ufficiently small, there exists a speed
limit which is the same in a large pipe and in free air. By some
beautiful experiments, MM. Violle and Vautier have clearly shown that
any disturbance in the air melts somewhat quickly into a single wave
of given form, which is propagated to a distance, while gradually
becoming weaker and showing a constant speed which differs little in
dry air at 0 deg. C. from 331.36 metres per second. In a narrow pipe the
influence of the walls makes itself felt and produces various effects,
in particular a kind of dispersion in space of the harmonics of the
sound. This phenomenon, according to M. Brillouin, is perfectly
explicable by a theory similar to the theory of gratings.

CHAPTER III

PRINCIPLES

Sec. 1. THE PRINCIPLES OF PHYSICS

Facts conscientiously observed lead by induction to the enunciation of
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 hi