INFECTIONS OF THE DIGESTIVE TRACT 143 



from the distillate containing the skatol. Various dyes 

 have been tried with a view to obtaining a standard 

 solution which will retain its color unchanged. Ex- 

 perience has shown that the best standard color solution 

 is one obtained from a solution of skatol. Although 

 such a solution fades after a few days, especially when 

 exposed to the light, and may assume a greenish tint, 

 in the dark it may last several weeks without undergoing 

 appreciable change. Moreover, as the skatol standard 

 solution is readily prepared, there is little disadvantage 

 from being compelled to renew the color solution from 

 time to time. This solution may be conveniently pre- 

 pared by dissolving five milligrams of skatol in water and 

 acting upon it with an excess of dimethylamidobenzalde- 

 hyde. It commonly requires from one hundred to one 

 hundred and fifty cubic centimeters of chloroform to 

 extract completely the blue coloring matter which has 

 already been described. The quantity of coloring mat- 

 ter present is sufficient to impart a deep blue color to 

 this volume of chloroform. Ten cubic centimeters of 

 this solution are placed in the receptacle of the Duboscq 

 colorimeter and used as a standard for comparison with 

 the chloroform color solution obtained from the distil- 

 late to be tested. The matching of the colors can 

 usually be made very closely. In cases where the 

 quantity of skatol is so small that the trace of indol 

 present influences the color of the chloroform solution, 

 changing it to violet or even purple, it is more difficult 

 to obtain a satisfactory matching of colors. In this 

 case is may be necessary to add a small quantity of indol 



