ANIMAL FLUIDS AND TISSUES, 361 



Bilirerdin, C 32 H 36 N 4 8 , is a green powder existing in green 

 biles; it responds to Gmelin's test. 



Biliary acids. Glycocholic acid, C^H^NOg, and taurocholic acid, 

 C 26 II 45 N0 7 S, exist as sodium salts in the bile of man and most 

 animals. Both salts may be obtained as colorless crystals, which 

 dissolve in water, forming solutions of an acid reaction, and an 

 intensely bitter taste. Both acids are easily decomposed by 

 heating with alkalies or with diluted acids, also by the action of 

 putrefying material or by the chemical changes taking place in 

 the intestines. In all these cases is formed cholic acid, C^H^O,., 

 and a second product, which in the case of glycocholic acid is 

 glycocol, C 2 II 5 N0 2 , and in the case of taurocholic acid taurine, 

 C 2 H 7 N0 3 S. 



Pettenkofcr 's test. The biliary acids and their salts show a 

 characteristic reaction known as Pettenkofer's test. This reac- 

 tion is shown by adding some cane-sugar to the liquid substance 

 under examination, and adding concentrated sulphuric acid in 

 such a manner that the temperature does not rise above 70. 

 In the presence of biliary acids a beautiful cherry-red color is 

 developed, which gradually changes to dark reddish-purple. 



The bile acids are, however, not the only substances which 

 show this reaction, and it, therefore, becomes in many cases 

 necessary to separate the bile acids from other organic matter. 

 This separation is accomplished by evaporating the substance 

 under examination (urine, for instance), after having been mixed 

 with a small quantity of coarse animal charcoal, to dryness at 

 100. The residue is extracted with absolute alcohol, the fil- 

 tered alcoholic solution is again partially evaporated, and mixed 

 with 10 volumes of absolute ether. The biliary acids are solu- 

 ble in alcohol, but not in ether, or in ether containing one-tenth 

 of alcohol. After standing an hour or two, the biliary acids 

 will form a deposit, which is collected on a small filter, dissolved 

 in a little water, and mixed with a few drops of a strong solu- 

 tion of sugar. Upon the addition of sulphuric acid, with the 

 precaution above mentioned, the characteristic colors will indi- 

 cate the presence of the bile acids. 



Cholesterin, C 26 H 43 ,HO. This substance is classed by physiolo- 

 gists among the fats, because it is greasy and soluble in ether, 



