46 FRAGMENTS OF SCIENCE. 



the distribution of heat in the solar spectrum is not by 

 any means so striking as that just described, and the 

 reason, doubtless, is that prior to reaching the earth 

 the solar rays have to traverse our atmosphere. By the 

 aqueous vapour 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 per- 

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

 them as the atmospheric vapour 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 perfor- 

 mances of Archimedes during the siege of Syracuse had 

 any foundation in fact, the dark solar rays would have 

 been the philosopher's chief agents of combustion. On 

 a small scale we can readily produce, with the purely 

 invisible rays of the electric light, all that Archi- 

 medes 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 

 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 anything that ordinary fire can effect may 

 be accomplished at the focus of invisible rays ; the air 



