168 BACTERIOLOGY. 



the edge begins to redden. The knives are then set aside 

 to cool, with the edges turned up, on a block or over the 

 edge of the table, care being taken that the blade does 

 not touch anything. 



The partially cooled potato is now picked up with the 

 left hand, which previously has been dipped in mercuric 

 chloride, and cut into halves by a horizontal section with 

 a sterilized knife. Each half of the potato is carefully 

 placed in the moist chamber, with the cut surface turned 

 upward. 



The cut surface must not come into contact with the fingers, 

 paper or glass, since such contact is very likely to result in the plant- 

 ing- of bacteria on the potato. The moist chamber should not be 

 allowed to remain uncovered, even for a minute. The cut potatoes 

 remain in the dish until perfectly cool. If the subsequent transplan- 

 tation is carried on with a hot knife, or to a hot potato, it may result 

 in the killing- of the organism to be planted. The potatoes, while 

 cooling, give off aqueous vapor which condenses on the lid and may, 

 eventually, drop down and thus cause contamination. The con- 

 densed water should, therefore, be repeatedly removed from the 

 cover, by means of a piece of filter-paper. 



When cold, the potatoes are ready to be inoculated. A 

 small portion, the size of a pin-head, of the bacterial 

 growth is transferred by means of a sterilized and cooled 

 wire, or knife to one of the above pieces of a potato, and 

 then thoroughly spread over the surface. Care should be 

 taken to avoid the outer J inch. The potato is held in the 

 fingers of the left hand, which has been dipped in mercuric 

 chloride. The number of bacteria which is thus trans- 

 ferred to the surface of the potato is usually so great that 

 when they develop the entire surface is covered with a con- 

 tinuous growth. Such a growth is spoken of as a mass 

 culture. 



The object in planting the bacteria on the potato is not 

 so much to obtain a mass growth, as it is to spread a 

 minute portion over as large an area as possible. When, 



