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. Fortunately, with this apparatus the determinations of water
are very exact, and since the amount of water vaporized inside the
chamber is known it is possible to compute the heat required to vaporize
this water by knowing the heat of vaporization of water.
Since the earlier reports describing the first form of calorimeters were
written, there has appeared a research by one of our former associates,
Dr. A. W. Smith[11] who, recognizing the importance of knowing exactly
the heat of vaporization of water at 20°, has made this a special object
of investigation. When connected with our laboratory a number of
experiments were made by Doctors Smith and Benedict in an attempt to
determine the heat of vaporization of water directly in a large
calorimeter; but for lack of time and pressure of other experimental
work it was impossible to complete the investigation. Subsequently Dr.
Smith has carried out the experiments with the accuracy of exact
physical measurements and has given us a very valuable series of
observations.
Using the method of expressing the heat of vaporization in electrical
units, Smith concludes that the heat of vaporization of water between
14° and 40° is given by the formula