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RADIANT HEAT AND ITS RELATIONS. 227

class. Thus the white powder, which has shown itself so
powertul an absorber, has been specially selected on account
of its extreme perviousness to the visible rays, and its ex-
treme imperviousness to the invisible ones ; while the dark
powder was chosen on account of its extreme transparency
to the invisible, and its extreme Opacity to the visible rays.
In the case of the radiation from our fire, about 98 per cent.
of the Whole emission consists of invisible rays 3 the body,
therefore, which was most opaque to these triumphed as
an absorber, though that body was a white one.

I would here invite you to consider the manner in
which we obtain from natural facts what may be called
their intellectual value. Throughout the processes of N a-
ture there is interdependence and harmony, and the main
value of our science, considered as a mental discipline, con-
sists in the tracing of this interdependence and the demon-
stration of this harmony. The outward and visible phe-
nomena are with us the counters of the intellect; and our
science would not be worthy of its name and fame 'if it
halted at facts, however practically useful, and neglected
the laws which accompany and rule phenomena. Let us
endeavor, then, to extract from the experiment of Franklin
its full intellectual value, calling to our aid the knowledge
which our predecessors have already stored. Let us im-
agine two pieces of cloth of the same texture, the one
black and the other white, placed upon sunned snow. Fix-
ing our attention on the white piece, let us inquire whether
there is any reason to expect that it will sink into the
snow at all. There is knowledge at hand which enables
us to reply at once in the negative. There is, on the con-
trary, reason to expect that after a sufficient exposure the
bit of cloth will be found on an eminence instead of in a
hollow; that instead of a depression, we shall have a rela-
tive elevation of the bit of cloth. For, as regards the lu-
minous rays of the sun, the cloth and the snow are alike

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