62 THE CULTIVATION OF BACTERIA. 



is a much more difficult process than with gelatin, and must 

 be carried on in the steam sterilizer. After filtration the 

 agar is distributed among test-tubes for use, and is sterilized 

 by the same procedure as nutrient gelatin. 



Nutrient agar has a melting-point much higher than that 

 of gelatin, about 42 C., so that it may be used for the culti- 

 vation of those bacteria which grow only at or best at the 

 temperature of the human body. It possesses all the advan- 

 tages of gelatin, but is not so clear and transparent, and is not 

 liquefied by the secretions of any known bacteria. It is useful 

 also for the isolation of bacteria by means of plate cultures. 



The disadvantages of agar, when compared with gelatin, 

 lie chiefly in the greater difficulty of its preparation, and 

 especially of its filtration. 



3. The glycerin-agar culture is obtained by adding 5 per 

 cent, of neutral glycerin to nutrient agar before sterilization. 

 This makes a very favorable medium for the cultivation of 

 the tubercle bacillus and of some other pathogenic bacteria. 



4. A mixture of agar and gelatin in bouillon is sometimes 

 used so as to obtain the advantages of the two substances com- 

 bined. This mixture is prepared from bouillon in. the usual 

 way by dissolving in the bouillon 0.75 per cent, of agar and 

 5 per cent, of gelatin in the manner outlined in the foregoing 

 paragraphs. 



There are a number of special filtering apparatus constructed 

 for the purpose of facilitating the filtration of agar. None 

 in the author's estimation has any advantages over filtration 

 through absorbent cotton in the steam sterilizer. 



5. Potato Culture. 



Koch called attention to the great value of potato cultures 

 for differentiating species of bacteria. 



The mode of preparing potatoes is as follows : A sound 

 potato, of good size, with an intact epidermis, is chosen, and 

 thoroughly washed and scrubbed with a brush to remove all 

 dirt ; after which, with a sharp-pointed knife, all the eyes 

 and discolored parts are cut out of the potato. It is then 



