92 FRAGMENTS OF SCIENCE 



these powders acquire no heat; still one of them, sugar, is 

 heated so highly by the concentrated beam of the electric 

 lamp that it first smokes and then violently inflames, while 

 the other substance, salt, is barely warmed at the focus. 

 Placing two perfectly transparent liquids in test-tubes at 

 the focus, one of them boils in a couple of seconds, while 

 the other, in a similar position, is hardly warmed. The 

 boiling-point of the first liquid is 78 C., which is speedily 

 reached; that of the second liquid is only 48 C., which is 

 never reached at all. These anomalies are entirely due to 

 the unseen element which mingles with the luminous rays 

 of the electric beam, and indeed constitutes 90 per cent of 

 its calorific power. 



A substance, as many of you know, has been discov- 

 ered, by which these dark rays may be detached from the 

 total emission of the electric lamp. This ray- filter is a 

 liquid, black as pitch to the luminous, but bright as a dia- 

 mond to the non- luminous, radiation. It mercilessly cuts 

 off the former, but allows the latter free transmission. 

 When these invisible rays are brought to a focus, at a 

 distance of several feet from the electric lamp, the dark 

 rays form an invisible image of their source. By proper 

 means, this image may be transformed into a visible one 

 of dazzling brightness. It might, moreover, be shown, if 

 time permitted, how, out of those perfectly dark rays, 

 could be extracted, by a process of transmutation, all the 

 colors of the solar spectrum. It might also be proved that 

 those rays, powerful as they are, and sufficient to fuse 

 many metals, can be permitted to enter the eye, and to 

 break upon the retina, without producing the least lumi- 

 nous impression. 



The dark rays being thus collected, you see nothing at 



