the dye, is nearly as good an absorber of heat as
the snow around it. But to the absorption of the dark solar rays by
the undyed cloth, is now added the absorption of the whole of the
luminous rays, and this great additional influx of heat is far more
than sufficient to turn the balance in favour of the black cloth. The
sum of its actions on the dark and luminous rays, exceeds the action
of the snow on the dark rays alone. Hence the cloth will sink in the
snow, and this is the complete analysis of Franklin's experiments.

Throughout this discourse the main stress has been laid on chemical
constitution, as influencing most powerfully the phenomena of
radiation and absorption.

With regard to gases and vapours, and to the liquids from which these
vapours 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 extend this principle to solids I was met
by a multitude of facts, obtained by celebrated experimenters, which
seemed flatly to forbid such an extension. Mellon, for example, had
found the same radiant and absorbent power for chalk and lamp-black.
MM. Masson and Courtepee had 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 when bodies are reduced to an
extremely fine state of division, the influence of this state is so
powerful 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 an
oversight has run, the mere mention of which will show what caution is
essential in the operations of experimental philosophy; while an
experiments or two will make clear wherein the oversight consists.
Filling a brightly polished metal cube with boiling water, I determine
the quantity of heat emitted by two of the bright surfaces. As a
radiator of heat one of them far transcends the other. Both surfaces
appear to be metallic; what, then, is the cause of the observed
difference in their radiative power? Simply this: one of the surfaces
is coated with transparent gum, through which, of course, is seen the
metallic lustre behind; and this varnish, though so perfectly
transparent to luminous rays, is as opaque as