CHEMICAL BASIS OF THE ANIMAL BODY. 215 



addition of acid after which the characteristic colour makes its 

 appearance. It has also recently been shown that the reaction depends 

 upon the formation of f urfurol 1 by the action of the sulphuric acid 

 upon the sugar, the colour arising from the interaction of furfurol with 

 cholalic acid 2 . 



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 3 . A further element of uncertainty 

 is introduced by the fact that if the suspected solution be extremely 

 dilute no characteristic colour is obtained although bile-acids may be 

 present. All the above militate against the detection of bile-acids in 

 fluids such as urine, in which their determination is a matter of not 

 infrequent importance. The application of Pettenkofer's reaction in 

 its original form has hence been modified in details by many observers 

 with a view to rendering it more decisive and delicate. The decisive- 

 ness of the reaction is ensured by careful spectroscopic examination of 

 the absorption spectrum of the coloured solution, since the colours 

 produced by the majority of those substances which yield a reaction 

 resembling that produced by cholalic acid, show no absorption bands in 

 their spectra. Some few however do exhibit absorption bands which 

 fortunately occupy a different position in the spectrum from those 

 shown by cholalic acid (Udranszky). If the suspected solution is 

 extremely dilute, it may frequently be made to yield Pettenkofer's 

 reaction directly by a previous concentration on the water- bath. A 

 further modification which is applicable to dilute solutions is the 

 following. A little cane-sugar is dissolved in the solution and a strip 

 of filter paper dipped into it and then air-dried. When dry one drop 

 of concentrated sulphuric acid is applied to the paper with a glass rod. 

 If bile-salts are present (even to the extent of '03 p.c.), a distinct 

 violet stain may be observed on the paper after standing for a quarter 

 of a minute : the stain is most easily seen by transmitted light 4 . 

 Instead of sugar an aqueous (O'l p.c.) solution of furfurol may be used 

 to great advantage as follows. One drop of this solution is added to 

 1 c.c. of the suspected solution, either aqueous or alcoholic, in a test 



1 Also known as furfuraldehyde C 4 H 3 . COH, the aldehyde of pyromucic acid 

 C 4 H 3 O.COOH. 



2 Mylius, Zt. f. physiol. Chem. Bd. xi.^(1887), S. 492. 



3 For a complete list of these see Udranszky, loc. cit., S. 358. 



4 Strassburg, Pfliiger's Arch. Bd. iv. (1871), S. 461. 



