THE BIOMETER: HOW TO USE IT 117 
with a less concentrated alkaline solution (a weaker 
solution is necessary so that the chamber may be filled, 
with air which is not too dry). Driving the air into 
the nitrometer is accomplished either by increasing the 
pressure in carboy F, by introducing more alkali from the 
carboy above £, or by introducing more alkali through 
funnel instead of from another carboy. After each 
evacuation of the apparatus by a strong suction this 
air free from carbon dioxide is introduced from the 
nitrometer C or W into the chambers through the stop- 
cock 7. For ordinary experiments one can keep the 
pressure in the carboy F high enough so that air free 
from carbon dioxide can be driven into the nitrometer 
simply by opening stopcock 9 after each evacuation. 
How to test purity of air —In order to test whether 
or not the air in carboy F is free from carbon dioxide 
the following experiment is necessary. It may be 
stated that in all ordinary experiments we use exactly 
the same manipulation as the one now to be described. 
When the apparatus is perfectly dry, the pump being 
at work, open stopcock (or pinchcock) 9 so that the 
nitrometer is filled with the air freed of carbon dioxide 
from carboy F. (If no bubbles come out by opening this 
pinchcock it means that there is not enough pressure in 
the carboy. In that case open pinchcock 8 to let more 
alkali siphon down from the carboy above E, until 
about 200 to 300 c.c. of air can be obtained by opening 
pinchcock 9.) 
Shut stopcocks C and C’ and J. If the pressure 
pump is strong enough and all the joints are tight, 
chambers A and B should be under a strong negative 
pressure by now, so that when you open stopcock C 
