acid. The soda solution may conveniently contain the equivalent of one
milligramme of recrystallized oxalic acid (H_{2}C_{2}O_{4}.2H_{2}O) in
each cubic centimeter.


UREA.

Carbamide, as it is called by systematic chemists, or _urea_, is next
to water the largest constituent of urine, and forms about one-third
of its total solids. Derived from ammonic carbonate by abstracting two
molecules of the elements of water, it is readily converted by
putrefaction into that salt, and the urine under these circumstances
becomes strongly alkaline in reaction. Earthy phosphates then fall
naturally out of solution, so that the putrid fluid is always well
furnished with sediment. Nitrogen that has served its purpose as
muscle or other proteid leaves the animal economy chiefly in the form
of urea, and its proportion in the urine, therefore, is a fair index
of the activity of wasting influences.

For its determination Knop's sodic hypobromite method, on account of
its convenience, is now generally preferred. The volumetric process of
Liebig, which depends on the formation of an insoluble compound of
urea with mercuric nitrate, possesses no advantages and is troublesome
to work. The principle of the hypobromite process is simple. In a
strongly alkaline solution urea is broken up by sodic hypobromite, its
nitrogen being evolved in the gaseous state, and its carbon and
hydrogen oxidized to carbonic anhydride and water respectively. The
volume of free nitrogen obtained bears a direct ratio to the amount of
urea decomposed.