The boldest minds have an instinctive confidence in it, and it is the
principle which has most stoutly resisted that assault which the
daring of a few theorists has lately directed to the overthrow of the
general principles of physics. At every new discovery, the first
thought of physicists is to find out how it accords with the principle
of the conservation of energy. The application of the principle,
moreover, never fails to give valuable hints on the new phenomenon,
and often even suggests a complementary discovery. Up till now it
seems never to have received a check, even the extraordinary
properties of radium not seriously contradicting it; also the general
form in which it is enunciated gives it such a suppleness that it is
no doubt very difficult to overthrow.

I do not claim to set forth here the complete history of this
principle, but I will endeavour to show with what pains it was born,
how it was kept back in its early days and then obstructed in its
development by the unfavourable conditions of the surroundings in
which it appeared. It first of all came, in fact, to oppose itself to
the reigning theories; but, little by little, it acted on these
theories, and they were modified under its pressure; then, in their
turn, these theories reacted on it and changed its primitive form.

It had to be made less wide in order to fit into the classic frame,
and was absorbed by mechanics; and if it thus became less general, it
gained in precision what it lost in extent. When once definitely
admitted and classed, as it were, in the official domain of science,
it endeavoured to burst its bonds and return to a more independent and
larger life. The history of this principle is similar to that of all
evolutions.