LORD KELVIN AND THE DISSIPATION OF ENERGY
The gradual permeation of the field by the great
doctrine of conservation simply repeated the history
of the introduction of every novel and revolutionary
thought. Necessarily the elder generation, to whom
all forms of energy were imponderable fluids, must pass
away before the new conception could claim the field.
Even the word energy, though Young had introduced
it in 1807, did not come into general use till some time
after the middle of the century. To the generality of
philosophers (the word physicist was even less in favor
at this time) the various forms of energy were still
subtile fluids, and never was idea relinquished with
greater unwillingness than this. The experiments of
Young and Fresnel had convinced a large number of
philosophers that light is a vibration and not a substance;
but so great an authority as Biot clung to the
old emission idea to the end of his life, in 1862, and held
a following.
Meantime, however, the company of brilliant young
men who had just served their apprenticeship when the
doctrine of conservation came upon the scene had
grown into authoritative positions, and were battling
actively for the new ideas. Confirmatory evidence
that energy is a molecular motion and not an
“imponderable” form of matter accumulated day by day.
The experiments of two Frenchmen, Hippolyte L.
Fizeau and Léon Foucault, served finally to convince
the last lingering sceptics that light is an undulation;
and by implication brought heat into the same category,
since James David Forbes, the Scotch physicist,
had shown in 1837 that radiant heat conforms to the
same laws of polarization and double refraction that
govern light. But, for that matter, the experiments
that had established the mechanical equivalent of heat
hardly left room for doubt as to the immateriality
of this “imponderable.” Doubters had indeed, expressed
scepticism as to the validity of Joule's experiments,
but the further researches, experimental and
mathematical, of such workers as Thomson (Lord Kelvin),
Rankine, and Tyndall in Great Britain, of Helmholtz
and Clausius in Germany, and of Regnault in
France, dealing with various manifestations of heat,
placed the evidence beyond the reach of criticism.
Out of these studies, just at the middle of the century,
to which the experiments of Mayer and Joule had
led, grew the new science of thermo-dynamics. Out of
them also grew in the mind of one of the investigators
a new generalization, only second in importance to the
doctrine of conservation itself. Professor William
Thomson (Lord Kelvin) in his studies in thermo-dynamics was early impressed with the fact that
whereas all the molar motion developed through labor
or gravity could be converted into heat, the process is
not fully reversible. Heat can, indeed, be converted
into molar motion or work, but in the process a certain
amount of the heat is radiated into space and lost. The
same thing happens whenever any other form of energy
is converted into molar motion. Indeed, every transmutation
of energy, of whatever character, seems complicated
by a tendency to develop heat, part of which
is lost. This observation led Professor Thomson to his
doctrine of the dissipation of energy, which he formulated
before the Royal Society of Edinburgh in 1852,
and published also in the
Philosophical Magazine the
same year, the title borne being, “On a Universal
Tendency in Nature to the Dissipation of Mechanical
Energy.”
From the principle here expressed Professor Thomson
drew the startling conclusion that, “since any restoration
of this mechanical energy without more than
an equivalent dissipation is impossible,” the universe,
as known to us, must be in the condition of a machine
gradually running down; and in particular that the
world we live on has been within a finite time unfit for
human habitation, and must again become so within a
finite future. This thought seems such a commonplace
to-day that it is difficult to realize how startling
it appeared half a century ago. A generation trained, as
ours has been, in the doctrines of the conservation and
dissipation of energy as the very alphabet of physical
science can but ill appreciate the mental attitude of a
generation which for the most part had not even
thought it problematical whether the sun could continue
to give out heat and light forever. But those
advance thinkers who had grasped the import of the
doctrine of conservation could at once appreciate the
force of Thomson's doctrine of dissipation, and realize
the complementary character of the two conceptions.
Here and there a thinker like Rankine did, indeed,
attempt to fancy conditions under which the energy lost
through dissipation might be restored to availability,
but no such effort has met with success, and in time
Professor Thomson's generalization and his conclusions
as to the consequences of the law involved came to be
universally accepted.
The introduction of the new views regarding the nature
of energy followed, as I have said, the course of
every other growth of new ideas. Young and imaginative
men could accept the new point of view; older philosophers,
their minds channelled by preconceptions,
could not get into the new groove. So strikingly true
is this in the particular case now before us that it is
worth while to note the ages at the time of the revolutionary
experiments of the men whose work has been
mentioned as entering into the scheme of evolution of
the idea that energy is merely a manifestation of matter
in motion. Such a list will tell the story better
than a volume of commentary.
Observe, then, that Davy made his epochal experiment
of melting ice by friction when he was a youth of
twenty. Young was no older when he made his first
communication to the Royal Society, and was in his
twenty-seventh year when he first actively espoused
the undulatory theory. Fresnel was twenty-six when
he made his first important discoveries in the same
field; and Arago, who at once became his champion,
was then but two years his senior, though for a decade
he had been so famous that one involuntarily thinks of
him as belonging to an elder generation.
Forbes was under thirty when he discovered the polarization
of heat, which pointed the way to Mohr, then
thirty-one, to the mechanical equivalent. Joule was
twenty-two in 1840, when his great work was begun;
and Mayer, whose discoveries date from the same year,
was then twenty-six, which was also the age of Helmholtz
when he published his independent discovery of
the same law. William Thomson was a youth just past
his majority when he came to the aid of Joule before
the British Society, and but seven years older when he
formulated his own doctrine of the dissipation of energy.
And Clausius and Rankine, who are usually mentioned
with Thomson as the great developers of thermo-dynamics,
were both far advanced with their novel studies
before they were thirty. With such a list in mind, we
may well agree with the father of inductive science
that “the man who is young in years may be old in
hours.”
Yet we must not forget that the shield has a reverse
side. For was not the greatest of observing astronomers,
Herschel, past thirty-five before he ever saw a
telescope, and past fifty before he discovered the heat
rays of the spectrum? And had not Faraday reached
middle life before he turned his attention especially to
electricity? Clearly, then, to make this phrase complete,
Bacon should have added that “the man who is
old in years may be young in imagination.” Here,
however, even more appropriate than in the other case
—more's the pity—would have been the application
of his qualifying clause: “but that happeneth rarely.”
