146 NITRIFICATION 



garden soil, and keep at a temperature of 30 C. Test a small 

 amount of the solution every fourth day with Nessler's solu- 

 tion until no reaction occurs. Then add i c.c. of a 10 per 

 cent, solution of ammonium sulphate and allow this to nitrify, 

 testing every three or four days until the ammonium salt is 

 completely changed. Then transfer some of this nitrifying 

 solution by means of a platinum loop to a second flask. When 

 the solution in the latter gives no reaction with Nessler transfer 

 some of it to a third flask, and similarly some of the third to 

 the fourth, and so on. By cultivating several generations in a 

 medium devoid of organic matter, in this way it is possible to 

 eliminate most of the non-nitrifying bacteria present in the soil 

 and obtain fairly pure cultures of the nitrifying organisms. 



Try and obtain a pure culture which will produce nitrites or 

 nitrates when introduced into fresh ammoniacal solution, and 

 yet give no growth or turbidity when a drop is transferred to 

 bouillon tubes and incubated at 30 C. 



Ex. 78. To obtain pure cultures of the nitrifying organisms 

 their isolation must be carried out on "silica jelly" plates. 



(a) For the preparation of " silica jelly " Winogradsky and 

 Omeliansky recommend the addition of a solution of pure 

 " water glass " (sp. gr. 1.05 to 1.06) to an equal quantity of hydro- 

 chloric acid (sp. gr. i.i). 



Pour the solution of sodium silicate slowly into the dilute 

 hydrochloric acid (not the acid into the silicate solution), shaking 

 the mixture gently. The following reaction takes place, and a 

 solution of silicic acid is obtained, mixed with sodium chloride : 



Na 2 SiO 3 + 2HC1 = H 2 SiO 3 + NaCl 

 Silicic Sodium 

 acid chloride 



To get rid of the sodium chloride pour the mixture into a 

 parchment dialyser and float or suspend the latter in distilled 

 water ; the sodium chloride diffuses through the parchment into 

 the water, which should he changed twice a day : the silicic acid 

 is a colloidal gelatinous body and does not pass through, but 



