38 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
Newton in one of his celebrated letters to Bentley, has justly said, 
“That one body may act upon another at a distance, through a 
vacuum without the mediation of anything else by and through which 
their action may be conveyed from one to another, is so great an 
absurdity, that no man, who has in philosophical matters, a competent 
faculty of thinking, can ever fall into it.” From this it inevitably 
follows, that no body, or system of bodies can possess energy merely 
by virtue of its position, in other words by virtue of the distances of 
its parts from all other bodies. In this sense, therefore, potential 
energy involves a contradiction in terms. 
But if we regard potential energy as a convenient name for those 
kinds of energy whose nature is not yet understood, the term is con- 
venient and admissible, though liable to create considerable confusion. 
There are not therefore two distinct kinds of energy—energy of 
motion, and energy of position. The distinction can, in the nature 
of things, have no possible fundamental difference for its basis. But 
energy may be conveniently divided into two classes, namely, energy 
whose nature we in some measure understand—called kinetic—and 
energy—known on the other hand as potential—of whose nature we 
know comparatively little, but which we regard as dependent on 
position, not that this dependence is an ultimate physical fact, but 
because it is a secondary or conventional mark, which, in the absence 
of more definite knowledge, it is convenient to adopt. 
Heat then, being beyond doubt, a form of energy, it is important 
to determine in what forms of matter the heat energy resides, whether 
for instance, in heated bodies, the vibrations, by virtue of which the 
bodies are said to be hot are vibrations of the atoms or of the molecules. 
Notwithstanding the high authority of Tyndall to the contrary, 
there is good reason to suppose that heat properly so called, consists 
exclusively in molecular motion. To make out the probability of this 
apparently bold assertion, it is necessary to investigate the real nature 
of what is most erroneously called radiant heat, but which possesses 
no more of the characteristic qualities of heat than the motion of a 
hammer about to strike an anvil. Tyndall himself has conclusively ~ 
proved, not only that radiant heat is not matter as is confusingly 
suggested by the origin of the phrase, but what is more to the point, 
that it is nothing more or nothing less than a wave motion of the 
luminiferous ether, which prevades not only all interstellar, but also 
intermolecular and interatomic space. 
