THE LACTATES, AND UREA AS IT EXISTS IN URINE. 153 
The first deduction we would draw from all these facts is, 
that in general, urea does not exist in urine in a free state ; that 
in man, and, without doubt, in the carnivorous animals, it is 
principally combined with lactic acid ; in the ruminating ani- 
mals with hippuric acid ; and in birds and reptiles with uric 
acid, or a peculiar acid of which, according to M. Liebig, it 
serves as a radical. 
II. Convinced, by these observations, of the real import- 
ance of the lactate of urea, we return to the study of this pro- 
duct, and its direct or artificial preparation. 
It may be obtained either by direct combination of urea 
with lactic acid, or by double decomposition ; by acting on 
lactate of lime with oxalate of urea, or better, on the sulphate 
of urea by lactate of baryta. In either case it is necessary, in 
the first place to procure the lactic acid, or the lactates, in a 
perfectly pure state. The following is the process which we 
have found to succeed best. 
Lactic acid is constantly forming in liquors which undergo 
the viscous fermentation and become sour, as, for example, in 
the juice of the beet root, the sour waters of rice and starch, 
mucilaginous solutions and sour milk. We have preferred 
sour milk, since it may be obtained in large quantities, and at 
a low price, from farms at a distance from large cities. Cow's 
milk, which is scarcely acid when first drawn, soon sours in 
contact with the air ; the curd begins to separate ; the serum 
becomes cloudy ; and, if heated, the coagulation of the 
curd and albumen is completed. In this state it is passed 
through linen, clarified by white of egg, filtered, evaporated 
to a syrupy consistence, and set aside for some days, when a 
large quantity of sugar of milk is deposited. The liquid is 
again strained and evaporated, by which an additional quanti- 
ty of sugar of milk is deposited, and may be separated by 
straining or filtering. The residual liquid is now very acid ; 
it is to be mixed with five times its bulk of alcohol of 33°, 
which produces a precipitate consisting chiefly of sugar of 
milk. The liquid is allowed to settle, is decanted and filtered, 
VOL. VI. — NO. II. 20 
