METHODS OF CULTURE AND OF EXAMINATION. 



85 



thus formed the bits of potato lie. The lower portion of 

 the tube is filled with sterile salt-solution in order to keep 

 the surface of the potato moist. Instead 

 of salt-solution other fluids are used for 

 special purposes. Thus, bits of potato 

 that just dip into five per cent, glycerin- 

 solution furnish an excellent culture- 

 medium for tubercle-bacilli. 



As the cut surface of the potato is 

 more or less changed by protracted ex- 

 posure to the action of steam, it is better 

 to boil the whole potato, and then to 

 divide it. To this end the unpared 

 potatoes are thoroughly cleansed by 

 means of a brush and a solution of 

 mercuric chlorid, the so-called eyes are 

 cut out, and sterilization is practised for 

 two hours in the steam-chamber on each 

 of two successive days, or more simply 

 and better for an hour in the autoclave at 

 a temperature of 110 C. (230 F.), or for twenty minutes 

 at a temperature of 120 C. (248 F.). The potatoes must 

 be sterilized thoroughly because they are often the seat of 

 special bacilli characterized by the extraordinary resistance 



Fig. 15. Roux's test- 

 tube, specially designed 

 for potato-cultures. 



Fig. 16. Method of slicing potato (after Woodhead and Hare). 



of their spores (red potato-bacilli). Then the hands are 

 thoroughly disinfected by means of soap, alcohol, and 

 mercuric chlorid, and the still warm potato is removed 

 with the fingers moist with mercuric chlorid, and it is 



