POTATO. 43 



produced by certain bacteria. The indol reaction is demon- 

 strated with this medium. It is made as follows : 



Sodium chloride, 0.5 gram ; 



Peptone, 1 " 



Water, 100 c.c. 



Mix and boil thoroughly ; filter ; fill into tubes and sterilize. 

 The solution is clear and colorless. 



The indol reaction is determined as follows : Inoculate a 

 tube of Dunham's solution with the Bacillus coll communis 

 (or any other germ producing aromatics) and place it in the 

 incubator for twelve hours. Then add 1 c.c. of a 0.01 per 

 cent, solution of KNO and 10 drops of chemically pure 

 H 2 SO 4 . A pink or faint red color, gradually turning to 

 purple, indicates indol. 



Potato : This is a very serviceable and easily prepared 

 culture-medium, especially for the cultivation of moulds. 

 The potato is cut into slices and cylinders. Only sound 

 potatoes should be used. First the potato is washed well in 

 water and scrubbed with a small hand-brush, after which it 

 is placed in a 1 : 1000 mercuric chloride solution for an hour ; 

 then it is washed in sterile water, and the skin scraped off 

 with a sterile knife. Large potatoes are cut into slices about 

 one-quarter of an inch in thickness, and these slices are 

 placed in sterile Petri dishes. 



The cylinders are cut with an apple-corer or a cork-borer. 

 They are made about two inches long and five-eighths of an 

 inch in thickness. The ends are squared. Each cylinder is 

 cut into two equal parts by an oblique incision which will 

 give a large inclined surface. The cylinders are placed in 

 test-tubes with the slanting surface up. Special potato-tubes 

 can be used, or an ordinary wide and short tube, in the bottom 

 of which a little pledget of cotton has been placed. The 

 special tubes have a small chamber in the bottom formed by 

 a constriction of the tube. These tubes can be made in the 

 laboratory by heating a tube and constricting it with a wire. 

 Plug the tube with cotton. Sterilize both the tubes and 

 Petri dishes in the steam sterilizer for one hour, and the 



