586 THE CHEMISTR Y OF THE URINE. 
To demonstrate the presence of the small quantities of ammonia in 
human urine is not easy, owing to the ready production of the base 
by hydrolysis of urea, which must, obviously, lead to error. We must 
employ a method analogous to that used for its estimation. 
Estimation of ammonia (Schilling's method). — Twenty-five c.c. of urine are 
placed in a basin with vertical sides, and about 20 c.c. of milk of lime are 
added. A glass triangle is placed over the basin, and, upon it, another small 
vessel containing 20 c.c. of one-fifth normal sulphuric acid. These stand 
upon a glass slab, and are covered with a bell-shaped glass cover, fitting air- 
tight on the slab. The ammonia is liberated by the lime, without any 
decomposition of other nitrogenous constituents, and, in the course of three 
days, the whole is absorbed by the sulphuric acid, the degree of neutralisation 
being afterwards estimated by titration. If dilute hydrochloric acid be 
used instead of sulphuric, it may, after the experiment, be evaporated to 
dryness on the water bath, and the residue taken up with a small quantity of 
water. Platinic chloride added to this solution will demonstrate the presence 
of ammonia, by giving a yellow crystalline precipitate of ammonio-platinic 
chloride. 
Pathologically, the urinary ammonia may be increased, not only after the 
manner we have discussed, by abnormal acid production (as in diabetes and 
fevers), but also by conditions which reduce the proper activity of the 
hepatic cells, whereby the dehydrolysis of ammonium carbonate into urea is 
less complete than normally. 
(d) Uric acid. — Uric acid was first separated from human urine by 
Scheele, in 1776. It is present in the urine of most mammals, though 
from that of the dog and cat it has been shown to be frequently absent. 
In man the daily output in the urine varies considerably (from 0"2 grm. 
to 14 grm.), the average amount being - 8 grm. 
Chemical constitution. — Rightly to appreciate the physiology no less 
than the chemistry of uric acid, its close relationship to urea should be 
clearly understood. It yields the latter easily by a combined process of 
oxidation and hydrolysis. It belongs, in fact, to the class of substances 
known as diureides, in which the residue of two urea molecules are 
united to a carbon - containing nucleus. In the case of uric acid this 
nucleus contains a chain of three carbon atoms. 
The constitutional formula first suggested by Medicus — 
NH— C— NH N 
I II /CO 
CO C— NH X 
I I 
NH— CO 
has now received ample confirmation from the synthetic production of 
the acid by Horbaczewski, 1 and by Behrend and lioosen. 2 
The ureides are, in general, produced by the condensation of hydroxy- 
acids with urea. The hypothetical acid, which would yield uric acid 
by such simple condensation, would be a trihydroxyacrylic acid ; but 
this has never been prepared. 
Lactic acid also contains a three-carbon chain in its molecule, and, 
because of the important physiological relationships of this acid, it is of 
special interest to find that uric acid can be synthesised by linking urea 
1 Monatsh./. Chcm.. Wien, 18S7, B<1. viii. S. 201, 584. 
- Ber. d. deutsch. chcm. Gcsellsch., 1S8S, Bd. xxi. S. 999. 
