CHANGES IN THE REACTION OF MEDIA 205 



7 c.c. of the peptone solution, but which has not been inocu- 

 lated, add 10 drops of concentrated sulphuric acid. To 

 another similar tube add 1 c.c. of a 0.01 per 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 is 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 reac- 

 tion 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 



