SOLID TRANSPARENT MEDIA 



57 



ordinarily used in the meat-water, and then, when well dis- 

 solved, the concentrated meat-water infusion is added and the 

 two solutions boiled together. 



Glycerin-agar. The addition of 4 to 6 per cent, of glycerin 

 to nutrient agar greatly enhances its value as a culture-medium. 



Gelatin-agar. A mixture of 5 per cent, gelatin and 0.75 

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



Blood-serum. Blood-serum, being rich in albumin, coagu- 

 lates 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 supervised, it were best to have the site of 

 the wound and the knife sterilized, and sterile flasks (Fig. 24) at 

 hand to receive the blood directly as it flows. 



The blood is placed on ice forty-eight 

 hours, and the serum is drawn out with 

 sterile pipets 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 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. 26). 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 con- 

 tracting on cooling allows more gas to enter again (Fig. 27). 



Fig. 24. Flask 

 to receive blood- 

 serum. 



