60 THE CULTIVATION OF BACTERIA. 



The disadvantages of agar, when compared with gelatin, 

 lie chiefly in the greater difficulty of its preparation, and 

 especially of its nitration. 



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 

 washed again in water and put into a solution of 1 : 500 

 bichloride of mercury for an hour, after which it is cooked in 

 the steam sterilizer for forty minutes on each of two suc- 

 cessive days. Just before inoculation the potato is split into 

 halves by means of a sterilized knife and allowed to fall into 

 a moist chamber cut surface up. 



The moist chamber consists of a double glass dish, the upper 

 one of which is larger than the lower. This chamber should 

 be thoroughly rinsed with a 1 : 500 solution of bichloride of 

 mercury before use. 



Some precautions are necessary to prevent contamination 

 of the potato from external germs. The hand that cuts the 



