e in the middle cavity is very
little changed during the experiment.

By any contrivance that could be devised, we could not prevent the
access of the external air into the interior cavity when the atmosphere
was 9 deg. or 10 deg. (52 deg. or 54 deg.) above zero. The air confined
in the cavity being in that case specifically heavier than the external
air, escapes downwards through the pipe x y, Fig. 3, and is replaced by
the warmer external air, which, giving out its caloric to the ice,
becomes heavier, and sinks in its turn; thus a current of air is formed
through the machine, which is the more rapid in proportion as the
external air exceeds the internal in temperature. This current of warm
air must melt a part of the ice, and injure the accuracy of the
experiment: We may, in a great degree, guard against this source of
error by keeping the stop-cock u continually shut; but it is better to
operate only when the temperature of the external air does not exceed
3 deg., or at most 4 deg., (39 deg. to 41 deg.); for we have observed,
that, in this case, the melting of the interior ice by the atmospheric
air is perfectly insensible; so that we may answer for the accuracy of
our experiments upon the specific heat of bodies to a fortieth part.

We have caused make two of the above described machines; one, which is
intended for such experiments as do not require the interior air to be
renewed, is precisely formed according to the description here given;
the other, which answers for experiments upon combustion, respiration,
&c. in which fresh quantities of air are indispensibly necessary,
differs from the former in having two small tubes in the two lids, by
which a current of atmospheric air may be blown into the interior cavity
of the machine.

It is extremely easy, with this apparatus, to determine the phenomena
which occur in operations where caloric is either disengaged or
absorbed. If we wish, for instance, to ascertain the quantity of caloric
which is disengaged from a solid body in cooling a certain number of
degrees, let its temperature be raised to 80 deg. (212 deg.); it is then
placed in the interior cavity f f f f, Fig. 2. and 3. of the calorimeter,
and allowed to remain till we are certain that its temperature is reduced
to zero (32 deg.); the water produced by melting the ice during its
cooling is collected, and carefully weighed; and this weight, divided
by the volume of the body submitted to experiment, multiplie