46 FRAGMENTS OF SCIENCE. 



the distribution of heat in the solar spectrum is not 

 any means so striking as that just described, and the 

 reason, doubtless, is that prior to reaching the 

 the solar rays have to traverse our atmosphere. By th< 

 aqueous vapour there diffused, the summit of the 

 representing the sun's invisible radiation is cut off. 

 similar lowering of the mountain of invisible heat 

 observed when the rays from the electric light are pei 

 mitted to pass through a film of water, which acts upoi 

 them as the atmospheric vapour acts upon the rays 

 the sun. 



7. Combustion by Invisible Rays. 



The sun's invisible rays far transcend the visib 

 ones in heating power, so that if the alleged perfor 

 mances of Archimedes during the siege of Syracuse 

 any foundation in fact, the dark solar rays would hav 

 been the philosopher's chief agents of combustion. 

 a small scale we can readily produce, with the pure! 

 invisible rays of the electric light, all that Archi 

 medes is said to have performed with the sun's to 

 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 tb 

 air. Placing between the luminous focus and the sourc< 

 of rays our solution of iodine, the light of the cone i 

 entirely cut away ; but the intolerable heat experiencec 

 when the hand is placed, even for a moment, at th< 

 dark focus, shows that the calorific rays pass unimpedec 

 through the opaque solution. 



Almost anything that ordinary fire can effect ma] 

 be accomplished at the focus of invisible rays ; the 



