94 BACTE.EIOLOGY 



obtained in this way. For most purposes it is better to put pieces 

 of potato in test-tubes where they are more perfectly protected 

 from contamination, as suggested by Bolton.^ The potato is 

 carefully washed, a slice removed from each end, and a cylinder 

 is cut out with a cork-borer or with a test tube cut off near its 

 bottom. This cylinder is divided diagonally into two pieces. The 

 pieces are washed in running water for twelve to eighteen hours. 



They are placed in test-tubes containing a httle 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 in cotton, or in a 

 specially devised form of tube with a constriction at the bottom). 

 The tubes are plugged, and sterilized in the autoclave at 110° C. 

 for 30 minutes. Potato is best when freshly prepared; it is likely 

 to become dry and discolored with keeping. 



Milk. — Milk fresh as possible is placed in a covered jar, 

 steamed 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 par- 

 ticles of dirt, both of which are to be avoided. About 7 to 10 

 c.c. are to be run into each test tube. The tube is plugged with 

 cotton, and sterilized in the autoclave at no" C. for 30 minutes. 



The coagulation of milk, which is accomplished by certain 

 bacteria, is a very valuable differential point. A little litmus 

 tincture may be added to the tubes of milk before sterihzing, 

 until they acquire a blue color, to indicate whether or not acids 

 are formed by the bacteria which are afterward cultivated in 

 the milk. Better results are obtained by sterilizing the litmus 

 solution separately and adding it to the sterilized milk with 

 aseptic technic. 



Dunham's Peptone Solution. 



Peptone 10 grams. 



Sodium chloride 5 grams. 



Water i liter. 



Boil, filter, sterilize in the usual manner. 

 . ' Bolton, The Medical News, Vol. 1, 1887, p. 318. 



