200 BA CTERIOLOG Y. 



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 \v ; .th the dilute acid, but 

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

 lihood of its presence being masked by other color- 

 reactions. 



Muir and Ritchie recommend the use of ordinary 

 fuming or yellow nitric acid for this test. In this 

 method two or three drops of the acid are added to 

 the culture under consideration. If indol be present, 

 the red color appears as a result of the reducing action 

 of the nitrous acid upon it. The defect in this method 



