The nitrate of urea crystallizes in flat rhom oidal tables, is sparingly soluble in water, retty freely in alcohol even when cold, very ightly in pure ether. It is wholly vola ized by heat; when digested with carbon le of baryta suspended in water, effervescence ,.Lsues; on evaporating to dryness, exhausting with hot alcohol arId evaporating slowly, long prisms of urea are obtahied.
Other methods have been proposed for esti mating the urea, and when the proportion is small the substitution of oxalic for nitric acid funti,shes results of greater accuracy ; but the urea must then be separated by absolute alco hol instead of rectified spirit from its other ac companiments; the oxalic acid is rubbed to a thin cream with water, and a portion of this, equal in bulk to the syrupy solution of urea, is added to the liquid, which is gently warmed ; it is then allowed to cool, and the whole im mersed in the frigorific mixture. The crystals are drained on calico and subjected as befbre to strong pressure; the tirm dry cake of oxalate of urea and oxalic acid is allowed to digest at a temperature of 100° F. for about eight hours, with rather more than its own weight of chalk and six or eight times its weight of water. Effervescence ensues, an insoluble oxalate of lime forms, and urea is dissolved ; the solution is filtered and evaporated' to dryness ; long crystals of nearly pure urea form, from the weight of which the quantity is ascertained : they ought to dissolve completely in absolute alcohol ; anything undissolved is oxalate of ammonia.
If the amount of urea be very small, as is sometimes the case with diabetic urine or the serum of the blood, Dr. Rees recommends the employment of ether instead of alcohol to the dry residue. Urea is very sparingly soluble in this menstruum and will be obtained with the fats ; the ethereal liquid evaporated and the residue treated with water, filtered and again evaporated, furnishes long delicate prisms of urea; this_ process, however, must be re garded more as a test of the presence of urea rather than as a means of estimating its quan tity. Sometimes urobenzoic (hippuric) acid occurs in diabetic urine, and it would then be extracted by ether along with the urea and crystallize with it ; but it is easily separated and distinguished by the sparing solubility of its crystals id cold water.
Sugar.—It is best to take a separate portion of the fluid to determine the quantity of sugar. A jar, with its aperture ground smooth, gradu ated to tenths of a cubic inch, and capable of containing from 12 to 20 cubic inches, is turned with its mouth upwards and filled to within an inch or two of the top with mercury ; from 100 to 200 grs. of the liquid for analysis is accu
rately weighed and transferred to the jar with the usual precautions; 8 or 10 grs. of yeast are introduced, and the jar accurately filled with water ; the mouth is closed by a glass valve, which is retained in its place by a piece of linen or any other convenient means; the jar is inverted in a basin of mercury and the valve removed. The apparatus is set aside for two or three days in a temperature of about 70°, till the fermentation is complete. The quantity of gas is now accurately read off'; the temperature of the room and height of the barometer care fully noted, as well as the difference between the level of the mercury within and without the jar. As the liquid will be saturated with its own bulk of carbonic acid at the observed temperature and pressure, we must in our cal culations add the bulk of the liquid to that of the gas actually observed. Having reduced the bulk of gas to the standard 'pressure of 30 inches and the temperature of 60° F., accord ing to the rules given in all works on physics, we deduce the quantity of sugar, since 100 cubic inches of carbonic acid are furnished by the decomposition of 106.6 grs. of grape sugar.
If mercury be not at hand, we may, by adapting a tube containing chloride of cal cium, and a bulb apparatus charged with strong solution of potash," the weight of which is accurately known, to a tubulated retort, ob tain a result of considerable accuracy. In a half-pint retort, so adjusted, 500 g-rs. of urine are placed, and 30 or 40 grs. of yeast added ; through the tubulure of the retort a straight tube passes, and dips below the surface of the fluid ; the upper extremity is closed by a cork. It is set aside to ferment at a temperature of 70° ; the chloride of calcium retains the mois ture, and the potash ley absorbs the carbonic acid ; by the increase of weight vre know the quantity of carbonic acid formed. As at the close of the experiment the apparatus will be full of carbonic acid, it must be displaced by re moving the cork from the straight tube passing through the tubidure of the retort, and then gently drawing air through the apparatus for some minutes by careful suction at the extre mity of the potash apparatus, in the manner to be described presently when treating of the process of ultimate analysis. 100 grs. of car bonic acid indicate '225 grs. of diabetic sugar.