RADIATION. 179 



gested further inquiry. Sulphur dissolved in bisulphate of 

 carbon was found almost perfectly transparent. The dense 

 and deeply-colored 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 bro- 

 mine, 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 per- 

 mitted to do so, it was found that a layer of dissolved 

 iodine, sufficiently opaque to cut off the light of the mid- 

 day 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 

 renders attainable far more powerful foci of invisible rays 

 than could possibly be obtained by the method of Sir Wil- 

 liam 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, 

 luminous and non-luminous, we can intercept the former by 

 the iodine, and do what w r e please with the latter. Ex- 

 periments of this character, not only with the iodine solu- 



