112 
ON HEAT AS A MODE OF MOTION. 
“ Now from this our first vintage it follows, that the form or true definition of 
heat (heat, that is, in relation to the universe, not simply in relation to man) is in 
a few words as follows:—Heat is a motion, expansive, restrained, and acting in 
its strife upon the smaller particles of bodies. But the expansion is thus modi¬ 
fied ; while it expands all ways, it has at the same time an inclination upwards. 
And the struggle in the particles is modified also ; it is not sluggish, but hurried 
and with violence.” 
Not without reason, therefore, do we bow with reverence before the wisdom 
of the ancients. u To the scientific public” (writes Tyndall) “the names of 
the builders of this new philosophy are already familiar. As experimental con¬ 
tributors, Rumford, Davy, Faraday, and Joule stand prominently forward. 
As theoretic writers (placing them alphabetically) we have Clausius, Helmholtz, 
Kirchhoff, Mayer, Rankine, Thomson ; and in the memoirs of these eminent 
men the student who desires it must seek a deeper acquaintance with the sub¬ 
ject. MM. Regnault and Seguin also stand in honourable relationship to the 
Dynamical theory of heat, and M. Yerdet has recently published two lectures 
on it, marked by the learning for which he is conspicuous. To the English 
reader it is superfluous to mention the well-known and highly-prized work of Mr. 
Grove.” 
Passing by all preliminary dissertations, let us hasten to the vital question at 
issue. Heat can be, and is (for the fact is patent to the most casual observer) 
generated by mechanical action. Why should heat be generated by mechanical 
action, and what is the real nature of the agent thus generated? 
Two theories have been offered as an answer :— 
1. The Material theory, in which heat is supposed to be a kind of matter, a 
subtle fluid stored up in the inter-atomic spaces of bodies : thus Gmelin speaks 
of heat combining with bodies as one ponderable substance does with another ; 
and many other eminent chemists treat the subject from the same point of view. 
All substances possess in a greater or less degree an apparent power of storing 
up heat,—lead, for instance, in which heat is generated by compression. Adopting 
the material theory, the explanation is as follows :—the uncompressed lead was 
imagined to have a higher capacity for heat than the compressed substance ; the 
size of its atomic storehouse is diminished by compression, and hence, when the 
lead is squeezed, a portion of that heat which, previous to compression, was hidden, 
must make its appearance, for the compressed substance can no longer hold it 
all. Thus the idea of new heat being generated was rejected, the quantity of heat 
in the universe being considered as constant as that of ordinary matter, and thus 
the utmost we can do by mechanical and chemical means, is to store up this 
heat, or to drive it from its lurking-places into open light of day. 
2. The Dynamical or the Mechanical theory of heat, in which heat is not be¬ 
lieved to be matter, but an accident or condition of matter; namely, a motion 
of its ultimate particles, which is almost the verbal definition of Locke, 
who said, “ Heat is a very brisk agitation of the insensible parts of the object, 
which produce in us that sensation from whence we denominate the object hot: 
so what in our sensation is heat , in the object is nothing but motion .” 
Sir H. Davy (Works, vol. ii. p. 11) recounts a remarkable experiment. Ice 
is solid water, and the solid has only one-half the capacity for heat that liquid 
water possesses. A quantity of heat which would raise a pound of ice ten de¬ 
grees in temperature, would raise a pound of water only five degrees. Further, 
to simply liquefy a mass of ice, an enormous amount of heat is necessary, this 
heat being so utterly absorbed or rendered latent as to make no impression upon 
the thermometer: liquid water therefore at its freezing temperature, possesses a 
vastly greater amount of heat than ice at the same temperature. Davy reasoned 
thus:—“ If I, by friction, liquefy ice, I produce a substance tvhich contains a far 
greater absolute amount of heat than the ice ; and in this case, it cannot with 
