THE COPLEY MEDALIST OF 1871. 333 
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, "endeavored to prove that when two atoms com- 
bine 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 me- 
chanical force expended by the atoms in falling toward 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 me- 
chanical power, prepared for action, as a watch-spring is 
when wound up. Suppose, for the sake of illustration, that 
8 Ibs. of oxygen and 1 Ib. of hydrogen were presented to 
one another in tjie gaseous state, and then exploded; the 
heat evolved would be about 1 degree Fahr. in 60,000 Ibs. 
of water, indicating a mechanical force, expended in the 
combination, equal to a weight of about 50,000,000 Ibs. 
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 mechan- 
ical 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 pressure, and therefore performing no 
work. He thus established anew the relation between the 
heat consumed and the work done. From five different 
series of experiments he deduced five different mechanical 
