72 PRACTICAL BACTERIOLOGY 



Replace the cotton plug, and allow the gas in the closed branch 

 to flow into the bulb and mix with the air present there. The plug 

 is removed, and a lighted match inserted into the mouth of the bulb. 

 The intensity of the explosion varies with the amount of air present 

 in the bulb. 



CXXXIII. METHOD OF DETECTING INDOL IN 

 CULTURES OF BACTERIA. 



1. Cultivate the organism from twenty-four to forty-eight hours at 

 37 C. in Dunham's Peptone Solution (see 82, p. 51), using four 

 tubes kept under exactly the same conditions. 



2. Apply the test as follows : Take two tubes, each containing 

 7 c.c. of the peptone solution, but ' not inoculated.'' To one add 10 

 drops of concentrated sulphuric acid, to the other 1 c.c. of O'Ol per 

 cent, solution of sodium nitrite, and afterwards 10 drops of concen- 

 trated sulphuric acid. In five or ten minutes if no rose colour 

 appears, then indol is absent. 



3. To two ' inoculated ' tubes add 10 drops of concentrated sul- 

 phuric acid, and in five or ten minutes, if no rose colour appears, add 

 1 c.c. of the sodium nitrite solution ; if no rose colour appears, then the 

 ' indol reaction ' is absent. 



4. When the rose colour appears with the addition of the concen- 

 trated sulphuric acid alone, then indol has been formed, and likewise 

 a reducing' body. 



5. When the rose colour appears only with the addition of both 

 the concentrated sulphuric acid and the nitrite solution, then indol 

 has been formed during the growth of the organism, but no nitrites. 



CXXXIV. TEST FOR NITRITES IN CULTURES. 



ILOSVAY'S MODIFICATION OF GRIESS'S METHOD. 

 The following reagents are employed : 



A. 



Naphthylamine ... 01 gramme. 



Aqua distillata . . . 20*0 c.c. 



Acetic acid (25 per cent, solution) . 150'0 c.c. 

 Dissolve the naphthylamine in 20 c.c. of boiling water, allow it to 

 cool, and mix with the acetic acid solution. 



