By the expression _specific caloric_ of bodies, we understand the
respective quantities of caloric requisite for raising a number of
bodies of the same weight to an equal degree of temperature. This
proportional quantity of caloric depends upon the distance between the
constituent particles of bodies, and their greater or lesser degrees of
cohesion; and this distance, or rather the space or void resulting from
it, is, as I have already observed, called the _capacity of bodies for
containing caloric_.
_Heat_, considered as a sensation, or, in other words, sensible heat, is
only the effect produced upon our sentient organs, by the motion or
passage of caloric, disengaged from the surrounding bodies. In general,
we receive impressions only in consequence of motion, and we might
establish it as an axiom, _That_, WITHOUT MOTION, THERE IS NO SENSATION.
This general principle applies very accurately to the sensations of heat
and cold: When we touch a cold body, the caloric which always tends to
become in equilibrio in all bodies, passes from our hand into the body
we touch, which gives us the feeling or sensation of cold. The direct
contrary happens, when we touch a warm body, the caloric then passing
from the body into our hand, produces the sensation of heat. If the hand
and the body touched be of the same temperature, or very nearly so, we
receive no impression, either of heat or cold, because there is no
motion or passage of caloric; and thus no sensation can take place,
without some correspondent motion to occasion it.
When the thermometer rises, it shows, that free caloric is entering into
the surrounding bodies: The thermometer, which is one of these, receives
its share in proportion to its mass, and to the capacity which it
possesses for containing caloric. The change therefore which takes place
upon the thermometer, only announces a change of place of the caloric
in those bodies, of which the thermometer forms one part; it only
indicates the portion of caloric received, without being a measure of
the whole quantity disengaged, displaced, or absorbed.
The most simple and most exact method for determining this latter point,
is that described by Mr de la Place, in the Memoirs of the Academy, No.
1780, p. 364; a summary explanation of which will be found towards the
conclusion of this work. This method consists in placing a body, or a
combination of bodies, from which caloric is disengaging, in the midst
of a hollow sphere of ice; and the quantity of ice melted becomes an
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