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RADIANT HEAT AND ITS RELATIONS. 229

on chemical constitution, as influencing most powerfully
the phenomena of radiation and absorption. With regard
to gases, vapors, and to the liquids from which these va-
pors are derived, it has been proved by the most varied
and conclusive experiments that the acts of radiation and
absorption are molecular—that they depend upon chemical
and not upon mechanical condition. In attempting to ex-
tend this principle to solids I was met by a multitude of
facts obtained by celebrated experimenters, which seemed
flatly to forbid such extension. Melloni, for example, found
the same radiant and absorbent power for chalk and lamp-
black. MM. Masson and Courtépée performed a most
elaborate series of experiments on chemical precipitates of
various kinds, and found that they one and all manifested
the same power of radiation. They concluded from their
researches, that where bodies are reduced to an extremely
fine state of division the influence of this state is so power-
ful \ as entirely to mask and override whatever influence
may be due to chemical constitution.

But it appears to me that through the whole of these
researches a serious oversight has run, the mere mention
of which will show you what caution is essential in the
operations of experimental philosophy. Let me state
wherein I suppose this oversight to consist. I have here
a metal cube with two of its sides brightly polished. I
fill the cube with boiling water and determine the quan-
tity of heat emitted by the two bright surfaces. One of
them far transcends the other as a radiator of heat. Both
surfaces appear to be metallic; what, then, is the cause of
the observed difference in their radiative power ? Simply
this: I have coated one of the surfaces with transparent
gum, through which, of course, is seen the metallic lustre
behind. Now this varnish, though so perfectly transparent
to luminous rays, is as Opaque as pitch or lamp-black to
non-luminous ones. It is a powerful emitter of dark rays;

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