630 THE CHEMISTR Y OF THE URINE. 



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 used as a drug, the urine has 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 

 carboluria 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 be 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 INOKGANIC 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 grms. of the acid, 



