ion to which this leads. It proves the aether at the focus
to be practically detached from the air,--that the most violent
aethereal motion may there exist, without the least aerial motion.
But, though you see it not, there is sufficient heat at that focus to
set London on fire. The heat there is competent to raise iron to a
temperature at which it throws off brilliant scintillations. It can
heat platinum to whiteness, and almost fuse that refractory 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 radiation or
absorption, the elementary atoms possess but little power. This might
be illustrated by a long array of facts; and one of the most singular
of these is furnished by the deportment of that extremely combustible
substance, phosphorus, when placed at the dark focus. It is
impossible to ignite there a fragment of amorphous phosphorus. But
ordinary phosphorus is a far quicker combustible, and its deportment
towards radiant heat is still more impressive. It may be exposed to
the intense radiation of an ordinary fire without 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 red heat, without
ignition. Notwithstanding the energy of the aethereal 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 powerful waves of low
refrangibility, and consequently cannot be affected by their heat.

The knowledge we now possess will enable us to analyse 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 placed bits of cloth of various colours upon snow, exposed
them to direct sunshine, and found that they sank to different depths
in the snow. The black cloth sank deepest, the white did not sink at
all. Franklin inferred from this experiment that black-bodies are the
best absorbers, and white ones the worst absorbers, of radiant heat.
Let us test the generality of this conclusion. One of these two cards
is coated with a very dark powder, and the other with a perfectly
white one. I place the powdered surfaces before a fire, and leave
them there until they