CHANGES IN THE REACTION OF MEDIA 223 



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 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 it is more permanent, and there is never any likelihood 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 con- 

 sideration. 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 is that it reveals only the presence 

 of indol, and fails to indicate whether or not reducing bodies 

 were coincidently formed with the indol. As a test for indol 

 alone it is convenient and entirely trustworthy. 



Reducing Power of Bacteria. The power to reduce chemical 

 compounds from a higher to a lower state may be said to 

 be common to all bacteria. In some bacteria, perhaps the 

 majority, it is most conspicuously manifested in connection 

 with substances containing sulphur, hydrogen sulphide being 

 formed. In other bacteria it is best seen in connection with 



