MANUAL OF BACTERIOLOGY. 



The pieces are washed in running water for several hours. 

 They are placed in test-tubes containing a little water to keep the 

 potato moist, and are supported from the bottom on a piece of 

 glass tubing about i to 2 cm. in length (or on cotton, or in a 

 specially devised form of tube with a constriction at the bot- 

 tom). The tubes are plugged, and sterilized as with other 

 media. Sterilization, however, must be 

 thorough on account of the danger of con- 

 tamination with the extremely resistant spores 

 of the potato bacillus. Potato is best when 

 freshly prepared; it is likely to become dry 

 and discolored with keeping. It is a very 

 useful ^medium; certain growths on it, like 

 those of the bacillus of typhoid fever or of 

 glanders, and those of chromogenic bacteria, 

 are very characteristic. 



Milk. Milk fresh as possible is placed in 

 a covered jar, sterilized for fifteen minutes, 

 and then kept on ice for twenty-four hours. 

 At the end of that time the middle portion is 

 removed by means of a siphon. The upper 

 and lower layers must not be taken; the 

 upper part contains cream, and the lower 

 part particles of dirt, both of which are to be 

 avoided. 



The reaction should be corrected to -f i 

 if the milk is found to be too acid. About 7 

 to 10 c.c. are to be run into each test-tube. The tube is 

 plugged with cotton and sterilized as usual. When milk is 

 contaminated with spores of the hay or potato bacillus it is 

 sometimes very difficult to sterilize, a fact of much importance 

 in connection with the feeding of children, where the frac- 

 tional method of sterilization and the use of the autoclave are 

 impracticable. 



FIG. 17. Tube 

 containing potato. 



