URINE AND ITS CONSTITUENTS. 739 



placed in the centre of the remaining stain, when concentric color 

 rings appear. The second portion of chloroform solution is evap- 

 orated and the residue used for making the reactions, as described 

 above. 



Melanin (melanogen), the black pigment of the skin of the negro, has been 

 found in the urine of persons suffering from melanotic cancer and certain wast- 

 ing diseases. Urine containing melanin darkens on standing, turns black on 

 the addition of either nitric or chromic acid, and forms with bromine-water a 

 yellow precipitate rapidly turning dark. 



Alkaptonic acids. Two of these acids occur in the urine of certain 

 otherwise healthy persons, and seem to be without clinical signifi- 

 cance. The acids are : Jiomogentisic acid, dioxphenyl-acetic acid, 

 C 6 H 3 (OH) 2 .CH 2 .CO 2 H, and uroleucic acid, dioxyphenyl-lactic acid, 

 C G H 3 (OH) 2 .C 2 H 3 .OH.C0 2 H. 



Both acids reduce Fehling's solution, as also ammoniacal silver nitrate solu- 

 tion, but not bismuth solution ; they are optically inactive, do not form an 

 osazone, and do not ferment with yeast. 



To test for alkaptonic acids, the urine should be acidified with hydrochloric 

 acid and then extracted with ether. The ethereal solution is evaporated, the 

 residue dissolved in water, and heated with Millon's reagent. In the presence 

 of alkaptonic acids a purple-red color is observed. 



Diazo-reaction. Some abnormal constituent (which has not yet 

 been isolated) is found in the urine of certain diseases. The presence 

 of this unknown substance is indicated by a very characteristic reac- 

 tion with diazo-benzene-sulphonic acid, which compound is produced 

 by the action of nitrous acid on sulphanilic acid. Two solutions are 

 required : a. 5 grammes of sulphanilic acid dissolved in a mixture 

 of 50 c.c. of hydrochloric acid and 1000 c.c. of water; 6. a 0.5 

 per cent, solution of sodium nitrite. To perform the reaction 50 

 parts of a and 1 part of b are mixed, and equal volumes of the reagent 

 and of urine are mixed in a test-tube and saturated with ammonia. 

 In those cases in which the reaction is positive the solution assumes 

 a carmine-red color, which, on shaking, must also be visible in the 

 foam. If the test-tube is allowed to stand twenty-four hours, a 

 greenish precipitate is formed. Normal urine, thus treated, shows a 

 deep-yellow or orange to orange-red color; the precipitated phos- 

 phates as well as the foam are colorless. On Plate VIII., 8, the 

 color of the diazo-reaction is represented. Normal urine may show 

 the orange-red on the left, but the carmine-red on the right is char- 

 acteristic of the diazo-reaction. 



If, instead of mixing the urine and reagent with ammonia water, 



