84 BACTERIOLOGY. 



1. They are taken as they come to the market — old 

 potatoes being usually recommended, and carefully 

 scrubbed under the water-tap with a .stiff brush until 

 all adherent dirt has been removed ; "the eyes" and all 

 discolored or decayed parts are carefully removed with 

 a pointed knife. They are then to be placed in a solu- 

 tion of corrosive sublimate of the strength of 1 : 1000 

 and allowed to remain there for twenty minutes ; at the 

 end of this time, without rinsing off the sublimate, they 

 are placed in a covered tin bucket with a perforated 

 bottom and sterilized in the steam sterilizer for forty- 

 five minutes. On the second and third days the sterili- 

 zation is repeated for fifteen to twenty minutes each 

 day. They must not be removed from the sterilizing 

 bucket until sterilization is complete. At the end of 

 this time they are ready for use. When prepared in 

 this way, they are usually intended to be cut into two 

 halves, and the cultivation of the organisms is to be 

 conducted upon the flat surfaces of the sections. 



This method requires some care to prevent con- 

 tamination during manipulation. The hand which is 

 to take up the potato from the bucket, which until 

 now has remained covered, is first disinfected in the 

 sublimate solution for ten minutes, the potato is then 

 taken up between the thumb and index finger, and sev- 

 ered into two by a knife which has just been sterilized 

 in the free flame until it is quite hot. The blade of the 

 knife is passed not quite through the potato, but nearly 

 so. A large glass culture-dish for the reception of the 

 two halves of the potato having been disinfected for 

 twenty minutes with 1 : 1000 sublimate solution and 

 then drained of all the adherent solution, is at hand 

 ready for the bits of potato ; the cover is removed, and 



