Page:On Radiation.djvu/31

 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 all 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 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 more powerful foci of invisible rays than could possibly be obtained in the above experiment by Sir William Herschel. For to form his spectrum he was obliged to operate upon solar h'ght which had passed through a narrow slit or through a small aperture, the amount of the obscure heat admitted 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 Rh