118 
ON HEAT AS A MODE OF MOTION. 
“HEAT AS A MODE OE MOTION. 
“I trust you will permit me to make a few remarks in connection with the brief review 
of my lectures on heat which appeared in your last number. It is quite possible that 
my mode of treating the subject may be open to even graver criticism than that which 
your reviewer has bestowed upon it, but I should be extremely sorry if ail alleged per¬ 
sonal defect on my part should stand between the readers of the ‘ Athenaeum ’ and a 
generalization which they ought to be among the earliest to understand and appreciate. 
The dynamical theory of heat, forming as it does the most important part of the theory 
of the interaction of natural forces, is deemed by the most competent authorities of this 
age fit to stand, in point of importance, beside the Newtonian theory of gravitation. It 
is held without misgiving, by the leading natural philosophers of all the countries of 
Europe. Had the theory still to be discussed, I should never have presumed to bring it 
before the public as I have done. But proofs of it of the most varied and conclusive 
character have been accumula+ing for the last twenty years, until they have at last dis¬ 
armed all opposition. This theory must not only be the future guide of the practical 
engineer in his application of heat as a motive power, but in its more purely intellectual 
bearings the theory must profoundly affect the whole course of philosophic thought and 
inquiry. It, moreover, opens up views of vital phenomena, and of the organization of 
the material universe, which cannot be regarded with indifference by any thinking man ; 
and it is for these reasons that I have endeavoured, as far as in me lay, to divest the sub¬ 
ject, without injuring it, of technical difficulties, and thus to render it accessible to the 
intellectual public of England. 
“ The passage which your reviewer has done me the honour to quote is the most highly 
coloured in the book. It reads, indeed, more like romance than science; and nothing is 
more natural than that it should stagger those who have not closely followed the deve¬ 
lopments of modern physics. It may be, and probably is, open to the charge of unne¬ 
cessary iteration, but certainly no coolness or calmness on my part could add to the sub¬ 
stantial truth of the statements therein contained. When, for example, I say that ‘ every 
tire that burns and every lamp that glows dispenses light and heat which originally be¬ 
longed to the sun,’ I mean to express a fact , and not a figure of speech. The heat of 
every fire in London is as much a part of the sun’s heat as if every glowing coal had 
been taken by a tongs from the body of the sun and put into our grates ; and the sun’s 
heat has been diminished by the exact quantity emitted by our fires. This seems very 
wonderful, but it is only a small part of the wonders which the dynamical theory of 
heat lays open to our view. The lifting of a weight from the earth is a commonplace 
act; but it is an act of the same mechanical quality as that performed by the sun when 
he lifts the oxygen from the carbon of carbonic acid gas, and permits the latter to store 
itself in the boles, branches and leaves of our trees. It is the solar force thus invested 
in the vegetable, that afterwards becomes the source of all animal power. Muscular 
force is simply the sun’s force transmuted. The helmsman stands at the wheel, but he 
cannot add one jot or tittle to the motion of the ship. lie directs, but cannot create. 
Equally ineffectual is human volition, or the organic processes of the animal body, to 
generate one unit of mechanical force. All such force is primarily derived from the 
sun. These are the latest verities of science, and, read in their light, the passage quoted 
by your reviewer will not appear extravagant. I may add, that these considerations are 
confined to the twelfth lecture of the course, and that the work embraces the relation¬ 
ship of the dynamical theory to all the phenomena of heat. It is, moreover, so written 
that the reader who wishes it may also make himself acquainted with the old theory. 
Indeed, the old theory comes out with greater vividness by its contrast with the new one. 
“ John Tyndall. 
“ Royal Institution , April 27, 1863.” 
Enough upon the theory ; let me end with quoting the last passage in the 
lectures. I think the most fastidious reader will not pass censure on the style. 
“To nature nothing can be added; from nature nothing can be taken away; 
the sum of her energies is constant, and the utmost man can do in the pursuit 
of physical truth, or in the applications of physical knowledge, is to shift the 
constituents of the never-varying total, and out of them to form another. The 
law of conservation rigidly excludes both creation and annihilation. Waves 
