INDOL PRODUCTION. 193 



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 pro- 

 duced, 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 salts of nitrous 

 acid. The sulphuric acid liberates this acid from its 

 salts and permits of its reducing action being brought 

 into play. 



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- 

 maining cultures in the same way. 



The test is sometimes made by allowing concentrated 

 acid to flow down the sides and collect at the bottom of 

 the tube ; the reaction is then seen as a rose-colored 

 zone overlying the line of contact of the acid and cul- 

 ture medium. This method is open to the objection 

 that if indol is present in only a very limited amount, 

 the rose color produced by it is apt to be masked by a 

 brown color that results from the charring action of the 

 concentrated acid on the other organic matters in the 

 culture medium, so that its presence may in this way 

 escape detection. In view of this, Petri recommends 

 the use of dilute sulphuric acid. He states that when 

 indol is present the characteristic rose color appears a 

 little more slowly with the dilute acid, but is more per- 



