RADIATION. 69 



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 matter 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 ob- 

 stacles 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 bisulphide of car- 

 bon, possesses the power of detaching, with extraor- 

 dinary 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 em- 

 ployed to filter the beams of the electric light, and'to 

 form foci of invisible rays so intense as to produce al- 

 most all the effects obtainable in an ordinary fire. Com- 

 bustible bodies were burnt, and refractory ones were 

 raised to a white heat, by the concentrated invisible rays. 

 Thus, by exalting their ref rangibility, the invisible rays 

 of the electric light were rendered visible, and all the 

 colours of the solar spectrum were extracted from utter 

 darkness. The extreme richness of the electric light in 

 invisible rays of low ref rangibility was demonstrated, 

 one-eighth only of its radiation consisting of luminous 

 rays. The deadness of the optic nerve to those invisible 

 rays was proved, and experiments were then added to 

 show that the bright and the dark rays of a solid body, 



