THE MEDIA OF THE CULTIVATION OF BACTERIA. 61 



potato should be thoroughly sterilized in a 1 : 1000 bichloride 

 of mercury bath. It is better also after washing the potato 

 and before submitting it to the bichloride bath to wrap it in 

 some thin tissue paper, and to keep it in this paper 

 until ready for inoculation. Fio^ll. 



Preparation of a Potato for Test-tube Culture. By 

 means of a cork-borer (Fig. 10) a cylinder is cut 

 from a sound potato. This cylinder is cut obliquely 



FIG. 10. 



Nest of cork-borers, used to cut potatoes for test-tube cultures. 



into two pieces, and each placed into a large test- 

 tube (Fig. 11), in which it is sterilized and cooked 

 on three successive days for a half-hour in a steam 

 sterilizer. 



6. Potato-paste is sometimes used for cultivation. 

 For this purpose a potato is boiled, peeled, and 

 mashed with a little sterilized water, placed in a 

 suitable glass dish, and sterilized for one-half hour 

 on three successive days in a steam sterilizer. 



7. Bread-paste is a useful medium for the growth 



of moulds, and is made in the same way as potato- Pota tube. test 

 paste. 



III. The Most Commonly Used Special Culture-Media. 



In making final distinctions between the different species 

 of bacteria the following special media are occasionally used : 



1. The Peptone Solution. 



Dry peptone, 1 part; 



Sodium chloride, J " ; 



Distilled water, 100 parts. 



This is filtered, decanted into test-tubes, and sterilized in the 

 steam sterilizer. 



