ued 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.

INTERCHANGE OF AIR IN THE FOOD-APERTURE.

The volume of air in the food-aperture between the two glass doors is
approximately 5.3 liters. When the door on the inside is opened and the
material placed in the food-aperture and the outer door is subsequently
opened, there is by diffusion a passage outward of air of the
composition of the air inside of the chamber, and the food-aperture is
now filled with room air. When the inner door is again opened this room
air enters the chamber and is replaced by air of the same composition as
that in the chamber. It is seen, then, that there may theoretically be
an interchange of air here which may have an influence on the results.
In severe work experiments, where the amount of carbon dioxide in the
air is enormously increased, such interchange doubtless does take place
in measurable amounts and correction should undoubtedly be made. In
ordinary rest experiments, where the composition of the air in the
chamber is much more nearly normal, this correction is without special
significance. Furthermore, in the two forms of calorimeter now in use,
the experiments being of but short duration, provision is made to render
it unnecessary to open the food-aperture during the experiment proper.