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RADIANT HEAT AND ITS RELATIONS. 225

to be practically detached from the air—that the most vio-
lent ethereal motion may there exist without the least
aerial motion. But though you see it not, there is suffi-
cient heat at that focus to set London on fire. The heat
there at the present moment is competent to raise iron to a
temperature at which it throws ofl' brilliant scintillations.
It can heat platinum to whiteness and almost fuse that re—
fractory metal. It actually can fuse gold, silver, copper,
and aluminium. The moment, moreover, that wood is
placed at the focus it bursts into a blaze.

It has been already affirmed that whether as regards ra-
diation or absorption the elementary atoms possess but little
power. This might be illustrated by a long array of facts 5
and one of the most singular of these is furnished by the
deportment of that extremely combustible substance, phos-
phorus, when placed at this dark focus. It is impossible to
ignite there a fragment of amorphous phosphorus. But or-
dinary phosphorus is a far quicker combustible, and its de-
portment to radiant heat is still more impressive. It may
be exposed to the intense radiation of an ordinary fire with-
out bursting into flame. It may also be exposed for twenty
or thirty seconds at an obscure focus of sufficient power to
raise platinum to a White heat, without ignition. Notwith-
standing the energy of the ethereal waves here concentrated,
notwithstanding the extremely inflammable character of
the elementary body exposed to their action, the atoms of
that body refuse to partake of the motion of the waves, and
consequently cannot be powerfully affected by their heat.

The knowledge which we now possess will enable us to
analyze with profit a practical question. White dresses are
worn in summer because they are found to be cooler than
dark ones. The celebrated Benjamin Franklin made the fol-
lowing experiment: He placed bits of cloth of various colors
upon snow, exposed them to direct sunshine, and found that
they sank to different depths in the snow The black cloth

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