ORGANIC ANALYSIS. 
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. Boiling 
water removes from the residue every thing 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 1°25, which is a convenient strength 
for precipitating the nitrate of urea. The eva- 
‘porating basin with the impure solution of urea 
His next placed in a vessel containing a frigorific 
‘Mixture composed of 1 oz. of nitre, 1 of sal 
t ammoniac, each finely powdered, and 4 oz. of 
‘water; when the basin and its contents have 
deen thus cooled, colourless nitric acid, sp. gr. 
1°25, rather more than equal in bulk to the 
Solution of urea, is added drop by drop, stir- 
ie : ; 
Ting carefully ; if added too quickly, the tempe- 
ature 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 
hree 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 
iiterimg paper, and then subjected to strong 
wressure. ‘The mother-liquor and excess of 
cid are thus almost entirely expelled; a slightly 
iscoloured firm dry mass of nitrate of urea is 
procured, which may be exposed for some time 
© a temperature of 212°, and then weighed ; 
00 parts contain 48°78 of urea. The amount 
# impurity which this nitrate contains is usu- 
ly very small, as is easily proved by ultimate 
ni lysis. 
| The nitrate of urea crystallizes in flat rhom- 
bidal tables, is sparingly soluble in water, 
etty freely in alcohol even when cold, very 
nightly in pure ether. It is wholly vola- 
ized by heat; when digested with carbon- 
e of baryta suspended in water, effervescence 
Sues; on evaporating to dryness, exhausting 
* Proof-spirit may be substituted for water as a 
vent, but it leaves more animal extractive mat- 
, though it dissolves less of the casein. ~ 
799 
with hot alcohol ard 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. 
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 
