58 
UREA. 
Urea is very soluble in water and in alcohol, but dissolves 
sparingly in ether. When burnt upon a platina spatula it 
emits a peculiarly nauseous smell, which is often sufficient to 
show its presence. It combines with nitric and oxalic acids, 
but not with sulphuric acid; we have observed, however, 
that the presence of the latter appears to hasten the formation 
of nitrate of urea in dilute solutions. The combination of 
nitric acid with urea when viewed under the microscope is 
seen to consist of superposed crystallised plates, or rectangular 
and rhombohedral prisms. If nitric acid be added to urine 
concentrated in a watch-glass, thick lozenge-shaped crystals 
often occur, two of them aggregating in the shape of a cross, 
or several joining to produce an arborescent structure. Nitrate 
of urea is sparingly soluble in cold water ; it dissolves in hot 
alcohol, but not in rectified ether. When oxalic acid is added 
to concentrated urine, tabular groups of crystals similar to 
the former are obtained, occasionally assuming the form of 
very flat prisms. When the crystallisation has been rapid, 
we may detect groups of elongated crystals resulting from 
small imbricated prisms. Along with these, several other 
crystalline forms occur, as spherical masses of laminated 
crystals. The crystals of oxalate of urea have a peculiar 
yellow colour ; they transmit polarised light. 
Liebig has discovered lately a very accurate method for 
ascertaining the amount of urea contained in fluids. For this 
purpose he prepares a normal solution of acid nitrate of mer- 
cury in water, and determines, by pouring it from a graduated 
tube, how much of this solution will precipitate a given 
quantity of urea. By adding this fluid to an unknown solu- 
tion of urea until the whole of it has precipitated, the 
quantity of the urea precipitated and therefore contained in 
the solution will be at once detected. The amount of urea 
obtained from urine by this method is slightly larger than 
that yielded by the other process. — Traite de Chimie Anato- 
mique et Physiologique, fyc. Par C. Robins et F. Verdeil; Paris , 
1853. 
