72 



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 

 -Tube avoided. 



The reaction should be corrected to -|- 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. 



containing potato. 



