RADIATION. 60 



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 obstacles to the calor- 

 ific waves. This deportment of the simple gases directed 

 our attention to other elementary bodies, the examina- 

 tion of which led to the discovery that the element iodine, 

 dissolved in bisulphide 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 sub- 

 stance was then employed to filter the beams of the 

 electric light, and to form foci of invisible rays so in- 

 tense as to produce almost all the effects obtainable in 

 an ordinary fire. Combustible bodies were burnt, and 

 refractory ones were raised to a white heat, by the con- 

 centrated invisible rays. Thus, by exalting their re- 

 frangibility, 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 rich- 

 ness of the electric light in invisible rays of low re- 

 frangibility 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, raised gradually to 



