NITRIFICATION 147 



remains in the dialyser. (Great care must be taken to procure 

 a sound piece of parchment or dialyzing membrane, or it will 

 not be possible to obtain a solution of silicic acid of the right 

 strength to form a firm solid jelly.) Test small portions of the 

 solution taken from the dialyzer with silver nitrate from time to 

 time, and when no milkiness is given with this reagent the solution 

 may be poured into a stoppered flask and sterilized : it does not 

 keep well and is best used in a day or two, since it is liable to 

 lose its clearness and become unfit for use. 



(b) When required for the preparation of " silica jelly " plates, 

 for the cultivation of the nitrite organism, the silicic acid ob- 

 tained by the above method is mixed with the following solution, 

 which must be sterilized : 



Ammonium sulphate . % . . . . 3 gr. 



Di-potassium phosphate . . . .1 gr. 



Sodium chloride . . . . ' .2 gr. 



Magnesium sulphate 05 gr. 



Ferrous sulphate . . . . .04 gr. 



Water (twice distilled) . . . . 100 c.c. 

 To 50 c.c. of the silicic acid solution add 2.5 c.c. of the above 

 and enough of a sterilized mixture of fine magnesium carbonate 

 and water to make the whole milky. 



(c) (i) Inoculate as rapidly as possible with a loopful of the 

 purest culture obtained by the successive cultivation of the 

 nitrifying organism, as in Ex. 77, and after shaking gently pour 

 at once into a sterilized Petri dish and allow the liquid to 

 solidify ; incubate at 30 C., or 



(ii) Pour the mixed silicic acid and ammoniacal solution into 

 a Petri dish, and after the "jelly" has "set" place a drop of 

 inoculating medium on the surface and very gently spread it in 

 streaks with a sterilized bent glass rod, taking care not to tear 

 the surface of the jelly. 



As the silicic acid solution gelatinizes in a short time when 

 mixed with the ammonium sulphate solution, the mixing and 

 inoculation must be carried out rapidly. Incubate at 30 C., 





