UREA. 
57 
crystals of urea, at the end of the operation, to sprout from 
one side of the beaker to the other, being interlaced in a 
variety of ways. Large leaf-like masses of crystals are often 
seen adhering to the others, and resting upon their thickest 
extremity, or that connected with the glass. From their 
being mixed with a little common salt, the crystals of urea 
deliquesce very readily as soon as the mother-liquor is 
decanted ; but another crystallisation in water will produce 
them in the pure state. The acid liquor may now be used for the 
extraction of the peculiar acid. From the above method for 
the extraction of urea, we may safely conclude that by far the 
largest proportion of this substance exists in the free state, as 
one of the immediate principles of human urine. 
If an alcoholic extract of urine be concentrated, needle-like 
crystals of urea are deposited, mixed with the inorganic salts 
also dissolved by the alcohol ; these crystals will occasionally 
lie parallel to each other, but generally coalesce into stellate 
groups adhering to the bottom of the capsule. We have fre- 
quently observed this crystallisation to occur in the concen- 
trated alcoholic extract of dog’s urine, which contains a large 
proportion of urea. If a drop of a solution of urea be evaporated 
on a glass plate, the crystals will assume a peculiar arborescent 
structure, transmitting, very readily, polarised light. It also 
often happens in this case that a number of parallel crystals 
of urea will be connected to each other transversely by shorter 
crystals. If urine be evaporated to the consistence of a syrup, 
and left undisturbed for a day or two, a crystallisation of 
urea will appear, assuming the form of long prisms or large 
striated needles, with a granular surface. 
To extract urea from blood the fibrine has first to be 
removed, and then the fluid, mixed with an equal bulk of 
water, is heated to the boiling point and strained through 
calico. The serum free from fibrin, from albumen, and from 
colouring matter, is evaporated on the water-bath to a 
thickish consistence. The addition of alcohol to the residue 
will cause an abundant precipitate to appear, and the filtered 
alcoholic solution is treated with oxalic acid, to separate the 
fatty matters which have now become insoluble. The fluid 
is again filtered and rinsed with ether ; the latter floating on 
the water will contain all the hippuric acid of the blood ; it 
must be decanted ; the aqueous solution neutralised with car- 
bonate of lime is finally evaporated to dryness under the air- 
pump. If this dry residue be treated with alcohol, it will 
yield an alcoholic solution of urea, from which the latter can 
be easily obtained in the crystallised state or combined with 
nitric or oxalic acid. 
xxvii. 
8 
