With an apparatus as elaborate as is the respiration calorimeter and its
accessories, the calculation of results presents many difficulties, but
the experience of the past few years has enabled us to lessen materially
the intricacies of the calculations formerly thought necessary.
The total amount of water-vapor leaving the chamber is determined by
noting the increase in weight of the first sulphuric-acid vessel in the
absorber system. This vessel is weighed with a counterpoise and hence
only the increment in weight is recorded. A slight correction may be
necessary here, as frequently the absorber is considerably warmer at the
end of the period than at the beginning and if weighed while warm there
may be an error of 0.1 to 0.2 gram. If the absorbers are weighed at the
same temperature at the beginning and end, this correction is avoided.
The amount of carbon dioxide absorbed from the ventilating air-current
is found by noting the changes in weight of the potash-lime can and the
last sulphuric-acid vessel. As shown by the weights of this latter
vessel, it is very rare that sufficient water is carried over from the
potash-lime to the sulphuric acid to cause a perceptible change in
temperature, and no temperature corrections are necessary. It may
occasionally happen that the amount of carbon dioxide absorbed is
actually somewhat less than the amount of water-vapor abstracted from
the reagent by the dry air-current as it passes through the can. The
conditions will then be such that there will be a loss in weight of the
potash-lime can and a large gain in weight of the sulphuric-acid vessel.
Obviously, the algebraic sum of these amounts will give the true weight
of the carbon dioxide absorbed.
The amount of oxygen admitted is approximately measured by noting the
loss in weight of the oxygen cylinder. Since, however, in admitting the
oxygen from the cylinder there is a simultaneous admission of a small
amount of nitrogen, a correction is necessary. This correction can be
computed either by the elaborate formulas described in the publication
of Atwater and Benedict[24] or by the more abbreviated method of
calculation which has been used very successfully in all short
experiments in this laboratory. In either case it is necessary to know
the approximate percentage of nitrogen in the oxygen.