STRUCTURE AND FUNCTION OF THE KIDNEYS. 369 



marked spectroscopic absorption band at the junction of green and 

 blue, best seen in acid solutions; and by giving a green fluorescence 

 when excess of ammonia with a little chloride of zinc is added to it. 

 The very vexed question of the relation of the pigments of urine to bile 

 pigments turns largely upon the spectroscopic appearances of urobilin; 

 for orange-colored solutions having the same absorption band as urobilin 

 may be prepared from bile pigments in two different ways i., by reduc- 

 tion with sodium amalgam Hydrobilirubiu (Maly); ii., by oxidation 

 with nitric acid Choletelin (Jaffe), and both these bile derivatives give 

 a fluorescence with ammonia and a drop of chloride of zinc. It is not 

 satisfactorily settled which of these, if either, is the same as urobilin of 

 urine. It is worth noting that choletelin may be oxidized a stage fur- 

 ther; it then loses its absorption band, remaining however of a yellow 

 color. It is very possible that the urochrome of normal urine may be 

 this oxidized choletelin, and that the presence of the absorption band of 

 urobilin in urines may mean that some of the pigment is in the stage of 

 choletelin; i.e., that its oxidation is not quite completed. 



Those who believe urobilin to be identical with hydrobilirubin sup- 

 pose that the bilirubin is reduced by the putrefactive processes in the 

 intestines, and is conveyed in its reduced form by the blood stream to the 

 kidneys. 



3. Uro-erythrin is the pigment which is found in the pink deposits 

 of urates which are sometimes seen in urines; it communicates a rich 

 red-orange color to urine when in solution, and its solutions have two 

 broad faint absorption bands in the green. 



4. Uromelanin. When urine is boiled with strong acids it darkens 

 to a reddish-brown color. This change, once ascribed to the formation 

 of a new pigment uromelanin, is now believed to be due to the presence 

 in urine of pyrocatechin and allied bodies which are capable of taking 

 up oxygen when boiled with acids, yielding C0 2 and brown or black re- 

 sidual products. 



5. Indigo is found rarely in urines, to which it may communicate a 

 blue or green color. Urine frequently contains a compound which is 

 either a glucoside, Indican; or more probably a salt of indoxyl-sulphuric 

 acid. It yields indigo blue when treated with strong hydrochloric acid 

 and left to stand for some hours exposed to the air; the indigo may be 

 separated by treatment with boiling chloroform, which takes it up, 

 forming a blue solution. 



There is a similar compound of skatol and sulphuric acid which is 

 sometimes recognized in the urine, by the production of a red color when 

 nitric acid is added to it. 



Many medicinal substances color the urine, for instance: Rhubarb, 

 Santonin, Senna, Fuchsine, Carbolic Acid. 



Bromides and Iodides yield Bromine or Iodine, when nitric acid is 



