52 



ESSENTIALS OF BACTERIOLOGY. 



Cotton can be used here instead of filter-paper, or filtering 

 entirely dispensed with by making use simply of decantation. 



'As agar is seldom clear, a little more or less opaqueness will 

 not harm. The test-tubes are filled as with the gelatine, and 

 sterilized in the same manner. While cooling, some of the 

 tubes can be placed in a slanting position, so as to obtain a larger 

 surface to work upon. 



Water of condensation will usually separate and settle at the 

 bottom, or a little white sediment remain encysted in the centre ; 

 this cannot easily be avoided, nor does it form any serious obstacle. 

 The crude agar should first be rinsed in clear water, and then 

 in five per cent, acetic acid and clear water again, to rid it of im- 

 purities. 



Glycerine Agar. The addition of 4 per cent, to 6 per cent, of 

 glycerine to nutrient agar greatly enhances its value as a culture 

 medium. 



Gelatine- Agar. A mixture of 5 per cent, gelatine and 0.75 

 per cent, agar combines in it some of the virtues of both agents. 

 Blood Serum. Blood serum being rich in albumen coagulates 

 very easily at 70 C., and if this temperature is not exceeded, 

 a transparent, solid substance is obtained 

 upon which the majority of bacteria develop, 

 and some with preference. 



Preparation of Nutrient Blood Serum. 

 If the slaughter of the animal can be super- 

 vised, it were best to have the site -of the 

 wound and the knife sterilized carefully, 

 and then sterile flasks placed to receive the 

 blood directly as it flows. 



It is placed on ice forty-eight hours, and 

 then the serum is drawn out with sterile 

 pipettes into test-tubes ; these are placed 

 obliquely in an oven where the temperature 

 can be controlled and maintained at a cer- 

 tain degree. See Fig. 25. 



Incubators or Brood-ovens. Incubators 

 or brood-ovens, as such ovens are called, 

 consist essentially of a double-walled zinc 

 or copper chest, the space between the walls filled with water. 

 The oven is covered with some impermeable material to pre- 



FIG. 24. 



Flask to receive blood 

 serum. 



