18 MICRO-ORGANISMS IN WATER 



volume of a sterile solution of tlie following compo- 

 sition is added : — ■ 



The solution of the sulphates and 

 chloride should be sterilised sepa- 

 rately from the solution of the phos- 

 phate and carbonate ; these two 

 solutions are then preserved in a 

 sterile condition, and equal volumes 

 of each are taken for mixing with 

 the sterile solution of silicic acid. 



On thoroughly mixing the silicic and saline solutions 

 gelatinisation takes place in from five to fifteen minutes. 

 The carbonate of soda in the above may often be ad- 

 vantageously replaced by magnesium carbonate, but 

 the medium is then not transparent — a matter which 

 is of no consequence, however, for most purposes for 

 Avhich this jelly is used. The material containing the 

 micro-organisms for cultivation is introduced into and 

 thoroughly distributed throughout the above mixture 

 before gelatinisation has taken place. Or a " streak 

 culture ' may be made on the surface of the jelly after 

 solidification has taken place. 



Blood-serum. — The blood of an animal is carefully 

 collected in a sterilised vessel, neglecting a small quan- 

 tity which first flows from the wound, and is then kept 

 as cool as possible until coagulation commences. When 

 the first few drops of coloured serum make their appear- 

 ance they are removed with a sterilised pipette. The 

 liquid serum, which subsequently becomes expressed in 

 the course of from twenty-four to thirty-six hours (during 

 which time it must be kept in a refrigerator), is then 

 transferred by means of a sterilised pipette to either 

 sterilised test-tubes, covered glass dishes, or culture 

 flasks, as the case may be. If due care is exercised in 

 these manipulations the serum thus obtained is perfectly 

 sterile. In general, however, it will require sterilisation, 



