8 



may exist, since these frequently harbor bacteria which 

 are highly resistant to destruction. Place the potatoes, 

 thus prepared, in a solution of mercuric chloride (1-1000) 

 for | to 1 hour; transfer to a tin pail with a perforated 

 bottom, and then place tin's in a steam sterilizer. Heat 

 for f to 1 hour, the potatoes should be well cooked. 

 Remove the pail and allow the potatoes to partially cool. 



A "moist chamber" is prepared by placing around 

 filter paper on the bottom of the lower dish and moisten- 

 ing it with mercuric chloride, allowing any excess of the 

 solution to drain off. Three potato knives are sterilized 

 by heating in the flame till the edge begins to redden. 

 They are then set aside to cool, with the edge up, on a 

 block or over the edge of the table so 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 

 upwards. Contact with the cut surface must be avoided. 



To inoculate, transfer a small portion of the bacterial 

 growth by means of a sterilized and cooled wire or knife 

 to the potato, and then thoroughly spread this material 

 over the surface avoiding the outer 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 transferred to the surface of the potato is 

 usually so great that when they develop the entire surface 

 is covered with a continuous growth. To isolate bac- 

 teria, therefore, it is necessary to resort to dilution cul- 

 ture. 



For this purpose a small amount of material is taken 

 from the surface of the inoculated potato, or No. 1, by 

 means of a sterilized knife, and transferred in the same 

 manner as before to the surface of potato No. 2, where it is 

 likewise spread thoroughly, and evenly, over the surface. 

 The number of bacteria transplanted to the second potato 



