CONVERSION OF BENZOIC INTO HIPPURIC ACID. 7 
organic and inorganic substances. It may be produced 
artificially from inorganic substances, and is itself readily 
converted into carbonate of ammonia by taking up the ele- 
ments of one atom of water. It combines with some acids, 
such as the nitric and oxalic acids, and performs in these 
compounds the part of a base. 
Lithic or uric acid forms only a very small part of the 
urine ; being about one-seventieth of the solid substances, 
and only one-ten thousandth part of the whole urine. It 
is very insoluble in water, and this is the reason that it. for 
the most part deposits from urine on cooling. By oxida- 
tion it yields, among other products, urea. It forms the 
greater part of urinary concretions. 
The urine of herbivorous animals does not contain any 
uric acid, but another acid, called hippuric acid, from '1**0$ 
a horse, in whose urine it was first discovered. It only 
contains seven per cent, of nitrogen, while uric acid con- 
tains thirty-three per cent. By oxidation, it yields, among 
other products, benzoic acid. 
The urine of the horse sometimes contains benzoic acid 
instead of hippuric. Benzoic acid is generally obtained 
from gum-benzoin by subjecting it to a gentle heat ; when 
it sublimes in crystalline needles. It will be observed that 
it contains no nitrogen. It belongs to a series of com- 
pounds, which in a historical point of view are of interest, 
as containing the first well established radical in organic 
chemistry ; that is to say, an imaginary body, which, though 
never isolated by itself, may be carried through a series of 
combinations with most of the simple elementary bodies. 
The question now naturally arises, how is this transfor- 
mation of benzoic acid into hippuric effected? We have 
seen that the normal quantity of uric acid is too small to 
suffice, if its formation depended on this acid, and, besides, 
their formation seems independent of each other. That 
the transformation should be effected in the stomach and 
intestines during the digestion is less probable : it seems 
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