I 



BACTERIA OCCURRING IN SEWAGE 67 



organisms have, therefore, to be isolated by means of a silica- 

 jelly plate, proceeding, according to Kiihne, as follows : 



A solution of potassium silicate of about 5 per cent, strength 

 is mixed with 10 per cent, hydrochloric acid, placed in a parch- 

 ment-paper dialyzer, and floated for two or three days on 

 running water, protected from dust, until, after floating the 

 dialyzer on distilled water for thirty minutes, the water is 

 practically neutral, and gives only a faint reaction with nitrate 

 of silver, showing that the chloride and free acid have been 

 washed out. The solution of silicic acid should be clear and 

 mobile. If there are a few white flocks in it they may be 

 strained out ; if many, the solution is spoilt. Two solutions 

 are made, containing the following quantities of nutritive in- 

 organic constituents in grammes : 



(a) Ammonium sulphate, 0*4 ; magnesium sulphate, 0*05 ; 

 calcium chloride, 0*0005 > distilled water, 50 c.c. 



{b) Potassium phosphate, o'l ; sodium carbonate, 0*075 ; 

 distilled water, 50 c.c. 



They are separately sterilized, mixed in a sterile flask, and 

 closed with a sterilized stopper. The silicic solution is boiled 

 gently down till a sample, on cooling and mixing with one- 

 third of its volume of the above saline solution, sets in about 

 ten minutes into a sufficiently firm jelly. The saline solution 

 is apt to deposit slightly, but, if kept sterile, is not rendered 

 unfit for use. The liquid to be tested is mixed with the two 

 solutions, and at once poured into Petri dishes to set, provision 

 being made for an ample supply of oxygen. Plates of magnesia- 

 gypsum containing the above salts are used by Omeliansky.^ 



MacConkey's medium, which has to a great extent displaced 

 the use of carbolic acid for determining coli organisms, depends 

 upon the inhibitive action of bile salt. It may be prepared as 

 follows : To distilled water add 2 per cent, of peptone and 

 o*5 per cent, of sodium taurocholate ; dissolve as far as possible 

 by heating, then add 0*5 per cent, of glucose, and, after filtering, 

 tint with litmus solution. The fluid should be clear and 

 distinctly blue. It is sterilized in quantities of about 10 c.c. 

 in test-tubes containing a small inverted test-tube or " fermen- 

 tation tube." During sterilization the latter is completely 

 filled with the culture material, so that subsequently, if the 

 growth of an organism is accompanied by the production of 

 gas, it will collect, partially displacing the fluid. 



^ Cheiu. Ceiiir., 1899, ii. 725. 



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