ding the
possible action of temperature on the weight of a body; and if this be
really so, we may reassure ourselves, and from the point of view of
practical application may continue to look upon matter as
indestructible.

The principles of physics, by imposing certain conditions on
phenomena, limit after a fashion the field of the possible. Among
these principles is one which, notwithstanding its importance when
compared with that of universally known principles, is less familiar
to some people. This is the principle of symmetry, more or less
conscious applications of which can, no doubt, be found in various
works and even in the conceptions of Copernican astronomers, but which
was generalized and clearly enunciated for the first time by the late
M. Curie. This illustrious physicist pointed out the advantage of
introducing into the study of physical phenomena the considerations on
symmetry familiar to crystallographers; for a phenomenon to take
place, it is necessary that a certain dissymmetry should previously
exist in the medium in which this phenomenon occurs. A body, for
instance, may be animated with a certain linear velocity or a speed of
rotation; it may be compressed, or twisted; it may be placed in an
electric or in a magnetic field; it may be affected by an electric
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.

Sec. 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 powe