if the alleged performances of Archimedes during the
siege of Syracuse had any foundation in fact, the dark solar rays
would have been the philosopher's chief agents of combustion. On a
small scale we can readily produce, with the purely invisible rays of
the electric light, all that Archimedes is said to have performed with
the sun's total radiation. Placing behind the electric light a small
concave mirror, the rays are converged, the cone of reflected rays and
their point of convergence being rendered clearly visible by the dust
always floating in the air. Placing between the luminous focus and
the source of rays our solution of iodine, the light of the cone is
entirely cut away; but the intolerable heat experienced when the band
is placed, even for a moment, at the dark focus, shows that the
calorific rays pass unimpeded through the opaque solution.

Almost anything that ordinary fire can effect may be accomplished at
the focus of invisible rays; the air at the focus remaining at the
same time perfectly cold, on account of its transparency to the
heat-rays. An air thermometer, with a hollow rack-salt bulb, would be
unaffected by the heat of the focus: there would be no expansion, and
in the open air there is no convection. The aether at the focus, and
not the air, is the substance in which the heat is embodied. A block
of wood, placed at the focus, absorbs the heat, and dense volumes of
smoke rise swiftly upwards, showing the manner in which the air itself
would rise, if the invisible rays were competent to heat it. At the
perfectly dark focus dry paper is instantly inflamed: chips of wood
are speedily burnt up: lead, tin, and zinc are fused: and disks of
charred paper are raised to vivid incandescence. It might be supposed
that the obscure rays would show no preference for black over white;
but they do show a preference, and to obtain rapid combustion, the
body, if not already black, ought to be blackened. When metals are to
be burned, it is necessary to blacken or otherwise tarnish them, so as
to diminish their reflective power. Blackened zinc foil, when brought
into the focus of invisible rays, is instantly caused to blaze, and
burns with its peculiar purple light. Magnesium wire flattened, or
tarnished magnesium ribbon, also bursts into flame. Pieces of
charcoal suspended in a receiver full of oxygen are also set on fire
when the invisible focus falls upon them; the dark rays after having
passed throu