INDOL PRODUCTION. 121 



cent, solution of sodium nitrite, and afterward 10 drops 

 of concentrated sulphuric acid. Observe the tubes for 

 five to ten minutes. No alteration in their color appears, 

 or at least there will be no production of a rose color. 

 They contain no indol. 



Treat in the same way, with the acid alone, two of 

 the tubes which have been inoculated. If no rose color 

 appears after five or ten minutes, add 1 c.c. of the sodium 

 nitrite solution. If now no rose color is produced, the 

 indol reaction may be considered as negative. No indol 

 is present. 



If indol is present, and the rose color appears after 

 the addition of the acid alone, it is plain that not only 

 indol has been formed, but likewise a reducing body. 

 This is found, by proper means, to be nitrous acid. 

 The sulphuric acid liberates it from its salts and permits 

 of its reducing action being seen. 



If the rose color appears only after the addition of 

 both the acid and the nitrite solution, then indol has 

 been formed during the growth of the organisms, but no 

 nitrites. 



Control the results obtained by treating the two re- 

 mainiug cultures in the same way. 



