CULTURE MEDIA. 49 



in which it is preserved from contamination by atmospheric germs 

 after being inoculated with some particular microorganism. Just 

 before such inoculation the potato is cut in halves with a sterilized 

 (by heat) table knife. The bacteria to be cultivated are placed upon 

 the cut surface and the potato is preserved in a glass dish (Fig. 20). 



A more convenient method, and one which secures the potato more 

 effectually from atmospheric organisms, is to cut a cylinder, about 

 an inch in diameter, from a sound potato, by means of a tin instru- 

 ment resembling a cork borer or apple corer. This cylinder is cut 

 obliquely into two pieces having the form shown in Fig. 22, and 

 each piece is placed in a large test tube having a cotton air filter, in 

 which it is sterilized. This method, first employed by Bolton, has 

 been slightly modified by Roux, who recommends that a receptacle 

 for catching the water which separates during the sterilizing process 

 be formed by making a constriction around the test tube an inch 

 above its lower extremity. This is done by the use of a blowpipe. 

 The cylinder of potato rests upon the constricted portion of the tube, 

 as shown in Fig. 21. 



Sometimes a potato paste is employed. The potatoes are boiled 

 for an hour and the skins removed, after which they are mashed 

 with a little sterilized water, placed in suitable plates, and sterilized 

 by exposure for half an hour on three successive days in the steam 

 sterilizer. Bread paste may be made in the same way, and is a very 

 favorable medium for the growth of certain bacteria and also for the 

 common moulds. 



