INFECTIONS OF THE DIGESTIVE TRACT 141 



amount of indol, however, which passes over after treat- 

 ment with the /3-naphthaqumone-sodium-monosulpho- 

 nate is so small that it is practically negligible, although 

 its presence is detectable through the red color which it 

 gives when acted upon by dimethylamidobenzaldehyde. 

 The distillate containing skatol is boiled with a solution 

 of Ehrlich's aldehyde in sulphuric acid. 1 A slight amount 

 of dilute hydrochloric acid is now added and has the 

 effect of intensifying the blue color produced by boiling 

 the skatol with the Ehrlich aldehyde solution. A little 

 experience is required to find the amount of hydro- 

 chloric acid which gives the maximal intensification of 

 the reaction. An excess of hydrochloric acid causes the 

 blue color to fade. It is important to use an excess of the 

 Ehrlich aldehyde solution in order to develop fully the 

 color reaction with skatol. The color obtained through 

 the action of Ehrlich's aldehyde upon skatol is purple- 

 blue rather than blue so long as the solution is hot. 

 On cooling it under the tap, the blue color asserts itself 

 more strongly, and the solution may become some- 

 what opalescent from the separation of uncombined 

 dimethylamidobenzaldehyde. Chloroform is now added 

 to the solution containing the blue product. On agita- 

 tion with the solution this carries out the blue color, 

 and the chloroform assumes a pure blue tint. By means 

 of a good colorimeter the quantity of skatol present 

 in the original solution may be approximated by the 

 intensity of the color reaction. 

 On evaporating the chloroform containing the blue 

 1 Five per cent, aldehyde in ten per cent, sulphuric acid. 



