46 FRAGMENTS OF SCIENCE 



black glass (which owes its blackness to the element car- 

 bon), were to a considerable extent transparent to calorific 

 rays of low refrangibility. These facts, harmonizing so 

 strikingly with the deportment of the simple gases, sug- 

 gested further inquiry. Sulphur dissolved in bisulphide 

 of carbon was found almost perfectly diathermic. The 

 dense and deeply- colored element bromine was examined, 

 and found competent to cut off the light of our most bril- 

 liant flames, while it transmitted the invisible calorific rays 

 with extreme freedom. Iodine, the companion element of 

 bromine, was next thought of, but it was found impracti- 

 cable to examine the substance in its usual solid condi- 

 tion. It, however, dissolves freely in bisulphide of car- 

 bon. There is no chemical union between the liquid and 

 the iodine; it is simply a case of solution, in which the 

 uncombined atoms of the element can act upon the radi- 

 ant heat. When permitted to do so, it was found that a 

 layer of dissolved iodine, sufficiently opaque to cut off the 

 light of the midday sun, was almost absolutely transparent 

 to the invisible calorific rays.* 



By prismatic analysis Sir William Herschel separated 

 the luminous from the non- luminous rays of the sun, and 

 he also sought to render the obscure rays visible by con- 

 centration. Intercepting the luminous portion of his spec- 

 trum, he brought, by a converging lens, the ultra-red rays 

 to a focus, but by this condensation he obtained no light. 

 The solution of iodine offers a means of filtering the solar 

 beam, or, failing it, the beam of the electric lamp, which 



' Professor Dewar lias recently succeeded in producing a medium highly- 

 opaque to light, and highly transparent to obscure heat, by fusing together 

 sulphur and iodine. 



