CHEMICAL BASIS OF THE ANIMAL BODY. 1289 



free acid and its salts are dextrorotatory: for the former, in 

 alcoholic solutions, (a) B = +29-0, for the latter (a) D = +25-7°. 



4. Taurocholic acid. C 26 H 45 NS0 7 . 



This acid is found to some extent in ox-bile, and is more 

 plentifully present in that of man. The bile of the dog con- 

 tains taurocholic acid alone, unmixed with glycocholic. 



Preparation. As already described in § 207, using dog-bile 

 as material. 



This acid is extremely soluble in water and in alcohol, but 

 not in ether, so also are its salts with the exception of the one 

 formed on the addition of basic lead acetate in presence of 

 ammonia, which is insoluble in water and in alcohol. The 

 acid and its salts are dextrorotatory; for the sodium salt in 

 alcoholic solution (a) D = +2-1.5°. If dissolved in water the 

 rotatory power is less, and in this respect it resembles glyco- 

 cholic acid. 



Pettenkofer's reaction for bile acids. 



The following is the more usual method of obtaining the reaction. 

 Bile, which may be very considerably diluted, or a dilute solution of 

 bile-salts or acids is mixed in a porcelain dish with a few drops of a 

 10 p.c. solution of cane-sugar. Concentrated sulphuric acid is now 

 added to the mixture with constant stirring to an extent not exceed- 

 ing 1 of its volume, the addition of the acid being so regulated that 

 the temperature of the mixture is not allowed to rise above 70° C. 

 Hereupon a brilliant cherry-red colour makes its appearance and 

 rapidly assumes a magnificent purple tint. On standing for some 

 time the colour becomes darker and of a more distinctly blue tint. 

 The reaction may also be obtained by the addition of first the acid 

 and then the sugar solution. The success of the test depends on the 

 careful avoidance of any excessive rise of temperature during the 

 addition of the sulphuric acid and more especially of any excess of 

 sugar which by being charred by the acid gives a brown coloration 

 and masks the typical purple. The purple solution if diluted with 

 alcohol (not with water which destroys the colour), shews with a 

 spectroscope a characteristic absorption spectrum consisting of two 

 absorption bands, one between D and E abutting on E and a second 

 adjoining the F line. In the earlier stages of the reaction a third 

 narrow band near D makes its appearance but disappears later on. 



It is important to remember that an extended series of substances 

 other than cholalic acid and the bile-acids (pigments and other sub- 

 stances which are charred by sulphuric acid) either interfere with 

 the brilliancy of the reaction or else themselves yield a purple colour 

 which closely resembles that due to the bile-acids. Among the latter 

 those of chief importance are proteids, amyl-alcohol, oleic acid, the 

 higher fatty acids and cholesterin. 



