418 APPLICATION OF METHODS OF BACTERIOLOGY 



of dry meal spread upon the surface of the medium. In 

 the lower part of the tube in which they are growing — i. e., 

 that part occupied by a few drops of fluid which has in part 

 been squeezed from the medium during the process of solidi- 

 fication, and is in part water of condensation — the colonies 

 may be seen to float as a thin pellicle upon the surface of the 

 fluid. 



The individuals composing the growth adhere so tena- 

 ciously together that it is with the greatest difficulty they 

 can be separated. In even the oldest and dryest cultures 

 pulverization is impossible. The masses can only be sepa- 

 rated and broken up by grinding in a mortar with the 

 addition of some foreign substance, such as very fine, 

 sterilized sand, or ground glass, etc. 



The cultures are of a dirty-drab or brownish-gray color 

 when seen on serum or glycerin-agar-agar. 



On potato they grow in practically the same way, though 

 the development is much more limited. On this medium 

 they are of nearly the same color as the potato on which 

 they are growing. When cultivated for a time on potato 

 they are said to lose their pathogenic properties. 



On milk-agar-agar they are of so nearly the same color 

 as the medium that, unless they are growing as character- 

 istic mealy-looking masses, considerably elevated above the 

 surface, their presence is less conspicuous than when on 

 other media. 



In bouillon they grow as a thin pellicle on the surface. 

 This may fall to the bottom of the fluid and continue to 

 develop, its place on the surface being taken by a second 

 pellicle. 



The tubercle bacillus does not develop on gelatin because 

 of the low temperature at which this medium must be used. 



