132 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



Fehling's test is not absolutely trustworthy. Often a normal urine will 

 decolorise Fehling's solution, though seldom a red precipitate is formed. 

 This is due to excess of urates and creatinine. Another substance called 

 glycuronic acid (Cj-HjoO^) is, however, very likely to be confused with 

 sugar by Fehling's test ; the cause of its appearance is sometimes the 

 administration of drugs (chloral, camphor, &c.); but sometimes it appears 

 independently of drug treatment. The cause of this is not known, but the 

 condition has not the serious meaning one attaches to diabetes ; hence, for 

 life assurance purposes, it is most necessary to confirm the presence of sugar 

 by other tests. 



Then, too, in the rare condition called alcaptonuria, confusion may 

 similarly arise. Alcapton is a substance which probably originates from 

 tyrosine by an unusual form of metabolism. It gives the urine a brown tint, 

 which darkens on exposure to the air. It is an aromatic substance, and the 

 researches of Baumann and Wolkow ^ have identified it with homogenti- 

 sinic acid (G6H3.(OH)2CH2.COOH). 



The best confirmatory tests for sugar are the phenyl-hydrazine 

 test (see Lesson XIII.), and the ferinentation test, which is performed 

 as follows : — 



Half fill a test-tube with the urine and add a little German yeast. 

 Fill up the tube with mercury ; invert it in a basin of mercury, and 

 leave it in a warm place for twenty-four hours. The sugar will 

 undergo fermentation : carbonic acid gas accumulates in the tube, 

 and the liquid no longer gives the tests for sugar, or only faintly, but 

 gives those for alcohol instead. A control experiment should be 

 made with yeast and water in another test-tube, as a small yield of 

 carbonic acid is sometimes obtained from impurities in the yeast. 



Sir W. Eoberts introduced a method for estimating sugar in urine, by the 

 diminution in specific gravity which it undergoes on fermentation. Every 

 degree lost in the specific gravity corresponds to one grain of sugar per fluid 

 ounce. Four ounces of urine are placed in a bottle, and a piece of German 

 yeast about the size of a small walnut is added. The bottle is closed with a 

 cork, through which a small hole is bored to allow the carbonic acid to 

 escape. This is put in a warm chamber (40° C), and beside it is placed 

 another similar bottle containing 4 ounces of the urine without any yeast. 

 After 18 to 24 hours, fermentation is complete, and the specific gravity of 

 both is taken : suppose that the specific gravity of the unfermented urine is 

 1040, and that of the urine which has undergone fermentation is 1030 : the 

 number of degrees lost is ten ; i.e. the urine contained 10 grains of sugar per 

 ounce. The percentage of sugar may be ascertained by multiplying the 

 degrees of specific gravity lost by 0*22 ; thus the percentage in the example 

 just given will be 0*22 x 10 = 2-2. The method, however, is too rough for 

 trustworthy observations to be made, and has dropped out of use. 



BILE IN THE TJIIINE 



This occurs in jaundice. The urine is dark-brown, greenish, or 

 in extreme cases almost black in colour. The most readily applied 

 test is Gmelin's test for the bile pigments. Pettenkofer's test for the 

 ' Zeit. physiol, Chem. xv. 228. 



