THE BILE ACIDS. 377 



The two hyocholalic acids show analogous differences to the two hyoglycocholic 

 acids. The formula of a-hyoglycocholic acid is C 27 H 43 N0 5 , that of fi-hyo- 

 glycocholic acid is C 26 H 43 N0 5 . 



Tauroclienocholic acid, 1 the principal bile acid of goose bile, has the 

 formula C 29 H 49 NSQ 6 , has not been crystallised, and is soluble in water and 

 alcohol. From this acid Heintz and Wislicenus 2 prepared chenocholic acid 

 (C 27 H 44 4 ) ; this is itself crystallisable with difficulty, but yields a barium salt, 

 which is insoluble in water and can easily be obtained in a crystalline form. 



Pettenkofers test for bile acids? When bile is gently warmed with 

 concentrated sulphuric acid and cane-sugar, a beautiful purple or 

 purplish-red colour develops, becoming deeper on standing. The colour 

 is due to an interaction between the bile salts, or cholalic acid, and a 

 substance called furfurol or furfuraldehyde developed by the action of 

 the strong, sulphuric acid on the cane-sugar; 4 hence the test may 

 be more satisfactorily carried out where only traces of bile salts are 

 suspected, by using a solution of furfurol (1 per 1000) instead of cane- 

 sugar. 



To carry out the test in the ordinary manner, add to a drop or two of the 

 bile, or fluid suspected of containing bile acids, a drop of strong sulphuric 

 acid, taking care that any great rise in temperature does not occur ; spread the 

 mixture out in a thin film in a porcelain capsule, and either add a drop of a 

 10 per cent, solution or a small crystal of cane-sugar ; if the violet colour does 

 not appear at once, warm very gently. To carry out the test with furfurol, one 

 drop of a solution of furfurol (1 per 1000) is added to 1 c.c. of an alcoholic 

 solution of bile salts, and 1 c.c. of concentrated sulphuric acid is added 

 cautiously to this, so as not to overheat. In this manner -J^-^-Q of a milligramme 

 of cholalic acid may be detected. 5 



The test with sugar may be easily spoiled by overheating or when 

 too much sugar is used, which favours carbonisation. The presence of 

 sulphurous acids or nitrous fumes in the sulphuric acid is also unfavour- 

 able to the reaction. Strong phosphoric acid may be used instead of 

 sulphuric acid. 



Many other substances give a similar reaction. Pettenkofer himself 

 was aware that proteids gave a similar colour, though much less easily. 

 By subsequent observers 6 a large number of substances giving colour re- 

 actions with furfurol have been described ; amongst these many phenols 

 and aromatic bases are included, some of which are also found in 

 the urine, v. Udranszky 7 gives a list of over forty substances which 

 give colour reactions with furfurol, but none except a-naphthol show 

 the reaction with the same delicacy as the bile salts. That the 

 coloured substance so produced is not in all cases the same, is shown 

 by the fact that some possess no absorption spectrum, and that the 

 spectra of the others differ from one another. In this way the 

 spectrum of the colour given by the bile salts may be distinguished 



1 Marsson, Arch. d. Pharm., Bd. Ivii. S. 138. 



2 Ann. d. 'Ptiys. u. Chem., Leipzig, 1859, Bd. cviii. S. 547. 



3 Ann. d. Chem., Leipzig, 1844, Bd. Hi. S. 90. 



4 Mylius, Ztschr. f. physiol. Chem., Strassburg, 1887, Bd. xi. S. 492. 



5 v. Udrdnszky, ibid., 1888, Bd. xii. S. 355. 



6 Baeyer, Ber. d. deutsch. chem. Gesellsch., Berlin, 1872, Bd. v. S. 26; Stenhouse, 

 Ann. d. Chem., Leipzig, 1870, Bd. clvi. S. 197 ; Schiff, ibid., Bd. cci. S. 355. 



7 Loc. cit. Drechsel, Journ. f. praH. Chem., Leipzig, Bd. xxvii. S. 424. 



