ORGANIC ANALYSIS. 



799 



with which it can be confounded being an alka- 

 line solution of albumen, which resembles it 

 in this and in some other particulars. Acetic 

 acid in small excess causes its immediate 

 coagulation ; a great excess of the acid redis- 

 solves the coagulum. The fibrin, fats, and 

 albumen having been separated in the manner 

 already described, we may proceed to deter- 

 mine the casein by adding a few drops of 

 acetic acid and evaporating to dryness. Coiling 

 water removes from the residue every thine; ex- 

 cept saline matters and casein ; this residue is 

 dried, weighed, incinerated, and the salts de- 

 ducted; a portion of casein is apt to redissolve 

 in the water, so that this process is not perfectly 

 accurate.* 



Urea. It is best, when practicable, to make 

 a separate analysis for this principle; if this be 

 impossible, we may proceed with the fluid from 

 which albumen, casein, and fats have been se- 

 parated, as already mentioned. In either case 

 the liquid is evaporated to dryness, and the re- 

 siduum digested with alcohol; we thus obtain 

 a solution of urea freed from every thing ex- 

 cept chlorides, sugar, some "extractive" mat- 

 ters, besides fats and lactates, unless previously 

 removed by ether. The alcohol having been 

 driven off, the residue is dissolved in sufficient 

 water to produce a liquid of syrupy consistence. 

 Colourless nitric acid is diluted till of a specific 

 gravity of T25, which is a convenient strength 

 for precipitating the nitrate of urea. The eva- 

 porating basin with the impure solution of urea 

 is next placed in a vessel containing a frigonfic 

 mixture composed of 1 oz. of nitre, 1 of sal 

 ammoniac, each finely powdered, and 4 oz. of 

 water; when the basin and its contents have 

 been thus cooled, colourless nitric acid, sp. gr. 

 l - 25, rather more than equal in bulk to the 

 solution of urea, is added drop by drop, stir- 

 ring carefully ; if added too quickly, the tempe- 

 rature rises, a little effervescence ensues, and 

 part of the urea is decomposed. A flaky precipi- 

 tate shows nitrate of urea ; the whole is al- 

 lowed to remain in the freezing mixture for 

 three or four hours, or even longer; the nitrate 

 of urea is then collected on linen, the linen is 

 folded up, placed between several layers of 

 filtering paper, and then subjected to strong 

 pressure. The mother-liquor and excess of 

 acid are thus almost entirely expelled ; a slightly 

 discoloured firm dry mass of nitrate of urea is 

 procured, which may be exposed for some time 

 to a temperature of 212, and then weighed; 

 100 parts contain 48'78 of urea. The amount 

 of impurity which this nitrate contains is usu- 

 ally very small, as is easily proved by ultimate 

 analysis. 



The nitrate of urea crystallizes in flat rhom- 

 boidal tables, is sparingly soluble in water, 

 pretty freely in alcohol even when cold, very 

 slightly in pure ether. It is wholly vola- 

 tilized by heat; when digested with carbon- 

 ate of baryta suspended in water, effervescence 

 ensues ; on evaporating to dryness, exhausting 



* Proof-spirit may be substituted for water as a 

 solvent, but it leaves more animal extractive mat- 

 ter, though it dissolves less of the casein. 



with hot alcohol and evaporating slowly, long 

 prisms of urea are obtained. 



Other methods have been proposed for esti- 

 mating the urea, and when the proportion is 

 small the substitution of oxalic for nitric acid 

 furnishes 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 before 

 to strong pressure ; the firm 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 in cold water. 



Sugar. It is best to take a separate portion 

 of the fluid to determine the quantity of sugar. 

 Ajar, 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 



