circumstances the weight is no longer the same after as before the
reaction. In particular, the weight of a solution of salts of copper
in water is not the exact sum of the joint weights of the salt and the
water. Such experiments are evidently very delicate; they have been
disputed, and they cannot be considered as sufficient for conviction.
It follows nevertheless that it is no longer forbidden to regard the
law of Lavoisier as only an approximate law; according to Sandford and
Ray, this approximation would be about 1/2,400,000. This is also the
result reached by Professor Poynting in experiments regarding 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