68 CULTURES IN 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 (Hihorn’s) supported upon 
a glass base as shown in the accompanying 
figure taken from the catalogue of Himer & 
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 
AGE es 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 asa receptacle for the culture liquid 
when it is forced out of the closed arm by the accumulation of gas at 
its upper extremity. 
“umn 
