34 FRAGMENTS OF SCIENCE. 
there diffused, the summit of the peak representing the 
sun's invisible radiation is cut off. A similar lowering of the 
mountain of invisible heat is observed when the rays from 
the electric light are permitted to pass through a film of 
water, which acts upon them as the atmospheric vapor acts 
upon the rays of the sun. 
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 hud 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 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 en- 
tirely 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 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 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 em- 
bodied. 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 per- 
fectly dark focus dry paper is instantly inflamed; chips of 
wood arespeedily burned up; lead, tin, and zinc are fused; 
and disks of charred paper are raised to vivid incandes- 
cence. 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 
