once sealed the chamber and closed
it tightly, no nitrogen can enter other than that admitted with the
oxygen, and hence the residual amount of nitrogen remains unaltered save
for this single exception. If care is taken to keep an accurate record
of the amount of nitrogen admitted with the oxygen, the nitrogen
residual in the chamber at any given time is readily computed. While
from an absolute mathematical standpoint the accuracy of this
computation can be questioned, here again we are seeking an accurate
record of differences rather than an absolute amount, and whether we
assume the volume of the air in the chamber to contain 20.4 per cent of
oxygen or 21.6 per cent is a matter of indifference. It is of importance
only to note the increases in the amount of nitrogen, since these
increases represent decrease in the residual oxygen and it is with the
changes in the residual oxygen that we particularly have to do.

INFLUENCE OF FLUCTUATIONS IN TEMPERATURE AND PRESSURE ON THE APPARENT
VOLUME OF AIR IN THE SYSTEM.

The air, being confined in a space with semi-rigid walls, is subjected
naturally to variations in true volume, depending upon the temperature
and barometric pressure. If the air inside of the chamber becomes
considerably warmer there is naturally an expansion, and were it not for
the tension-equalizer there would be pressure in the system. Also, if
the barometer falls, there is an expansion of air which, again, in the
absence of the tension-equalizer, would produce pressure in the system.
It is necessary, therefore, in calculating the true volume of air, to
take into account not only the apparent volume, which, as is shown
above, is always a constant amount at the end of each period, but the
changes in temperature and barometric pressure must also be noted. Since
there is a volume of about 1,400 liters, a simple calculation will show
that for each degree centigrade change in temperature there will be a
change in volume of approximately 4.8 liters. In actual practice,
however, this rarely occurs, as the temperature control is usually
inside of 0.1 deg. C. and for the most part within a few hundredths. A
variation in barometric pressure of 1 millimeter will affect 1,400
liters by 1.8 liters.

In actual practice, therefore, it is seen that if the barometer falls
there will be an expansion of air in the system. This will tend to
increase the volume by raising the rubber diaphragm on the
tension-equalizer, the u