The carbon-dioxide production, water-vapor elimination, and oxygen
absorption of the subject during 1 or 2 hour periods are recorded in a
general way by the amounts of carbon dioxide and water-vapor absorbed by
the purifying vessels and the loss of weight of the oxygen cylinder;
but, as a matter of fact, there may be considerable fluctuations in the
amounts of carbon dioxide and water-vapor and particularly oxygen in the
large volume of residual air inside the chamber. With carbon dioxide and
water-vapor this is not as noticeable as with oxygen, for in the 1,300
liters of air in the chamber there are some 250 liters of oxygen, and
slight changes in the composition of this air indicate considerable
changes in the amount of oxygen. Great changes may also take place in
the amounts of carbon dioxide and water-vapor under certain conditions.
In some experiments, particularly where there are variations in muscular
activity from period to period, there may be a considerable amount of
carbon dioxide in the residual air and during the next period, when the
muscular activity is decreased, for example, the percentage composition
of the air may vary so much as to indicate a distinct fall in the amount
of carbon dioxide present. Under ordinary conditions of ventilation
during rest experiments the quantity of carbon dioxide present in the
residual air is not far from 8 to 10 grams. There are usually present in
the air not far from 6 to 9 grams of water-vapor, and hence this
residual amount can undergo considerable fluctuations. When it is
considered that an attempt is made to measure the total amount of carbon
dioxide expired in one hour to the fraction of a gram, it is obvious
that fluctuations in the composition of residual air must be taken into
consideration.
It is extremely difficult to get a fair sample of air from the chamber.
The air entering the chamber is free from water-vapor and carbon
dioxide. In the immediate vicinity of the entering air-tube there is air
which has a much lower percentage of carbon dioxide and water-vapor than
the average, and on the other hand close to the nose and mouth of the
subject there is air of a much higher percentage of carbon dioxide and
water-vapor than the average. It has been assumed that the composition
of the air leaving the chamber represents the average composition of the
air in the chamber. This assumption is only in part true, but in rest
experiments (and by far the largest number of experiments are rest
experiments) the changes in the composition of the residual air are so
slow and so small that this assumption is safe for all practical
purposes.
Another difficulty presents itself in the matter of determining the
amount of carbon dioxide and water-vapor; that is, to make a
satisfactory analysis of air without withdrawing too great a volume from
the chamber. The difficulty in analysis is almost wholly confined to the
determination of water-vapor, for while there are a large number of
methods for determining small amounts of carbon dioxide with great
accuracy, the method for determining water-vapor to be accurate calls
for the use of rather large quantities of air. From preliminary
experiments with a sling psychrometer it was found that its use was
precluded by the space required to successfully use this instrument, the
addition of an unknown amount of water to the chamber from the wet bulb,
and the difficulties of reading the instrument from without the chamber.
Recourse was had to the determination of moisture by the absolute
method, in that a definite amount of air is caused to pass over
pumice-stone saturated with sulphuric acid. It is of interest here to
record that at the moment of writing a series of experiments are in
progress in which an attempt is being made to use a hair hygrometer for
this purpose.
The method of determining the water-vapor and carbon dioxide in the
residual air is extremely simple, in that a definite volume of air is
caused to pass over sulphuric acid and soda-lime contained in U-tubes.
In other words, a small amount of air is caused to pass through a small
absorbing-system constructed of U-tubes rather than of porcelain vessels
and silver-plated cans. Formerly a very elaborate apparatus was employed
for aspirating the air from the chamber through U-tubes and then
returning the aspirated air to the chamber. This involved the use of a
suction-pump and called for a special installation for maintaining the
pressure of water constant. More recently a much simpler device has been
employed, in that we have taken advantage of the pressure in the
ventilating air-system developed by the passage of air through the
blower. After forcing a definite quantity of air through the reagents in
the U-tubes, it is then conducted back to the system after having been
measured in a gas-meter.
This procedure is best noted from fig. 30. The connected series of three
U-tubes on the rack on the table is joined on one end by well-fitting
rubber connections to the tube leading from the mercurial manometer and
on the other end to the rubber tube A leading to the gas-meter. On
lowering the mercury reservoir E, the mercury is drained out of the tube
D and air passes through both arms of the tube and then through the
three U-tubes. In the first of these it is deprived of moisture, and in
the last two of carbon dioxide. The air then enters the meter, where it
is measured and leaves the meter through the tube B, saturated with
water-vapor at the room temperature. To remove this water-vapor the air
is passed through a tower filled with pumice-stone drenched with
sulphuric acid. It leaves the tower through the tube C and enters the
ventilating air-pipe on its way to the calorimeter.
The method of manipulation is very simple. After connecting the U-tubes
the pet-cock connecting the tube C with the pipe is opened, the mercury
reservoir E is lowered, and air is allowed to pass through until the
meter registers 10 liters. By raising the reservoir E the air supply is
shut off, and after closing the stop-cock at C the tubes are
disconnected, a second set is put in place, and the operation repeated.
The U-tubes are of a size having a total length of the glass portion
equal to 270 millimeters and an internal diameter of 16 millimeters.
They permit the passage of 3 liters of air per minute through them
without a noticeable escape of water-vapor or carbon dioxide. The
U-tubes filled with pumice-stone and sulphuric acid weigh 90 grams. They
are always weighed on the balance with a counterpoise, but no attempt is
made to weigh them closer than to 0.5 milligram.