ature
conditions may easily be such that the air entering is warmer than the
outcoming air, in which case heat will be imparted to the calorimeter,
or the reverse conditions may obtain and then heat will be brought away.
To avoid this difficulty, arrangements are made for arbitrarily
controlling the temperature of the air as it enters the calorimeter.
This temperature control is based upon the fact that the air leaving the
chamber is caused to pass over the ends of a series of thermal junctions
shown as O in fig. 25. These thermal junctions have one terminal in the
outgoing air and the other in the ingoing air, and consequently any
difference in the temperature of the two air-currents is instantly
detected by connecting the circuit with the galvanometer. Formerly the
temperature control was made a varying one, by providing for either
cooling or heating the ingoing air as the situation called for. The
heating was done by passing the current through an electric lamp placed
in the cross immediately below the tension equalizer J. Cooling was
effected by means of a current of water through the lead pipe E closely
wrapped around the air-pipe, water entering at F and leaving at G. This
lead pipe is insulated by hair-felt pipe-covering, C. More recently, we
have adopted the procedure of passing a continuous current of water,
usually at a very slow rate, through the lead pipe E and always heating
the air somewhat by means of the lamp, the exact temperature control
being obtained by varying the heating effect of the lamp itself. This
has been found much more satisfactory than by alternating from the
cooling system to the heating system. In the case of the air-current,
however, it is unnecessary to have the drop-sight feed-valve as used for
the wall control, shown in fig. 13.

THE HEAT OF VAPORIZATION OF WATER.

During experiments with man not all the heat leaves the body by
radiation and conduction, since a part is required to vaporize the water
from the skin and lungs. An accurate measurement of the heat production
by man therefore required a knowledge of the amount of heat thus
vaporized. One of the great difficulties in the numerous forms of
calorimeters that have been used heretofore with man is that only that
portion of heat measured by direct radiation or conduction has been
measured and the difficulties attending the determination of water
vaporized have vitiated correspondingly the estimates of the heat
production. Fo