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 chamber. We have on the one hand the warm
surface of the man's body, averaging not far from 32° C. On the other
hand we have the cold water in the heat-absorbers at a temperature not
far from 12° C. Obviously, the air in the immediate neighborhood of
these two localities is considerably warmer or colder than the average
temperature of the air. The disposition of the electric-resistance
thermometers about the chamber has, after a great deal of experimenting,
been made such as to permit the measurement as nearly as possible of the
average temperature in the chamber. But this is at best a rough
approximation, and we must rely upon the assumption that while the
temperatures which are actually measured may not be the average
temperature, the fluctuations of the average temperature are parallel to
the fluctuations in the temperatures measured. Since every effort is
made to keep these fluctuations at a minimum, it is seen that the error
of this assumption is not as great as might appear at first sight.
However, the calculation of the residual amount of oxygen in the chamber
is dependent upon this assumption and hence any errors in the assumption
will affect noticeably the calculation of the residual oxygen.
Attempts to compare the determination of the oxygen by the exceedingly
accurate Sondén apparatus with that calculated after determining the
water-vapor and carbon dioxide, temperature and pressure of the air in
the chamber have thus far led to results which indicate one of three
things: (1) that there is not a homogeneous mixture; (2) that during the
time required for making residual analyses, _i. e._, some three or four
minutes, there may be a variation in the oxygen content in the air of
the chamber due to the oxygen continually added from the cylinder; (3)
that the oxygen supplied from the cylinder is not thoroughly mixed with
the air in the chamber until some time has elapsed. That is to say, with
the method now in use it is necessary to fill the tension-equalizer to a
definite pressure immediately at the end of each experimental period.
This is done by admitting oxygen from the cylinder, and obviously this
oxygen was not present in the air when analyzed. A series of experiments
with a somewhat differently arranged system is being planned in which
the oxygen will be admitted to the respiration chamber directly and not
into the tension-equalizer, and at the end of the experiment the
tension-equalizer will be kept at such a point that when the motor is
stopped the amount of oxygen to be added to bring the tension to a
definite point will be small.
Under these conditions it is hoped to secure a more satisfactory
comparison of the analyses as made by means of the Sondén apparatus and
as calculated from the composition of the residual air by the
gravimetric analysis. It remains a fact, however, that no matter with
what skill and care the gasometric analysis is made, either
gravimetrically or volumetrically, the calculation of the residual
amount of oxygen presents the same difficulties in both cases.
CALCULATION OF TOTAL OUTPUT OF CARBON DIOXIDE AND WATER-VAPOR AND OXYGEN
ABSORPTION.
From the weights of the sulphuric-acid and potash-lime vessels, the
amounts of water-vapor and carbon dioxide absorbed out of the
air-current are readily obtained. The loss in weight of the oxygen
cylinder increased by 0.4 per cent (see page 88) gives the weight of
oxygen admitted to the chamber. It remains, therefore, to make proper
allowance for the variations in composition of the air inside the
chamber at the beginning and end of the different periods. From the
residual sheets the amounts of water-vapor, carbonic acid, and oxygen
present in the system at the beginning and end of each period are
definitely known. If there is an increase, for example, in the amount of
carbon dioxide in the chamber at the end of a period, this increase must
be added to the amount absorbed out of the air-current in order to
obtain the true value for the amount produced during the experimental
period.
A similar calculation holds true with regard to the water-vapor and
oxygen. For convenience in calculating, the amounts of water-vapor and
carbon dioxide residual in the chamber are usually expressed in grams,
while the oxygen is expressed in liters. Hence, before making the
additions or subtractions from the amount of oxygen admitted, the
variations in the amount of oxygen residual in the system should be
converted from liters to grams. This is done by dividing by 0.7.