RADIATION. 41 



fectly diathermic. The dense and deeply-coloured ele- 

 ment bromine was examined, and found competent to 

 cut off the light of our most brilliant 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 impracticable to 

 examine the substance in its usual solid condition. It 

 however dissolves freely in bisulphide of carbon. 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 radiant 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 sepa- 

 rated the luminous from the non-luminous rays of the 

 sun, and he also sought to render the obscure rays visi- 

 ble by concentration. Intercepting the luminous por- 

 tion of his spectrum 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 renders attainable far 

 more powerful foci of invisible rays than could possibly 

 be obtained by the method of Sir William Herschel. 

 For to form his spectrum he was obliged to operate 

 upon solar light which had passed through a narrow slit 

 or through a small aperture, the amount of the obscure 

 heat being limited by this circumstance. But with our 

 opaque solution we may employ the entire surface of 

 the largest lens, and having thus converged the rays, 



* Professor Dewnr has recently succeeded in producing a 

 medium highly opaque to light, and highly transparent to ob- 

 scure heat, by fusing together sulphur and iodine. 



