22 



increase of indol as shown in the following chart, where the reactions 

 of greatest intensity are compared with the colorimetric scale of 

 standard sodium-nitrite dilutions: 



0.00001 gm. N. 



0. 000005 gm. N. 



0. 000002.5 gm. N. 



000001 gm. N. 



0. 0000005 gm. N. 



0.00000025 gm. N. . 



8 Days. 



Throughout the following experiments the test for indol was 

 made by adding 0.5 cubic centimeter of a freshly prepared 0.01 

 per cent solution of sodium nitrite and 0.5 cubic centimeter of 

 pure sulphuric acid. Cholera red was tested for by adding 0.5 

 cubic centimeter of pure sulphuric acid. 



(c) Such an albuminous medium, moreover, seems to absorb 

 nitrites from the laboratory atmosphere much more rapidly than 

 distilled water does, for this "peptone" solution, which at first 

 gave reactions for nitrites corresponding to about 0.0000005 gram 

 nitrogen, after two weeks at 18° to 28° gave reactions correspond- 

 ing to about 0.0000025 gram nitrogen — an increase quite suffi- 

 cient to yield an indol reaction when pure sulphuric acid alone is 

 added to a twenty-four-hour culture of B. colt bll . 



This increase of nitrites was not due to an increase in the con- 

 centration of the medium, for the initial level of the medium had 

 been marked on each tube and very little evaporation had taken 

 place. Further, this increase in the amount of nitrites in the 

 medium containing nitrates is sufficient to obscure somewhat the 

 bright vermilion-red color of the cholera-red reaction. 



