56 



ESSENTIALS OF BACTERIOLOGY. 



FIG. 25. 



Flask to receive blood 



out with sterile pipettes into test-tubes; these are placed ob- 

 liquely in an oven where the temperature can be controlled 

 and maintained at a certain degree. See 

 Fig. 26. 



Incubators or Brood-ovens. Incubators 

 or brood-ovens consist essentially of a double- 

 walled zinc or copper chest, the space between 

 the walls being filled with water. 



The oven is covered with some imperme- 

 able material to prevent the action of the 

 surrounding atmosphere. (Fig, 27.) It is 

 supplied with a thermometer and a regu- 

 lator. The regulator is connected with a 

 Bunsen burner, and keeps the temperature 

 at a certain height. 



There are several forms of regulators in 

 use, and new ones are invented continually. 

 The size of the flame in some is regulated 

 by the expansion of mercury, which, as it 

 rises, lessens the opening of the gas supply, 

 The mercury contracting on cooling allows more gas to enter 

 again. (Fig. 28.) 



Koch has invented a safety burner, by which the gas supply is 

 shut oft' should the flame accidentally have gone out. 



Coagulation of Blood Serum. The tubes of blood serum 

 having been placed in the oven, are kept at a tempera- 

 ture of 65 to 68 C., until coagulation occurs ; then removed 

 and sterilized. 



Sterilization of Blood Serum. The tubes are placed 3 to 4 

 days in incubation at 58 C., and those tubes which show any 

 evidences of organic growth are discarded. 



If now, at the end of a week, the serum remains sterile at the 

 ordinary temperature of the room, it can be used for experi- 

 mental purposes. 



Perfectly prepared blood serum is transparent, of a gelatine- 

 like consistence, and straw-color. It will not liquefy by heat, 

 though bacteria can digest it. Water of condensation always 

 forms, which prevents the drying of the serum. Blood serum 



