78 CULTURES OF BACTERIA. 
run the mixture six or eight times through the 
length of the tube, returning the remaining gas to 
the closed branch before removing the thumb, 
Measure the amount of gas remaining ; the differ- 
ence between this and the former measurement 
shows in millimeters the amount of carbon dioxid 
absorbed by the alkali. The remaining gas, mostly 
hydrogen, may be transferred to the bulb and 
exploded by a flame. ‘The proportion of hydrogen 
to carbon dioxid is usually expressed in the form 
of a fraction called the gas formula, as ; 
2 
The fermentation-tube affords a ready method of 
determining the oxygen requirement of bacteria. 
Growth, indicated by cloudiness, in the bulb only, 
is to be found only among obligatory aérobes ; 
in the closed branch only, among obligatory anaé- 
robes; while growth in both, only among the 
facultative anaérobes. 
VII. ANAEROBIC CULTURES. 
Growth under the mica plate or cover-glass’ and 
in the fermentation-tube are methods for the deter- 
mination of the aérobic properties of organisms. 
For the growth of strictly anaérobic forms special 
methods have been devised : 
I. Place the cultures in a vessel from which the 
air can be withdrawn and hydrogen substituted. 
2. Buchner’s Method.—Use two test-tubes, one 
inside the other. ‘The outer one is partially filled 
1 See page 75. 
