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XXII
HERMANN VON HELMHOLTZ
known to us as the Principle of the Conservation of Energy.
It had long before been suspected that in the unending
succession of phenomena there was something else besides
matter which persisted, which could neither be created nor
destroyed, something immaterial and scarcely tangible. At
one time it seemed to be quantity of motion measured in this
way or that, at another time force, or again an expression
compounded of both.
In place of these obscure guesses Helmholtz brought
forward distinct ideas and fixed relations which led immedi-
ately to a wealth of general and special connections. Magni-
ficent were the views which the principle opened up into the
past and future of our planetary system; in every separate
investigation, even the most restricted, its applications were
innumerable. For forty years it has been so much expounded
and extolled that no man of culture can be quite ignorant of
it. It is noteworthy that about this time other heads began
to think more clearly of these things; and it came about that
as far as the phenomena of heat were concerned other men
had anticipated Helmholtz by a few years without his know-
ing it.
It would be far from his wish to detract from the
fame of these men; but it should not be forgotten that their
researches were almost entirely restricted to the nature of
heat, whereas the significance and value of the principle lie
precisely in the fact that it is not limited to this or that
natural force, but that it embraces all of them and can even
serve as our pole-star amongst unknown forces.
It is not generally known that in his mature years Helm-
holtz has returned to the work of his youth and has still
further developed it. The law of the conservation of energy,
general though it is, nevertheless appears to be only one half of
a still more comprehensive law. A stone projected into empty
space would persist in a state of uniform motion, and thus its
energy would remain constant: to this corresponds the con-
servation of energy in any system, however complicated that
system may be. But the stone would also tend to retain its
direction and to travel in a straight line: to this behaviour
there is a corresponding general behaviour on the part of
every moving system. In the case of purely mechanical
systems it has long been known that every system, according