THE EXAMINATION OF CULTURES SI 
open bulb. It is then returned. This should be repeated 
several times. Remove the thumb when the open bulb is full, 
and the liquid will rush up into the closed branch to fill the 
space occupied by the CO, which has been absorbed by the 
caustic soda. Measure the portion of the tube first occupied 
with gas and now filled with the liquid. This will indicate 
the quantity of CO,, The remainder of the gas is H. (There 
are also traces of other gases.) Its explosive property can be 
tested by filling the open bulb with water, covering it with the 
thumb and again bringing the gas to the open bulb, holding 
it close to a flame, and removing the thumb. A distinct 
explosion will be heard. 
The ratio of CO, to H can be determined from the meas- 
urements. Thus the total amount of gas in the closed branch 
=5cm. The amount absorbed (CO,)=2cm. The remain- 
ing gas, or 3cm.,=H. The ratio of CO, to H is, therefore, 
as 2:3, or CO,: H:: 2: 3. 
79. Making cover-glass preparations from milk cultures. 
Spread as thin a film of the milk culture as possible on the 
cover glass and allow it to dry in the air. Immerse the prep- 
aration in a watch glass or other receptacle containing a few 
cubic centimeters of ether and absolute alcohol in equal parts, 
which dissolves out the fat and fixes the film to the cover glass 
at the same time. Then remove and, after the ether and alco- 
hol have evaporated, stain as usual. The amount of albumen 
in the milk will usually cause a heavy background, which will 
require decolorizing with alcohol or weak acetic acid. 
