68 



CULTURES IX LIQUID MEDIA. 



mal death-point, consists in the application of heat and thus destroy- 

 ing all except the most resistant species. This method is especially 

 applicable when one of the species, only, forms spores. By subject- 

 ing the mixed culture to a temperature which is sufficient to destroy 

 all the vegetative cells in it, the more resistant spores are left and, 



under favorable conditions, may subsequently 

 vegetate and give us a pure culture of the 

 species to which they belong. 



Fermentation. The development of certain 

 bacteria is attended with an evolution of gas, 

 especially in media containing grape sugar or 

 glycerin. For the determination of the quantity 

 and kind of gas produced by a given micro- 

 organism the fermentation tube recommended 

 by Theobald Smith has special advantages. 

 This is a bent tube (Eihorn's) supported upon 

 a glass base as shown in the accompanying 

 figure taken from the catalogue of Eimer & 

 Amend. The graduation shown upon the up- 

 right arm is not essential for ordinary labora- 

 tory work. A liquid culture medium containing 

 one to two per cent of grape sugar is usually 

 used. This is introduced into the upright arm 

 of the fermentation tube, where it is held by atmospheric pressure. 

 A cotton plug is placed in the opening of the short and bulbous arm 

 of the tube, which is intended as a receptacle for the culture liquid 

 when it is forced out of the closed arm by the accumulation of gas at 

 its upper extremity. 



FIG. 38. 



