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184 FRAGMEN TS OF SCIENCE.

7. Combustion by Invisible Rays.

The sun’s invisible rays far transcend the visible ones
in heating power, so that if the alleged performances of
Archimedes during the siege of Syracuse had any founda-
tion in fact, the dark solar rays would have been the phi-
losopher’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 per-
formed 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 hand is placed, even for a moment, at the dark
focus, shows that the calorific rays pass unimpeded through
the opaque solution.

Almost any thing 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 ac-
count of its transparency to the heat-rays. An air-ther-
mometer, with a hollow rock-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 ether 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
upward, 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

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