56 
UREA. 
the form of ammoniacal compounds. This vicarious secretion 
comes into action whenever the kidneys are insufficient to 
remove all, or the greater part, of the urea contained in the 
blood. 
Extraction . — To extract urea from the urine, that fluid is 
first evaporated to almost one-tenth of its bulk, and then 
mixed with a small quantity of nitric acid free from nitrous 
acid. There occurs instantly, in a concentrated solution, or 
after a few minutes if it be diluted, a crystalline precipitate 
of nitrate of urea, which can be easily purified by repeated 
crystallisations. This compound will yield pure urea if it be 
treated with carbonate of lime, or carbonate of baryta, to 
separate the nitric acid as nitrate of lime or nitrate of baryta. 
This fluid, evaporated to dryness, is treated with alcohol, 
when the urea will crystallise by concentration. We have 
observed that the presence of sulphuric acid does not prevent 
the formation of nitrate of urea; on the contrary, it appears 
to hasten the combination, which assumes, in this case, a 
beautiful pearl lustre, much more remarkable than when 
nitric acid is made to react upon urea in pure water. It is 
sometimes advisable to use oxalic instead of nitric acid; the 
operations required in both cases are exactly the same. We 
have employed another method for separating urea from urine, 
which has enabled us to obtain this substance directly, with- 
out having recourse to a series of combinations and decom- 
positions. The urine is first evaporated on the water-bath, 
and the solid residue thoroughly dried over sulphuric acid, 
under the air-pump. The brittle mass is then to be treated 
with boiling absolute alcohol, until it yields no more colouring 
matter to that fluid. It is advisable, in order to obtain this 
extract, to employ repeated small quantities of alcohol, the 
solution being decanted after each operation. By this pro- 
cess the whole of the free urea contained in the urine examined 
may be extracted, together with a small quantity of common 
salt : test-paper will show this alcoholic solution to have a 
strong acid reaction. Sulphuric ether is then added to the 
acid alcoholic extract, care being taken to pour it gently down 
the sides of the glass to prevent the fluids from mixing. At 
first, a cloudy precipitate will appear at the line of contact of 
the two liquids, gradually extending below and above. Five 
or six hours afterwards the precipitate disappears, when the 
sides of the breaker will be found covered with beautiful, 
needle-like crystals of urea, sprouting from the sides and 
bottom of the glass to the centre of the fluid. Ether is now 
added until the precipitate ceases to appear, and another crop 
of crystals is obtained. We have frequently observed the 
