io8 Elements of Water Bacteriology. 



peptone solution is examined for indol by adding i c.c. of a 

 .02 per cent solution of sodium or potassium nitrite and 

 i c.c. of a i to i solution of sulphuric acid. Both the tube 

 and the reagents should be cooled on ice before mixing, 

 and the tube should be left in a cool place for an hour 

 afterward to allow time for the characteristic rose-red 

 color of nitroso -indol to develop. At the same time 

 (after four days) the nitrate tube is tested for nitrites by 

 adding a drop of each of the following solutions in suc- 

 cession: 



A. Sulphanilic acid .5 gram 



Acetic acid (25% sol.) 150-0 c.c. 



B. Naphthylamine chloride .1 gram 



Distilled water 20.00 c.c. 



Acetic acid (25% sol.) .- . . 150.0 c.c. 



A red or violet coloration indicates the presence of nitrites. 



In making the nitrite and indol tests it is important 

 to remember the possibility that appreciable amounts of 

 nitrite may be present in the media either derived from 

 the air or from the use of impure peptone in the indol 

 solution (Wherry, 1905). In the case of the nitrite reac- 

 tion, control tubes should always be tested from the same 

 batch of media and only a distinct red color should be 

 considered positive. 



The gelatin tube should be kept at 20 degrees for ten 

 days. It seems undesirable in practice to prolong the 

 test much beyond this point, although some slowly lique- 

 fying organisms are doubtless included, which would be 

 thrown out by a longer incubation. The extent of this 



