pitch, or lamp-black, to
non-luminous ones. It is a powerful emitter of dark rays; it is also
a powerful absorber. While, therefore, at the present moment, it is
copiously pouring forth radiant heat itself, it does not allow a
single ray from the metal behind to pass through it. The varnish
then, and not the metal, is the real radiator.

Now Melloni, and Masson, and Courtepee experimented thus: they mixed
their powders and precipitates with gum-water, and laid them, by means
of a brush, upon the surfaces of a cube like this. True, they saw
their red powders red, their white ones white, and their black ones
black, but they saw these colours _through the coat of varnish which
surrounded every particle_. When, therefore, it was concluded that
colour had no influence on radiation, no chance had been given to it
of asserting its influence; when it was found that all chemical
precipitates radiated alike, it was the radiation from a varnish,
common to them all, which showed the observed constancy. Hundreds,
perhaps thousands, of experiments on' radiant heat have been performed
in this way, by various enquirers, but the work will, I fear, have to
be done over again. I am not, indeed, acquainted with an instance in
which an oversight of so trivial a character has been committed by so
many able men in succession, vitiating so large an amounts of
otherwise excellent work. Basing our reasonings thus on demonstrated
facts, we arrive at the extremely probable conclusion that the
envelope of the particles, and not the particles themselves, was the
real radiator in the experiments just referred to. To reason thus,
and deduce their more or less probable consequences from experimental
facts, is an incessant exercise of the student of physical science.
But having thus followed, for a time, the light of reason alone
through a series of phenomena, and emerged from them with a purely
intellectual conclusion, our duty is to bring that conclusion to an
experimental test. In this way we fortify our science.

For the purpose of testing our conclusion regarding the influence of
the gum, I take two powders presenting the same physical appearance;
one of them is a compound of mercury, and the other a compound of
lead. On two surfaces of a cube are spread these bright red powders,
without varnish of any kind. Filling the cube with boiling water, and
determining the radiation from the' two surfaces, one of them is found
to emit thirty-n