52 FRAGMENTS OF SCIENCE 



converged, the cone of reflected rays and their point of 

 convergence being rendered clearly visible by the dust al- 

 ways 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 mo- 

 ment, 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 embodied. A block of wood, placed at the focus, ab- 

 sorbs 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 burned 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 other- 

 wise tarnish them, so as to diminish their reflective power. 

 Blackened zinc foil, when brought into the focus of invis- 

 ible rays, is instantly caused to blaze, and burns with its 

 peculiar purple light. Magnesium wire flattened, or tar- 



