The rigid walls of the calorimeter and piping necessitate some provision
for minor fluctuations in the absolute volume of air in the confined
system. The apparatus was not constructed to withstand great
fluctuations in pressure, and thin walls were used, but it is deemed
inadvisable to submit it even to minor pressures, as thus there would be
danger of leakage of air through any possible small opening.
Furthermore, as the carbon dioxide and water-vapor are absorbed out of
the air-current, there is a constant decrease in volume, which is
ordinarily compensated by the admission of oxygen. It would be very
difficult to adjust the admission of oxygen so as to exactly compensate
for the contraction in volume caused by the absorption of water-vapor
and carbon dioxide. Consequently it is necessary to adjust some portion
of the circulating air-current so that there may be a contraction and
expansion in the volume without producing a pressure on the system. This
was done in a manner similar to that described in the earlier apparatus,
but on a much simpler plan.
To the air-pipe just before it entered the calorimeter was attached a
copper can with a rubber diaphragm top. This diaphragm, which is, as a
matter of fact, a ladies' pure rubber bathing-cap, allows for an
expansion or contraction of air in the system of 2 to 3 liters. The
apparatus shown in position is to be seen in fig. 25, in which the tin
can I is covered with the rubber diaphragm J. If there is any change in
volume, therefore, the rubber diaphragm rises or falls with it and under
ordinary conditions of an experiment this arrangement results in a
pressure in the chamber approximately that of the atmosphere. It was
found, however, that even the slight resistance of the piping from the
tension-equalizer to the chamber, a pipe some 26 millimeters in diameter
and 60 centimeters long, was sufficient to cause a slightly diminished
pressure inside the calorimeter, inasmuch as the air was sucked out by
the blower with a little greater speed than it was forced in by the
pressure at the diaphragm. Accordingly the apparatus has been modified
so that at present the tension-equalizer is attached directly to the
wall of the calorimeter independent of the air-pipe.
In most of the experiments made thus far it has been our custom to
conduct the supply of fresh oxygen through pet-cock K on the side of the
tension-equalizer. This is shown more in detail in fig. 32, in which,
also, is shown the interior construction of the can. Owing to the fact
that the air inside of this can is much dryer than the room air, we have
followed the custom with the earlier apparatus of placing a vessel
containing sulphuric acid inside the tension-equalizer, so that any
moisture absorbed by the dry air inside the diaphragm may be taken up by
the acid and not be carried into the chamber. The air passing through
the pipe to the calorimeter is, it must be remembered, absolutely dry
and hence there are the best conditions for the passage of moisture from
the outside air through the diaphragm to this dry air. Attaching the
tension-equalizer directly to the calorimeter obviates the necessity for
this drying process and hence the sulphuric-acid vessel has been
discarded.
The valve H (fig. 25) is used to cut off the tension-equalizer
completely from the rest of the system at the exact moment of the end of
the experimental period. After the motor has been stopped and the slight
amount of air partly compressed in the blower has leaked back into the
system, and the whole system is momentarily at equal tension, a process
occupying some 3 or 4 seconds, the gate-valve H is closed. Oxygen is
then admitted from the pet-cock K until there is a definite volume in J
as measured by the height to which the diaphragm can rise or a second
pet-cock is connected to the can I and a delicate petroleum manometer
attached in such a manner that the diaphragm can be filled to exactly
the same tension each time. Under these conditions, therefore,
the apparent volume of air in the system, exclusive of the
tension-equalizer, is always the same, since it is confined by the rigid
walls of the calorimeter and the piping. Furthermore, the apparent
volume of air in the tension-equalizer is arbitrarily adjusted to be the
same amount at the end of each period by closing the valve and
introducing oxygen until the tension is the same.