NITRATES AND NITRITES 165 



fine bubbles. Later liquid clears and froth disappears, leaving 

 faint iridescent scum and whitish light sediment. 



Ex. 81. Prepare the following media : 



1. Nitrate Bouillon. 



(.) 



Extract of lean beef (see p. 56) . . i litre. 



Peptone . . . . . 10 gr. 



Sodium chloride . . . .. . 5 gr. 



nitrate i 3 gr. 



(*) 



Lemco's Extract . . ... . 5 gr. 



Peptone . . . . . 10 gr. 



Sodium nitrate . . . . 3 gr. 



Water . . . . . . . i litre. 



No. i usually gives best results. Make both just neutral with 

 sodium carbonate and sterilize as usual. 



2. Nitrate Gelatine. 



Nitrate bouillon as above i litre. 



Gelatine . . . . . .TOO gr. 



Neutralize with sodium carbonate; place 10 to 15 c.c. in tubes 

 and sterilize. 



3. Nirate Agar. 



Nitrate bouillon as above i litre. 



Agar-agar . . . . . , . 15 gr. 

 Neutralize as above, tube and sterilize. 



Ex. 82. (i) Add 5 grams of fresh horse dung to 



Water . . . . . . 100 gr. 



Sodium nitrate . . . . .3 gr. 



Incubate about 30 C. : notice frothing, and test for nitrate 

 with diphenylamine and sulphuric acid, as in Ex. 74, every two 

 days for a fortnight. Nitrate usually disappears in six or eight 

 days. 



(2) Repeat above, using 5 grams of chopped straw instead of 

 dung. 



