a certain number of laws or general hypotheses which are the
principles already referred to. These principal hypotheses are, in the
eyes of a physicist, legitimate generalizations, the consequences of
which we shall be able at once to check by the experiments from which
they issue.

Among the principles almost universally adopted until lately figure
prominently those of mechanics--such as the principle of relativity,
and the principle of the equality of action and reaction. We will not
detail nor discuss them here, but later on we shall have an
opportunity of pointing out how recent theories on the phenomena of
electricity have shaken the confidence of physicists in them and have
led certain scholars to doubt their absolute value.

The principle of Lavoisier, or principle of the conservation of mass,
presents itself under two different aspects according to whether mass
is looked upon as the coefficient of the inertia of matter or as the
factor which intervenes in the phenomena of universal attraction, and
particularly in gravitation. We shall see when we treat of these
theories, how we have been led to suppose that inertia depended on
velocity and even on direction. If this conception were exact, the
principle of the invariability of mass would naturally be destroyed.
Considered as a factor of attraction, is mass really indestructible?

A few years ago such a question would have seemed singularly
audacious. And yet the law of Lavoisier is so far from self-evident
that for centuries it escaped the notice of physicists and chemists.
But its great apparent simplicity and its high character of
generality, when enunciated at the end of the eighteenth century,
rapidly gave it such an authority that no one was able to any longer