In long-continued experiments, where there is a possibility of a
noticeable diminution in the percentage of oxygen in the chamber--a
diminution caused either by a marked fall in barometer, which expands
the air inside of the chamber and permits admission of less oxygen than
would otherwise be required, or by the use of oxygen containing a high
percentage of nitrogen, thus continually increasing the amount of
nitrogen present in the system--it is highly probable that there may be
such an accumulation of nitrogen as to render it advisable to provide
for the admission of a large amount of oxygen to restore the air to
approximately normal conditions. In rest experiments of short duration
this is never necessary. The procedure by which such a restoration of
oxygen percentage is accomplished has already been discussed
elsewhere.[25] It involves the rejection of a definite amount of air by
allowing it to pass into the room through the gas-meter and then making
proper corrections for the composition of this air, deducting the volume
of oxygen in it from the excess volume of oxygen introduced and
correcting the nitrogen residual in order to determine the oxygen
absorption during the period in which the air has been rejected.