30 FRAGMENTS OF SCIENCE. 



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 uucombined atoms 

 of the element can act upon the radiant heat. AY hen 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 t<f render the obscure rays visible by con- 

 centration. 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 invisi- 

 ble 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 circum- 

 stance. But with our opaque solution we may employ the 

 entire surface of the largest lens, and having thus con- 

 verged the rays, luminous and non-luminous, we can 

 intercept the former by the iodine, and do what we please 

 with the latter. Experiments of this character, not only 

 with the iodine solution, but also with black glass and 

 layers of lampblack, were publicly performed at the Royal 

 Institution in the early part of 1862, and the effects at the 

 foci of invisible rays, then obtained, were such as had 

 never been witnessed previously. 



In the experiments here referred to, glass lenses were 

 employed to concentrate the rays. But glass, though 

 highly transparent to the luminous, is in a high degree 

 opaque to the invisible heat-rays of the electric lamp, and 

 hence a large portion of those rays was intercepted by the 

 glass. The obvious remedy here is to employ rock-salt 

 lenses instead of glass ones, or to abandon the use of lenses 



* Professor Dewar has recently succeeded in producing a medium 

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

 fusing together sulphur and iodine. 



