184 FRAGMENTS OF SCIENCE. 



7. Combustion by Invisible Mays. 



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- 

 raometer, 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 



