Methods of Determination of Pathogenic Microbes. 125 



Gelatinized serum. 



A solid medium of frequent use is sterilized and 

 cooked blood serum. Many methods are employed 

 in collecting and sterilizing the serum ; these we re- 

 frain from describing here and limit ourselves to a 

 description of that which we are accustomed to use. 

 Blood drawn from the jugular of the horse or the ox 

 is collected in deep cylindrical glass vessels and left 

 in the cold for twenty-four hours. The clot contracts 

 and the serum comes to the surface; the latter is 

 collected by means of a pipette and introduced into 

 conical vials to be sterilized. 



Sterilization is obtained by conveying the serum 

 on several successive days into a water bath a 58°, 

 and, each day, leaving it there for two hours. The 

 bath is provided with a thermo-regulator by which a 

 temperature of 58° is quickly obtained, a condition 

 indispensable to prevent the germs from multiplying 

 too much and, in consequence, altering the coagula- 

 bility of the serum. After eight days heating the 

 liquid maybe considered sterile ; it is then distributed 

 in test tubes by means of a Chamberland pipette. 

 Other nutritive substances, peptones, glycerin, etc., 

 may be added to the serum before sterilization. 



The cooking of the serum takes place in double 

 walled ovens so arranged that the tubes may be in- 

 clined so as to spread out the liquid. These ovens 

 are regulated to 70°; the tubes are removed as the 

 serum in them becomes coagulated. Before being 

 used these tubes are tested during three days in the 

 incubator. 



