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of Edinburgh, Session 1862 - 63 . 
curing perpetual motion (i.e., a machine which not only keeps up 
its motion but does external work) by means of any of the known 
forces of nature ; and vice versa, taking this impossibility for 
granted, we may show that the forces exerted by two material 
particles on each other must be in the direction of the line joining 
the two, and must depend on their distance only. 
The first of the physical energies, distinct from visible motions, 
which was shown to be subject to the law of “conservation,” was 
Heat. Bacon, Locke, and others, long ago regarded heat in a 
material body as a species of motion ; but it was not proved to be 
so till a comparatively recent period, when Davy showed it conclu- 
sively by melting pieces of ice by rubbing them together in an 
enclosure cooled below the freezing point, Davy says, “ The 
immediate cause of the phenomenon of Heat is motion, and the 
laws of its communication are the same as the laws of the com- 
munication of motion.” Take, in connection with this, Newton’s 
second form of Action and Eeaction, and we have the Dynamical 
Theory of Heat ; requiring, of course, experimental data to connect 
the two forms of Energy quantitatively. Kumford, by measuring 
the heat produced in boring cannon, and comparing it with the 
work expended, made a near approach to the value of the mechani- 
cal equivaleut of heat — i.e., to an answer to the question, “How 
much work is required to produce a given amount of kinetic energy 
in the form of heat?” Other thinkers and experimenters made 
more or less accurate and useful advances, but in a very small way, 
till Joule, about twenty years ago, made the experimental treat- 
ment of the subject his own. He showed by varied yet accordant 
experiments, that 772 foot pounds of mechanical energy are equi- 
valent to the additional kinetic energy which a pound of water 
must acquire to raise its temperature from 60° F. to 61° E. He 
has extended his experimental work to others of the physical ener- 
gies, and arrived at many most startling results, several of which 
I intend to show to-night. 
The science of Thermodynamics, in which Carnot and Clapeyron 
made great steps before the immateriality of heat was generally 
recognised, has, since Joule’s experiments were made, received 
enormous developments from Clausius, Eankine, Thomson, and 
others; and Helmholz, in an admirable essay (JJeher die Erhaltung 
VOL V. 
Q 
