INDOL PRODUCTION. 229 



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 

 i. e., no indol has been formed as a product of the 

 growth of the bacteria. 



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 coincidently a reducing- 

 body. This is found, by proper means, to be 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 

 remaining cultures in the same way. 



The test is sometimes made by allowing concentrated 

 sulphuric 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 

 culture-medium. This method is open to the objection 

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

 the faint rose tint 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 recom- 

 mends the use of dilute sulphuric acid. He states that 

 when indol is present the characteristic rose color ap- 

 pears a little more slowly \vfth the dilute acid, but 

 it is more permanent, and there is never any like- 



