298 MODIFICATIONS OF PETTENKOFER'S REACTION. [BOOK II. 



temperature of the mixture does not rise above 70C., lest the cane 

 sugar be carbonised and the dark colour thus caused should conceal 

 the reaction. If bile acids are present, the fluid first of all becomes 

 opalescent, then the opalescence clears, and the liquid becomes suc- 

 cessively of a pale cherry-red, then of a dark carmine-red, and lastly 

 of a beautiful purple-violet tint. The reaction does not occur in- 

 stantly and the tube should be set aside for some minutes and then 

 examined before a negative conclusion is drawn from the experiment. 

 In order to avoid the troublesome carbonising of cane sugar which 

 is very apt to occur, Drechsel suggested a very useful modification 

 of Pettenkofer's test, which consisted in substituting phosphoric acid 

 for sulphuric acid 1 . To the liquid to be tested sugar is added and, 

 instead of sulphuric acid, phosphoric acid made by diluting the 

 syrupy acid with one-sixth of its volume of water ; the test tube 

 containing the mixture is then placed in boiling water, when the 

 characteristic reaction developes. 



A very important modification of Pettenkofer's reaction we owe 

 to Mylius 2 , who found that the reaction depended on the production 

 of furfurol which is generated by the action of sulphuric acid on the 

 sugar employed, and suggested that a solution of furfurol in water, 

 of 1 per mille, should be substituted for sugar. According to 

 v. Udranszky 3 to 1 c.c. of the alcoholic solution suspected to contain 

 a bile acid is added a single drop of 1 p.m. furfurol solution, and 

 then 1 c.c. of concentrated sulphuric acid, the mixture being if 

 necessary cooled. By this method a perceptible reaction is obtained 

 with quantities of cholalic acid not exceeding ^th ^th of a 

 milligramme. 



It has long been known that many substances, some of them 

 constituents of the animal body, give reactions with cane sugar or with 

 cane sugar and sulphuric acid or with sulphuric acid alone, which are 

 exceedingly similar to the reaction which is observed with the bile 

 acids. Albumin, acids of the fatty series, amyl-alcohol, morphia, 

 and phenol compounds which occur in the urine, all give reactions 

 with sugar and sulphuric acid, which resemble Pettenkofer's reaction 

 so closely that were it not for the method of spectroscopic observation 

 we should be unable to pronounce an opinion concerning the identity 

 or non-identity of the colouring matters which are produced in each 

 case. 



1 E. Drechsel, 'Eine Modification der Pettenkofer'schen Reaction auf Gallen- 

 sauren.' Journ. f. prakt. Chemie, Vol. xxiv. (1881), p. 44. 'Anwendung von Phosphor- 

 saure statt Schwefelsaure bei der Pettenkofer'schen Reaction auf Gallensauren.' Journ. 

 f. prakt. Chem., Vol. xxvii. (1883), p. 424. 



2 F. Mylius, ' Zur Kenntniss der Pettenkofer'schen Gallensaure-reaction.' Zeitschrift 

 fiirphysiol. Chemie, Vol. xi. (1887), p. 492. 



3 v. Udranszky, 'Ueber Furfurolreaktionen.' Zeitschrift f.physiol. Chemie, Vol. xn. 

 (1888), p. 355-395, and Vol. xm. (1889), p. 248. 



