34 PR A GMENTS 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 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 wo 

 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 are'speedily 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 



