It is hardly necessary to add that anything which any
_insulated_ body or system of bodies can continue to furnish
_without limitation_ cannot possibly be _a material substance_;
and it appears to me to be extremely difficult, if not quite
impossible, to form any distinct idea of anything capable of
being excited and communicated in the manner the heat was
excited and communicated in these experiments, except it be
MOTION.
It has been stated that, if Rumford had dissolved in acid the borings
and the sawn strips of metal, the capacity for heat of which he
determined, and had shown that the heat developed in the solution was
the same in the two cases, his chain of argument would have been
absolutely complete. Considering the amount of heat produced in the
experiments, there are few minds whose conviction would be
strengthened by this experiment, and it is only those who look for
faultless logic that will refuse to Rumford the credit of having
established the dynamical nature of heat.
Davy afterwards showed that two pieces of ice could be melted by being
rubbed against one another in a vacuum, but he does not appear to have
made as much as he might of the experiment. Mayer calculated the
mechanical equivalent of heat from the heat developed in the
compression of air, but he _assumed_, what afterwards was shown by
Joule to be nearly true, that the whole of the work done in the
compression was converted into heat. It was Joule, however, who first
showed that heat and mechanical energy are mutually convertible, so
that each may be expressed in terms of the other, a _given_ quantity
of heat always corresponding to the _same amount_ of mechanical
energy, whatever may be the intermediate stages through which it
passes, and that we may therefore define the mechanical equivalent of
heat as _the number of units of energy which, when entirely converted
into heat, will raise unit mass of water one degree from the freezing
point_.