1876.] 
555 
The British Association . 
or living force, had been erroneously applied. As instances of ordinary kinetic 
energy, or of mixed kinetic and potential energies, we might take the follow- 
ing: — A current of water capable of driving an undershot wheel; winds, 
which also were used for driving machinery ; the energy of water-waves or of 
sound waves; the radiant energy which comes to us from the sun, whether it 
affedted our nerves of touch or of sight (and therefore be called radiant heat 
or light) or produced chemical decomposition, as of carbonic acid and water in 
the leaves of plants, or of silver salts in photography (and be therefore called 
adtinism) ; the energy of motion of the particles of a gas, upon which its 
pressure depends, &c. When the motion is vibratory the energy is generally 
half potential, half kinetic. These explanations and definitions being pre- 
mised, we could translate Newton’s words into the language of modern 
science, as follows : — 
“ Work done on any system of bodies (in Newton’s statement the parts of 
any machine) has its equivalent in work done against friction , molecular forces, 
or gravity , if there be no acceleration ; blit if there be acceleration , part of the 
work is expended in overcoming the resistance to acceleration , and the additional 
kinetic energy developed is equivalent to the work so spent.''' 
But we had just seen that when work was spent against molecular forces, as 
in drawing a bow or winding up a spring, it was stored up as potential energy. 
Also it was stored up in a similar form when done against gravity, as in raising 
a weight. Hence it appeared that, according to Newton, whenever work is 
spent it is stored up either as potential or as kinetic energy, except, possibly, 
in the case of work done against fridtion, about whose fate he gave us no 
information. Thus Newton expressly told us that, except, possibly, when 
there is fridtion, work is indestrudtible, it is changed from one form of energy 
to another, and so on, but never altered in quantity. To make this beautiful 
statement complete, all that was requisite was to know what became of work 
spent against fridtion. Here experiment was requisite. Newton, unfortu- 
nately, seemed to have forgotten that savage men had long since been in the 
habit of making it whenever they wished to procure fire. The patient rubbing 
of two dry sticks together, or the drilling of a soft piece of wood with the 
slightly blunted point of a hard piece, was known to all tribes of savages as a 
means of setting both pieces of wood on fire. Here, then, heat was un- 
doubtedly produced, but it was produced by the expenditure of work. In fact 
work done against fridtion had its equivalent in the heat produced. This 
Newton failed to see, and thus his grand generalisation was left, though on 
one point only, incomplete. The converse transformation, that of heat into 
work, dated back to the time of Hero at least. But the knowledge that a 
certain process would produce a certain result did not necessarily imply even 
a notion of the “ why ;” and Hero as little imagined that in his seolipile heat 
was converted into work, as did savages that work could be converted into 
heat. But whenever any such conversion or transference took place there was 
necessarily motion : and the mere rate of conversion or transference of energy 
per unit length of that motion was in the present state of science conveniently 
called force. No confusion could arise from using such a word in such a 
sense. Rumford and Davy showed conclusively that the materiality of heat 
could not be maintained, and thus gave the means of completing Newton’s 
statement which, still farther extended and generalised by Colding and Joule, 
was now known as the law of the conservation of energy. The conception of 
kinetic energy was a very simple one, when visible motion alone was involved. 
And from motion of visible masses to those motions of the particles of bodies 
whose energy we call heat, was by no means a difficult mental transition. 
Heat and kinetic energy in general were no more “ modes of motion ” than 
potential energy of every kind, including that of unfired gunpowder, was a 
“ mode of rest !” In fa dt a “ mode of motion ” was, if the word motion be 
used in its ordinary sense, purely kinematical, not physical ; and if motion were 
used in Newton’s sense, it referred to momentum, not to energy. The concep- 
tion of potential energy, however, was not by any means so easy or diredt. 
In fadt, the apparently diredt testimony of our muscular sense to the existence 
of force made it at first much easier for us to c onceive of force than of pote 
