116 A CHEMICAL SIGN OF LIFE 
stopcock C and from B by opening the corresponding 
stopcock. Fill both chambers A and B with water 
acidulated with nitric acid (not more than 1 per cent), 
having both stopcocks open, then with distilled water five 
times, then once with alcohol, and once with alcohol-ether. 
Two funnels placed under these two chambers (see Fig. 1) 
are connected with a sink, to form an outlet for this 
waste water. The alcohol and alcohol-ether drained out 
of these chambers should be saved for re-use. Replace 
the stoppers S and R. Let the machine remain un- 
touched for drying while the suction pump is going and 
while the tissue for an experiment is being prepared. 
Five or ten minutes will be sufficient for complete drying 
if the water pump is in good order and the alcohol used 
has not contained too much water. 
It is very important that we should leave the appa- 
ratus in this condition until we are ready for an experi- 
ment, and that no stopcock should be touched, for this 
is the only condition under which all parts of the appara- 
tus will dry. 
How io obtain air free from carbon dioxide —It is very 
difficult to make air completely free from carbon dioxide 
by the ordinary method, i.e., by merely passing it through 
several alkaline bottles or alkaline towers. A simpler 
and surer method is shown together with apparatus III in 
Fig. 5 (p.131). Itis prepared by shaking air with a 20 per 
cent solution of sodium hydroxide in a tightly stoppered 
carboy F, supplied with suitable tubes, one of which is 
led to another carboy £, which is filled with about 10 
to 15 per cent alkali solution. When the air in carboy 
F is to be used, it is driven into the nitrometer C (appa- 
ratus III) or W (in case of the biometer), which is filled 
