The residual air inside of the chamber amounts to some 1,300 liters and
contains about 250 liters of oxygen. Consequently it can be seen that in
an 8-hour experiment the subject could easily live during the entire
time upon the amount of oxygen already present in the residual air. It
has been repeatedly shown that until the per cent of oxygen falls to
about 11, or about one-half normal, there is no disturbance in the
respiratory exchange and therefore about 125 liters of oxygen would be
available for respiration even if no oxygen were admitted. Inasmuch as
the subject when at rest uses not far from 14 to 15 liters per hour, the
amount originally present in the chamber would easily suffice for an
8-hour experiment. Moreover, the difficulties attending an accurate gas
analysis and particularly the calculation of the total amount of oxygen
are such that satisfactory determinations of oxygen consumption by this
method would be impossible. Furthermore, from our previous experience
with long-continued experiments of from 10 days to 2 weeks, it has been
found that oxygen can be supplied to the system readily and the amount
thus supplied determined accurately. Consequently, even in these short
experiments, we adhere to the original practice of supplying oxygen to
the air and noting the amount thus added.
The oxygen supply was formerly obtained from small steel cylinders of
the highly compressed gas. This gas was made by the calcium-manganate
method and represented a high degree of purity for commercial oxygen.
More recently we have been using oxygen of great purity made from liquid
air. Inasmuch as this oxygen is very pure and much less expensive than
the chemically-prepared oxygen, extensive provisions have been made for
its continued use. Instead of using small cylinders containing 10 cubic
feet and attaching thereto purifying devices in the shape of soda-lime
U-tubes and a sulphuric-acid drying-tube, we now use large cylinders and
we have found that the oxygen from liquid air is practically free from
carbon dioxide and water-vapor, the quantities present being wholly
negligible in experiments such as these. Consequently, no purifying
attachments are considered necessary and the oxygen is delivered
directly from the cylinder. The cylinders, containing 100 cubic feet
(2,830 liters), under a pressure of 120 atmospheres, are provided with
well-closing valves and weigh when fully charged 57 kilograms.
[Illustration: FIG. 31.--Diagram of oxygen balance and cylinder. At the
top is the balance arrangement, and at the center its support. At the
left is the oxygen cylinder, with reducing valve A, rubber tube D
leading from it, F the electro-magnet which opens and closes D, K the
hanger of the cylinder and support for the magnet, R the lever which
operates the supports for the cylinder and its counterpoise S, T' a box
which is raised and lowered by R, and T its surrounding box.]
It is highly desirable to determine the oxygen to within 0.1 gram, and
we are fortunate in having a balance of the type used frequently in this
laboratory which will enable us to weigh this cylinder accurately with a
sensitiveness of less than 0.1 gram. Since 1 liter of oxygen weighs 1.43
grams, it can be seen that the amount of oxygen introduced into the
chamber can be measured by this method within 70 cubic centimeters.
Even in experiments of but an hour's duration, where the amount of
oxygen admitted from the cylinder is but 25 to 30 grams, it can be seen
that the error in the weighing of the oxygen is much less than 1 per
cent.
The earlier forms of cylinders used were provided with valves which
required some special control and a rubber bag was attached to provide
for any sudden rush of gas. The construction of the valve and valve-stem
was unfortunately such that the well-known reduction valves could not be
attached without leakage under the high pressure of 120 atmospheres.
With the type of cylinder at present in use, such leakage does not occur
and therefore we simply attach to the oxygen cylinder a reduction-valve
which reduces the pressure from 120 atmospheres to about 2 or 3 pounds
to the square inch. The cylinder, together with the reduction valve, is
suspended on one arm of the balance. The equipment of the arrangement is
shown in fig. 31. (See also fig. 5, page 4.) The cylinder is supported
by a clamp K hung from the balance arm, and the reduction-valve A is
shown at the top. The counterpoise S consists of a piece of 7-inch pipe,
with caps at each end. At a convenient height a wooden shelf with
slightly raised rim is attached.
In spite of the rigid construction of this balance, it would be
detrimental to allow this enormous weight to remain on the knife-edges
permanently, so provision is made for raising the cylinders on a small
elevator arrangement which consists of small boxes of wood, T, into
which telescope other boxes, T'. A lever handle, R, when pressed
forward, raises T' by means of a roller bearing U, and when the handle
is raised the total weight of the cylinders is supported on the
platforms.
The balance is attached to an upright I-beam which is anchored to the
floor and ceiling of the calorimeter laboratory. Two large turnbuckle
eye-bolts give still greater rigidity at the bottom. The whole apparatus
is inclosed in a glass case, shown in fig. 5.