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RADIANT IIEAT AND ITS RELATIONS. 231

sparing no pains, shirking no toil to secure sound materials
for the edifice which it is our privilege to raise.

For the purpose of testing our conclusion regarding the
influence of the gum I take two powders of the same physi-
cal appearance; one of them is a compound of mercury and
the other a compound of lead. On two surfaces of this
cube are spread these bright-red powders without varnish
of any kind. Filling the tube with boiling water, and de-'
termining the radiation from the two surfaces, one of them
is found to emit thirty-nine rays, while the other emits
seventy-four. This, surely, is a great difference. Here,
however, is a second cube, having two of its surfaces coated
with the same powders, the only difference being that now
the powders are laid on by means of a transparent gum.
Both surfaces are now absolutely alike in radiative power.
Both of them emit somewhat more than was emitted by
either of the unvarnished powders, simply because the gum
employed is a better radiator than either of them. Exclud-
ing all varnish, and comparing white with white, I find
vast differences; comparing black with black, I find them
also different; and when black and white are compared, in
some cases the black radiates far more than the white,
while in other cases the white radiates far more than the
black. Determining, moreover, the absorptive power of
those powders, it is found to go hand-in-hand with their
radiative power. The good radiator is a good absorber, and
the bad radiator is a bad absorber. From all this it is evi-
dent that as regards the radiation and absorption of non-
luminous heat, color teaches us nothing; and that even as
regards the radiation of the sun, consisting as it does main-
ly of non-luminous rays, conclusions as to the influence of
color may be altogether delusive. This is the strict scien-
tific upshot of our researches. But it is not the less true
that in the case of Wearing apparel—and this for reasons
which I have given in analyzing the experiment of Frank-

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