630 THE CHEMISTRY OF THE UR1XE. 
filter paper ; the paper is stained yellow, and a drop of fuming nitric acid 
allowed to fall upon it produces the characteristic play of colours. When 
only traces of the pigment are present, Gmelin's test is best applied to the 
precipitate produced in the urine by the addition of lime-water with the 
subsequent passage of a stream of carbon dioxide ; the precipitate is filtered 
off, dried, and touched with nitric acid. 
Carboluria. — In poisoning by carbolic acid, and often to a less degree after 
the substance has been freely ttsed as a drug, the urine lias a greenish-brown 
or dark brown colour, which increases on exposure to the air. This colour is 
due to oxidation products of some of the aromatic substances present in normal 
urine, which have been dealt with on p. 607 et seq. They are excreted in much 
greater quantity after the administration of phenol. Pyrocatechin and 
hydrochinon are especially responsible for the colour effect. 
Alcaptonuria (cf. p. 607). — A phenomenon very similar to that present in 
carbohiria is seen in certain other conditions, where an alkaline urine, as it 
stands in the air, takes first a brown colour at the surface, which gradually 
spreads through the fluid, and may finally result in the whole urine becoming 
nearly black. Such urine always reduces copper solutions. The phenomenon 
was first observed by Boedecker in 1859, and it was later ascribed by him to a 
substance which he called alcapton. 
But alcapton, as already stated, is not a definite compound, and the colour 
phenomena are probably due to the action of oxygen upon some of the 
aromatic bodies present; probably, at times, upon pyrocatechin and uroleucic 
acid, but more often perhaps upon the homogentisic acid of Wolkow and 
Baumann (vide p. 606). 
Although thought to lie especially frequent in various forms of tuberculosis, 
alcaptonuria must not be looked upon as specifically associated with any parti- 
cular diseased conditions ; it indicates rather some peculiar independent changes 
of metabolism, and is not infrequently met with in conditions of apparent 
health. In one case where there was a tendency for homogentisic acid to 
appear in the urine, it was found that the quantity of this substance and the 
associated colour phenomenon might be enormously increased by administering 
tyrosine, and it is suggested that, when homogentisic acid or other aromatic 
substances appear in excess, it is due to the action of special micro-organisms 
on the tyrosine of the bowel. 
Drug pigments. — The urine may contain purely accidental pigments due to 
the use of drugs, notably of rhubarb, senna, logwood, and santonin. 
The Inorganic Constituents. 
To a large extent the inorganic constituents of the urine arise 
directly from the food, which always contains a large excess of salts. 
It does not follow, however, that the bases and acids are to be found in 
the urine in the same combinations as when ingested, and indeed an 
interchange of base and acid may occur in special circumstances between 
the salts of the food and those of the tissue fluids. Thus, excess of 
potassium in the food may lead to increased elimination of sodium 
(Bunge). The sulphates, moreover, form an exception to the general 
rule of direct origin from the ingesta, very small quantities of these 
salts being present in a normal dietary. The urinary sulphates are 
derived almost entirely from proteid metabolism ; a small proportion of 
the phosphates arising in the same way. 
Sulphuric acid and other sulphur compounds. — About 80 per 
cent, of the total sulphur in normal human urine is present in the 
fully oxidised form of sulphuric acid: from 2 to 2"5 gnus, of the acid, 
