796 
the original liquid, if it produce coagulation, 
shews the presence of casein. We need not 
seek for casein if the fluid shews an acid re- 
action, asit is coagulated by free acids in general. 
If mucus be present, some ambiguity may arise 
from the action of acetic acid, as this re-agent _ 
causes the coagulation of the mucus furnished 
7 the alimentary canal and its appendages, 
When present, however, in appreciable quan- 
tity, mucus always communicates to the fluid 
a certain degree of ropiness which leads us to 
suspect its presence. A confirmatory test for 
casein under such circumstances consists in 
adding a few grains of milk sugar and a little 
washed rennet; if the mixture be heated for an 
hour or two to about 120°, the casein coagu- 
lates completely. 
c. Fatty matters and cholesterin are revealed 
by the microscope, and may be separated by 
evaporating the fluid to dryness, digesting the 
residue with ether and filtering; by sponta- 
neous: evaporation of the ethereal solution, 
they are left behind with their usual physical 
characters. 
d. The presence of sugar is best discovered 
by mixing the suspected fluid with yeast and 
placing it in an inverted tube over mercury for 
twenty-four hours, at a temperature of from 
70° to 80° F., making at the same time a 
comparative experiment with an equal bulk of 
the same yeast diluted to the same extent with 
pure water. We cannot by yeast determine 
with certainty the presence of sugar in a pro- 
portion less than ;1;th of the liquid employed. 
A much more delicate test, and one which fur- 
nishes more immediate results, has lately been 
proposed by Trommer, founded upon the fact 
that organic bodies, to which free alkali has 
been added in excess, and especially solutions 
of grape and milk sugar, dissolve freshly pre- 
cipitated oxide of copper; when the saccharine | 
solutions are boiled they are decomposed, and 
sub-oxide of copper is deposited of a charac- 
teristic reddish brown colour. To apply this 
test, add to the suspected liquid a few drops 
of solution of caustic potass, so as to render 
it distinctly alkaline, then a small quantity of 
a dilute solution of sulphate of copper, agita- 
ting to dissolve the precipitate ; a liquid of a 
blue colour varying in intensity with the quan- 
tity of copper held in solution is obtained. 
Apply heat, and if sugar is present an ochre 
yellow or red precipitate of sub-oxide takes 
place, as soon as ebullition begins. I-have 
already mentioned that the presence of milk 
sugar has the same effect as grape or dia- 
betic sugar; other animal matters produce 
a similar change. It is, however, very deli- 
cate in its indications; a negative result 
therefore may be considered as decisive of the 
absence of sugar. If a precipitate occur, the 
presumption that we are examining a saccha- 
rine liquid ought always to be confirmed by 
recourse to the unequivocal expedient of fer- 
mentation. It is easy to concentrate the fluid 
if the sugar is very small in quantity; the only 
case in which any ambiguity can arise. Sugar 
of milk has never been found but in the secre- 
tion from which it derives its name. 
ORGANIC ANALYSIS. 
Jowed to crystallize from a moderately dilute — 
e. Urea can only be discovered satisfactorily 
by evaporating the suspected fluid to dryness 
and treating the residue with alcohol, again 
evaporating this alcoholic solution to dryness, 
redissolving in water,and adding nitric or oxalie 
acid with the precautions to be menti 
hereafter when treating specially of the deter- 
mination of urea. When common salt is al- — 
solution of urea, the form of the crystals is 
modified, and instead of obtaining cubes or 
octohedra the crystals developed assume a — 
more or less penniform appearance, often shew- 
ing the figure of a Maltese cross with serrated 
edges. These modifications have been proposed” 
as a test of the presence of urea: they are 
not, however, certain indications, though useful — 
as affording a presumption of its presence, 
When a fluid which contains urea is concen- 
trated by evaporation, and nitric acid is then 
added, by spontaneous evaporation on a glass” 
plate we obtain lamellar crystals of nitrate of 
urea in irregular rhomboidal plates with the 
acute angles often truncated. ; 
J. To detect the existence of uric 
and the urates, if albuminous principles ar 
present, the liquid is evaporated to dryne 
and the residue digested for some hours with 
solution of caustic potash, till every thin 
soluble has been taken up. It is dilutec 
filtered, and supersaturated with h ry dre 
chloric acid; a flocculent precipitate forms 
which is redissolved by excess of acid; am 
the uric acid separates; this is collec 
on a filter. We are enabled by this pre 
cess, where considerable quantities of 
acid are present, as in the excrements 
birds of prey, to obtain results of consider 
ble accuracy ; but where the proportion of t 
acid is very minute, it cannot be relied upo 
for quantitative experiments; the residue 
these cases must be subjected to microscop 
examination. If no azotised matters are 
sent, the mere addition of free hydrochlorit 
acid after a lapse of some hours usually cau 
a separation of rhombic crystals of \ 
or by evaporating to dryness and treating 
residue with water acidulated with hy 
chloric acid, the uric acid remains undissoly 
the residue is proved to contain this compo 
either by the appearance of its crystals ut 
the microscope, or by evaporating a small 
tion to dryness with nitric acid ona 
glass, when a red stain is left; a drop o 
monia added produces a fine crimson; if 
be evaporated to dryness, a drop of sc 
of caustic potash converts it into a beat 
purple, which is destroyed by the applic 
of heat. 4 
The methods for testing the other acid 
be described when treating of their quanti 
determination. 
Having now obtained an idea of t 
of the fluid we have to examine, we are 
to proceed with its “ 
Quantitative analysis. 
A thin porcelain or platinam 
