to detect its presence, a couple of ounces or so of filtered urine are reduced to one-half the original bulk by evaporation, a little hydrochloric acid added to the concentrated liquid, and the whole set aside in a cool place for some hours, when the interior of the vessel will be found studded with small brown crystals of impure uric acid. In

the case of urine having a moderately high specific gravity, concentration is unnecessary. After pouring off the supernatant liquid, the crystals are detached, washed with water, and dissolved in a few drops of warm potash. The resulting solution of urate of potassium is then filtered, and acidulated with hydrochloric acid, whereby a crystalline precipitate of uric acid is thrown down, which may be examined microscopically and by the action of nitric acid, as described below.

B. Preparation of uric acid. Uric acid cannot well be prepared, in any quantity, from normal human urine. But it may be easily obtained from the common brickdust urinary deposit, collected on a filter and washed with water; or from powdered uric calculi; or the excreta of serpents. Any one of these substances is boiled with caustic potash, the solution diluted with water, filtered, and supersaturated with hydrochloric acid, whereby a very considerable opacity is at first produced, which, however, speedily disappears, and is replaced by a dense crystalline precipitate, from which the supernatant liquid may be readily poured off.

7. Properties of uric acid. Uric acid furnishes two classes of salts, acid and neutral, and is consequently dibasic. Its formula is C,H,N,O,. The formula for a scarcely soluble or acid urate is C¿MH,NO2, and that for a soluble or neutral urate C,M,H,N,O,. Uric acid itself is extremely insoluble, both in water and alcohol; but is soluble in alkaline solutions, forming neutral urates, and reprecipitated therefrom on the addition of an acid. It always occurs in the crystalline state, the appearance of the crystals, however, being very various. Occasionally the acid is met with in its normal form of the rhombic prism, more frequently in rhombic plates with the obtuse angles more or less rounded off, or in acuminated doubly-convex lozenge-shaped plates, or in elongated flat plates with excavated ends. Some of the forms of uric acid crystals are shown in figs. 55 and 56. Uric acid dissolves readily, with effervescence, in nitric acid, and

on evaporating the solution to dryness an amorphous pinkish residue is left. This, when moistened with ammonia, assumes a

fine crimson colour, which is changed to violet on the addition of a small quantity of caustic potash.

(158.) HIPPURIC ACID.

a. Its preparation. 'Although this acid exists, in healthy human urine, in nearly the same proportion as uric acid, yet its presence therein does not so readily admit of demonstration. It can, however, be easily procured from the urine of herbivora, and from that of patients who have been taking benzoic acid as a medicine. To prepare it from either of these sources, the recent filtered urine is evaporated down to one-fourth of its bulk, and then treated with an equal volume of ordinary hydrochloric acid, when, on cooling, long prismatic needles of impure hippuric acid crystallise out. These, after being washed with a little cold water, are dissolved in boiling water, and the solution set aside to crystallise.

B. Properties of hippuric acid. The hippuric is a monobasic acid, represented by the formula C,H,NO,. It is soluble in water, alcohol, and ether. The crystals obtained by cooling the hot aqueous solution on a slide consist of delicate

prisms, often presenting the appearance of elongated six-sided plates, as shown in fig. 57.

By prolonged boiling with concentrated hydrochloric acid, hippuric or glyco-benzoic acid absorbs a molecule of water, and breaks up into benzoic acid and glycocine or sugar of gelatine, thus: C,H,NO, + H2O = C2H6O2 + C2H¿NO2. The larger proportion of the former product is dissipated by evaporation; but the glycocine may be detected by adding to the liquid a drop or so

Fig. 57.

of aqueous sulphate of copper and an excess of potash, whereby a deep blue-coloured solution is produced, unaffected by ebullition.

(159.) COLOURING AND EXTRACTIVE Matters.

a. Purpurine, &c. When healthy urine is boiled in a testtube with about one-fourth its bulk of hydrochloric acid, a deep brownish-purple colour is produced, due to the metamorphosis of a peculiar highly carbonised pink colouring matter, known as purpurine. The common pink deposits of alkaline urates owe their colour to this purpurine, which has a great tendency to become precipitated with them. Hence, when perfectly white urate of ammonia, boa's excrement, for instance, is boiled in urine containing much purpurine, it is deposited on cooling of a pink colour, from its carrying down some purpurine with it. When these coloured deposits, natural or artificial, are boiled in alcohol, the purpurine is dissolved, forming a pinkish-red solution. The relation in which purpurine stands to the yellow colouring matter of urine is not satisfactorily established.

B. Extractives. The remaining organic urinary constituents are called by this name. They generally amount to about one per cent. of the urine. Included among them is some principle containing sulphur in an unoxidised form, also kreatine and