not in use, a solid
rubber stopper on a brass rod is drawn into this opening, thus
producing an air-tight closure. When in actual use during the process of
weighing, a thin rubber diaphragm prevents leakage of air through this
opening. The escape of heat through the weighing-tube is minimized by
having this tube of hard rubber.

VENTILATING AIR-CURRENT.

[Illustration: FIG. 27.--Diagram of ventilation of respiration
calorimeter. The air is taken out at lower right-hand corner and forced
by the blower through the apparatus for absorbing water and carbon
dioxide. It returns to the calorimeter at the top. Oxygen can be
introduced into the chamber itself as need is shown by the tension
equalizer.]

The ventilating air-current is so adjusted that the air which leaves the
chamber is caused to pass through purifiers, where the water-vapor and
the carbon dioxide are removed, and then, after being replenished with
fresh oxygen, it is returned to the chamber ready for use. The general
scheme of the respiration apparatus is shown in fig. 27. The air leaving
the chamber contains carbon dioxide and water-vapor and the original
amount of nitrogen and is somewhat deficient in oxygen. In order to
purify the air it must be passed through absorbents for carbonic acid
and water-vapor and hence some pressure is necessary to force the gas
through these purifying vessels. This pressure is obtained by a small
positive rotary blower, which has been described previously in
detail.[18] The air is thus forced successively through sulphuric acid,
soda or potash-lime, and again sulphuric acid. Finally it is directed
back to the respiration chamber free from carbon dioxide and water and
deficient in oxygen. Pure oxygen is admitted to the chamber to make up
the deficiency, and the air thus regenerated is breathed again by the
subject.

BLOWER.

The rotary blower used in these experiments for maintaining the
ventilating current of air has given the greatest satisfaction. It is a
so-called positive blower and capable of producing at the outlet
considerable pressure and at the inlet a vacuum of several inches of
mercury. At a speed of 230 revolutions per minute it delivers the air at
a pressure of 43 millimeters of mercury, forcing it through the
purifying vessels at the rate of 75 liters per minute. This rate of
ventilation has been established as being satisfactory for all
experiments and is constant. Under the pressure of 43 millimeters o