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.