current or by one of heat; it may be traversed by a ray of light
either ordinary or polarized rectilineally or circularly, etc.:--in
each case a certain minimum and characteristic dissymmetry is
necessary at every point of the body in question.

This consideration enables us to foresee that certain phenomena which
might be imagined _a priori_ cannot exist. Thus, for instance, it is
impossible that an electric field, a magnitude directed and not
superposable on its image in a mirror perpendicular to its direction,
could be created at right angles to the plane of symmetry of the
medium; while it would be possible to create a magnetic field under
the same conditions.

This consideration thus leads us to the discovery of new phenomena;
but it must be understood that it cannot of itself give us absolutely
precise notions as to the nature of these phenomena, nor disclose
their order of magnitude.


§ 2. THE PRINCIPLE OF THE CONSERVATION OF ENERGY

Dominating not physics alone, but nearly every other science, the
principle of the conservation of energy is justly considered as the
grandest conquest of contemporary thought. It shows us in a powerful
light the most diverse questions; it introduces order into the most
varied studies; it leads to a clear and coherent interpretation of
phenomena which, without it, appear to have no connexion with each
other; and it supplies precise and exact numerical relations between
the magnitudes which enter into these phenomena.