machinery, or in ascending a
mountain, I apprehend that in proportion to the muscular effort put
forth for the purpose, a _diminution_ of the heat evolved in the system
by a given chemical action would be experienced.' The italics in this
memorable passage, written, it is to be remembered, in 1843, are Mr.
Joule's own.

The concluding paragraph of this British Association paper equally
illustrates his insight and precision, regarding the nature of
chemical and latent heat. 'I had,' he writes, 'endeavoured to prove
that when two atoms combine together, the heat evolved is exactly that
which would have been evolved by the electrical current due to the
chemical action taking place, and is therefore proportional to the
intensity of the chemical force causing the atoms to combine. I now
venture to state more explicitly, that it is not precisely the
attraction of affinity, but rather the mechanical force expended by
the atoms in falling towards one another, which determines the
intensity of the current, and, consequently, the quantity of heat
evolved; so that we have a simple hypothesis by which we may explain
why heat is evolved so freely in the combination of gases, and by
which indeed we may account "latent heat" as a mechanical power,
prepared for action, as a watch-spring is when wound up. Suppose, for
the sake of illustration, that 8 lbs. of oxygen and 1 lb. of hydrogen
were presented to one another in the gaseous state, and then exploded;
the heat evolved would be about 1 degree Fahr. in 60,000 lbs. of
water, indicating a mechanical force, expended in the combination,
equal to a weight of about 50,000,000 lbs. raised to the height of one
foot. Now if the oxygen and hydrogen could be presented to each other
in a liquid state, the heat of combination would be less than before,
because the atoms in combining would fall through less space.' No
words of mine are needed to point out the commanding grasp of
molecular physics, in their relation to the mechanical theory of heat,
implied by this statement.

Perfectly assured of the importance of the principle which his
experiments aimed at establishing, Mr. Joule did not rest content with
results presenting such discrepancies as those above referred to. He
resorted in 1844 to entirely new methods, and made elaborate
experiments on the thermal changes produced in air during its
expansion: firstly, against a pressure, and therefore performing work;
secondly, against no