NITRIFICATION 149 



and evaporate to dryness a second time. Then wash and 

 transfer to a Swedish filter paper, the weight of whose ash is 

 known. Wash the precipitate until the water gives no cloudi- 

 ness with silver nitrate solution. Burn the filter with the pre- 

 cipitate upon it in a weighed platinum or porcelain crucible and 

 heat to redness. Allow to cool, and weigh the silicic anhydride 

 (SiO 2 ). Calculate the amount of silicic anhydride in i c.c. of 

 the original solution. 



(b) Dilute the amount of water glass which you wish to use for 

 preparation of the jelly until the solution contains 4 or 5 per 

 cent, of silicic anhydride. Prepare hydrochloric acid of such 

 strength that i c.c. just neutralizes i c.c. of the "water glass" 

 solution, using methyl orange as an indicator. Take 104 c.c. of 

 the hydrochloric acid and add to it 100 c.c. of the "water glass " 

 solution : the excess of acid prevents coagulation during the 

 subsequent sterilization. The solution should be poured into 

 tubes or flasks and sterilized at 120 C. for fifteen minutes, or at 

 100 C. for twenty minutes, on three successive days : 10 to 20 

 c,c. are convenient amounts to place in tubes. 



Since the medium cannot be liquefied after once it has " set," 

 tubes for " slants " must be put in position as soon as possible 

 after mixing. 



(c) Inoculate the tubes from the flasks containing nitrifying 

 organisms (Ex. 77) and add to each 10 c.c. of sterilized solu- 

 tion, .5 c.c. of the ammonium sulphate solution (in Ex. 74) and 

 a drop or two of sodium carbonate solution, enough to make the 

 whole alkaline. Pour into Petri dish at once and allow to 

 solidify. 



(d) Inoculation may be made on the surface of the solidified 

 medium as in Ex. 78 (c. ii). 



Ex. 80. Isolate and grow the nitrate organism. Prepare 

 the following solution : 



Sodium nitrite (puriss) . . . . i gr. 

 carbonate (calcined) . . . i gr. 

 chloride . . . . . .5 gr. 



