RADIATION 75 



16. Summary and Conclusion 



* 



Let us now cast a momentary glance over the ground 

 that we have left behind. The general nature of light and 

 heat was first briefly described: the compounding of mat- 

 ter from elementary atoms, and the influence of the act of 

 combination on radiation and absorption, were considered 

 and experimentally illustrated. Through the transparent 

 elementary gases radiant heat was found to pass as through 

 a vacuum, while many of the compound gases presented 

 almost impassable obstacles to the calorific waves. This 

 deportment of the simple gases directed our attention to 

 other elementary bodies, the examination of which led to 

 the discovery that the element iodine, dissolved in bisul- 

 phide of carbon, possesses the power of detaching, with 

 extraordinary sharpness, the light of the spectrum from its 

 heat, intercepting all luminous rays up to the extreme red, 

 and permitting the calorific rays beyond the red to pass 

 freely through it. This substance was then employed to 

 filter the beams of the electric light, and to form foci of 

 invisible rays so intense as to produce almost all the effects 

 obtainable in an ordinary fire. Combustible bodies were 

 burned, and refractory ones were raised to a white heat, by 

 the concentrated invisible rays. Thus, by exalting their 

 refrangibility, the invisible rays of the electric light were 

 rendered visible, and all the colors of the solar spectrum 

 were extracted from utter darkness. The extreme richness 

 of the electric light in invisible rays of low refrangibility 

 was demonstrated, one-eighth only of its radiation consist- 

 ing of luminous rays. The deadness of the optic nerve to 

 those invisible rays was proved, and experiments were then 



