freezing point, its heat can hardly reach the middle cavity, being
arrested by the ice of the cover, Fig. 7. and of the external cavity;
but, if the temperature of the air be under the degree of freezing, it
might cool the ice contained in the middle cavity, by causing the ice
in the external cavity to fall, in the first place, below zero (32 ). It
is therefore essential that this experiment be carried on in a
temperature somewhat above freezing: Hence, in time of frost, the
calorimeter must be kept in an apartment carefully heated. It is
likewise necessary that the ice employed be not under zero (32 ); for
which purpose it must be pounded, and spread out thin for some time, in
a place of a higher temperature.
The ice of the interior cavity always retains a certain quantity of
water adhering to its surface, which may be supposed to belong to the
result of the experiment; but as, at the beginning of each experiment,
the ice is already saturated with as much water as it can contain, if
any of the water produced by the caloric should remain attached to the
ice, it is evident, that very nearly an equal quantity of what adhered
to it before the experiment must have run down into the vessel F in its
stead; for the inner surface of the ice 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 or 10 (52 or 54 ) 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 , or at most 4 ,
(39 to 41 ); 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.
235