RADIATION. 41 



fectly diathermic. The dense and deeply-coloured 

 element 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. 1 



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 

 concentration. Intercepting the luminous portion 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, 



1 Professor Dewar has recently succeeded in producing a medium 

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

 fusing together sulphur and iodine. 



