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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.

N ow Melloni, and Masson, and Courtépée, 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 pow-
ders red, their white ones White, and their black ones black,
but they saw these colors through the coat of varnish
which encircled every particle of their powders. When,
therefore, it was concluded that color 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. Hun-
dreds, perhaps thousands, of experiments on radiant heat
have been performed in this way by various inquirers, but
I fear the work will 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 in succession
by so many able men, and vitiated so large an amount of
otherwise excellent work.

Basing our reasonings, then, on demonstrated facts, we
arrive at the extremely probable conclusion that the envel-
ope 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 conse-
quences from experimental facts, is an incessant exercise
of the student of physical science. But having thus fol-
lowed for a time the light of reason alone through a series
of phenomena, and emerged from them with a purely intel-
lectual conclusion, our duty is to bring that conclusion to
an experimental test. In this way we fortify our science,

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