THK MEDIA FOR THE CULTIVATION OF BACTERIA. 61 



added drop by drop until the mixture is faintly alkaline, as 

 in the bouillon. The flask is again put over the water-bath 

 for an hour, after which the mixture i-s filtered hot through 

 absorbent cotton and sterilized in the steam sterilizer for 

 twenty minutes each day on three consecutive days. 



Great care must be exercised to introduce the medium into the 

 sterilizer only when steam is actively being generated, and not to 

 allow it to cool in the sterilizer. 



Advantages. Gelatin is a most excellent medium, and 

 remains solid at room temperature (22 or 24 C.), but is 

 readily liquefied when exposed to a higher temperature. 

 Not only does it serve as an excellent medium for the culti- 

 vation of nearly all bacteria that grow out of the incubator 

 temperature (36 to 37 C.), but it offers by the plate-method 

 one of the best culture-soils for isolating bacteria. 



As some of the bacteria liquefy gelatin and others do not, 

 it is useful also in the differentiation of these species. On 

 account of its clearness, its easy preparation, and the other 

 advantages mentioned above, it is one of the best culture- 

 soils available. 



The disadvantages lie chiefly in the fact that it liquefies at 

 a much lower temperature than that of the incubator, and 

 can not therefore be used for the cultivation of some patho- 

 genic bacteria which grow only at blood temperature. 



2. Agar. 



Nutrient agar is prepared much in the same manner as 

 gelatin, except that 20 or 30 grams of agar are added, in- 

 stead of the gelatin, to the meat-infusion, and the process 

 of cooking must be much more prolonged (four to five hours), 

 as the agar is much slower in dissolving. It is necessary 

 also sometimes, after dissolving the agar and neutralizing 

 the mixture, to add the white of one or two eggs in order 

 to clarify the solution before filtration. This is done by re- 

 moving the flask from the fire and allowing it to cool to a 

 temperature below 70 C. After clarification the mixture is 

 again cooked for one and one-half to two hours and filtered 

 through absorbent cotton two or three times. This filtration 



