To maintain the apparent volume of air through the whole system
constant, oxygen is admitted into the tension-equalizer until the same
tension is exerted on this part of the system at the end as at the
beginning. This is done by closing the valve connecting the
tension-equalizer with the system and admitting oxygen to the
tension-equalizer until the petroleum manometer shows a definite
tension. After the motor is stopped, at the end of the experimental
period, there is a small amount of air compressed in the blower which
almost instantly leaks back through the blower and the whole system
comes under atmospheric pressure, save that portion which is sealed off
between the two levels of the sulphuric acid in the two absorbing
vessels. A few seconds after the motor is stopped the valve cutting off
the tension-equalizer from the rest of the system is closed, the
pet-cock connecting this with the petroleum manometer is opened, and
oxygen is admitted by short-circuiting the electrical connections at the
two mercury cups. This is done by the hands of the observer and must be
performed very gently and carefully, as otherwise oxygen will rush in so
rapidly as to cause excessive tension. As the bag fills with gas, the
index on the petroleum manometer moves along the arc of a circle and
gradually reaches the desired point. At this point, the supply of oxygen
is cut off, the valve connecting the tension-equalizer with the main
system is opened, and simultaneously the needle-valve on the
reduction-valve of the oxygen cylinder is tightly closed, preliminary to
weighing the cylinder. At this point the motor can be started and the
experiment continued.
It is necessary, then, that the oxygen cylinder be weighed. This is done
after first closing the pet-cock on the end of the pipe conducting the
gas beneath the floor of the calorimeter room, slipping the glass joint
in the rubber pipe leading from the reduction valve to the pet-cock, and
breaking the connections between the two rubber pipes, the one from the
pet-cock and the other to the reduction valve, also breaking the
electrical connection leading to the magnet on the cylinder. The
cylinder is then ready to swing freely without any connections to either
oxygen pipe or electrical wires. It is then weighed, the loss in weight
being noted by removing the brass weights on the shelf attached to the
counterpoise. It is important to see that there is a sufficient number
of brass weights always on the shelf to allow for a maximum loss of
weight of oxygen from the cylinder during a given period. Since the
cylinders contain not far from 4 to 5 kilograms of oxygen, in balancing
the cylinders at the start it is customary to place at least 4 kilograms
of brass weights on the shelf and then adjust the counterpoise so as to
allow for the gradual removal of these weights as the oxygen is
withdrawn.
As soon after the beginning of the period as possible, the U-tubes are
weighed on the analytical balance, and if they have not gained too much
they are connected ready for the next analysis. If they have already
absorbed too much water or carbon dioxide, they are replaced by freshly
filled tubes.
Immediately at the end of the experimental period the barometer is
carefully set and read, and the reading is verified by another
assistant. Throughout the whole experiment an assistant counts the pulse
of the subject frequently, by means of the stethoscope, and records the
respiration rate by noting the lesser fluctuations of the tambour
pointer on the smoked paper. These observations are recorded every few
minutes in a book kept especially for this purpose.
A most excellent preservation of the record of the minor muscular
movements is obtained by dipping the smoked paper on the kymograph drum
in a solution of resin and alcohol. The lesser movements on the paper
indicate the respiration rate, but every minor muscular movement, such
as moving the arm or shifting the body in any way, is shown by a large
deflection of the pointer out of the regular zone of vibration. These
records of the minor muscular activity are of great importance in
interpreting the results of the chemical and physical determinations.