esidual 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.--Diagra