teel
cylinder is made at just such a rate as to compensate for the decrease
in volume of the air in the system due to the absorption of oxygen by
the subject, it can be seen that if the exact volume of the gas leaving
the cylinder were known it would be immaterial whether this gas were
pure oxygen, oxygen with some nitrogen, or oxygen with any other inert
gas not dangerous to respiration or not absorbed by sulphuric acid or
potash-lime. If 10 liters of oxygen had been absorbed by the man in the
course of an hour, to bring the system back to constant apparent volume
it would be necessary to admit 10 liters of such a gas or mixture of
gases, assuming that during the hour there had been no change in the
temperature, the barometric pressure, or the residual amounts of carbon
dioxide or water-vapor.

Under these assumed conditions, then, it would only be necessary to
measure the amount of gas admitted in order to have a true measure of
the amount of oxygen absorbed. The measure of the volume of the gas
admitted may be used for a measure of the oxygen absorbed, even when it
is necessary to make allowances for the variations in the amount of
carbon dioxide or water-vapor in the chamber, the temperature, and
barometric pressure. From the loss in weight of the oxygen cylinder, if
the cylinder contained pure oxygen, it would be known that 10 liters
would be admitted for every 14.3 grams loss in weight.

From the difference in weight of 1 liter of oxygen and 1 liter of
nitrogen, a loss in weight of a gas containing a mixture of oxygen with
a small per cent of nitrogen would actually represent a somewhat larger
volume of gas than if pure oxygen were admitted. The differences in
weight of the two gases, however, and the amount of nitrogen present are
so small that one might almost wholly neglect the error thus arising
from this admixture of nitrogen and compute the volume of oxygen
directly from the loss in weight of the cylinder.

As a matter of fact, it has been found that by increasing the loss in
weight of the cylinder of oxygen containing 3 per cent nitrogen by 0.4
per cent and then converting this weight to volume by multiplying by
0.7, the volume of gas admitted is known with great accuracy. This
method of calculation has been used with success in connection with the
large chamber and particularly for experiments of short duration. It has
also been introduced with great success in a portable type of apparatus
described els