entrates it at the focus. Upon
substituting balls of snow for heated bodies, they even went so far as
to prove that frigorific foci may be formed by way of reflection. Some
years afterwards Mariotte, a member of this Academy, discovered that
there exist different kinds of radiating heat; that the heat with which
rays of light are accompanied traverses all transparent media as easily
as light does; while, again, the caloric which emanates from a strongly
heated, but opaque substance, while the rays of heat, which are found
mingled with the luminous rays of a body moderately incandescent, are
almost entirely arrested in their passage through the most transparent
plate of glass!

This striking discovery, let us remark in passing, will show,
notwithstanding the ridicule of pretended savans, how happily inspired
were the workmen in founderies, who looked at the incandescent matter of
their furnaces, only through a plate of ordinary glass, thinking by the
aid of this artifice to arrest the heat which would have burned their
eyes.

In the experimental sciences, the epochs of the most brilliant progress
are almost always separated by long intervals of almost absolute repose.
Thus, after Mariotte, there elapsed more than a century without history
having to record any new property of radiating heat. Then, in close
succession, we find in the solar light obscure calorific rays, the
existence of which could admit of being established only with the
thermometer, and which may be completely separated from luminous rays by
the aid of the prism; we discover, by the aid of terrestrial bodies,
that the emission of caloric rays, and consequently the cooling of those
bodies, is considerably retarded by the polish of the surfaces; that the
colour, the nature, and the thickness of the outer coating of these
same surfaces, exercise also a manifest influence upon their emissive
power. Experience, finally, rectifying the vague predictions to which
the most enlightened minds abandon themselves with so little reserve,
shows that the calorific rays which emanate from the plane surface of a
heated body have not the same force, the same intensity in all
directions; that the _maximum_ corresponds to the perpendicular
emission, and the _minimum_ to the emissions parallel to the surface.

Between these two extreme positions, how does the diminution of the
emissive power operate? Leslie first sought the solution of this
important question. His observat