form of the power may be so changed, that an apparent conversion of
one into the other takes place. So we can change chemical force
into the electric current, or the current into chemical force.
The beautiful experiments of Seebeck and Peltier show the convertibility
of heat and electricity; and others by Oersted and myself show the
convertibility of electricity and magnetism. But in no case, not
even in those of the Gymnotus and Torpedo, is there a pure creation
or a production of power without a corresponding exhaustion of
something to supply it.'

These words were published more than two years before either Mayer
printed his brief but celebrated essay on the Forces of Inorganic
Nature, or Mr. Joule published his first famous experiments on the
Mechanical Value of Heat. They illustrate the fact that before any
great scientific principle receives distinct enunciation by
individuals, it dwells more or less clearly in the general
scientific mind. The intellectual plateau is already high, and our
discoverers are those who, like peaks above the plateau, rise a
little above the general level of thought at the time.

But many years prior even to the foregoing utterance of Faraday,
a similar argument had been employed. I quote here with equal
pleasure and admiration the following passage written by Dr. Roget
so far back as 1829. Speaking of the contact theory, he says:--
'If there could exist a power having the property ascribed to it by
the hypothesis, namely, that of giving continual impulse to a fluid
in one constant direction, without being exhausted by its own
action, it would differ essentially from all the known powers in
nature. All the powers and sources of motion with the operation of
which we are acquainted, when producing these peculiar effects, are