ewhere.[27] Under these conditions, therefore, it is
unnecessary to make any correction on the residual volume of nitrogen as
calculated at the beginning of the experiment. When a direct comparison
of the calculated residual amount of oxygen present is to be made upon
determinations made with a gas-analysis apparatus the earlier and much
more complicated method of calculation must be employed.

CRITICISM OF THE METHOD OF CALCULATING THE VOLUME OF OXYGEN.

Since the ventilating air-current has a confined volume, in which there
are constantly changing percentages of carbon dioxide, oxygen, and
water-vapor, it is important to note that the nitrogen present in the
apparatus when the apparatus is sealed remains unchanged throughout the
whole experiment, save for the small amounts added with the commercial
oxygen--amounts well known and for which definite corrections can be
made. Consequently, in order to find the amount of oxygen present in the
residual air at any time it is only necessary to determine the amounts
of carbon dioxide and water-vapor and, from these two factors and from
the known volume of nitrogen present, it is possible to compute the
total volume of oxygen after calculating the total absolute volume of
air in the chamber at any given time.

While the apparent volume of the air remains constant throughout the
whole experiment, by the conditions of the experiment itself the
absolute amount may change considerably, owing primarily to the
fluctuations in barometric pressure and secondarily to slight
fluctuations in the temperature of the air inside of the chamber.
Although the attempt is made on the part of the observers to arbitrarily
control the temperature of this air to within a few hundredths of a
degree, at times the subject may inadvertently move his body about in
the chair just a few moments before the end of the period and thus
temporarily cause an increased expansion of the air. The apparatus is,
in a word, a large air-thermometer, inside the bulb of which the subject
is sitting. If the whole system were inclosed in rigid walls there would
be from time to time noticeable changes in pressure on the system due to
variations in the absolute volume, but by means of the tension-equalizer
these fluctuations in pressure are avoided.

The same difficulties pertain here which were experienced with the
earlier type of apparatus in determining the average temperature of the
volume of air inside of the chambe