RADIATION. 205 



power must be referred not to period, but to some other 

 peculiarity of the elementary gas. The atomic group which 

 constitutes the molecule of defiant gas, produces many 

 thousand times the disturbance caused by the oxygen, be- 

 cause the group is able to lay a vastly more powerful hold 

 upon the ether than the single atoms can. The cavities and 

 indentations of a molecule composed of spherical atoms 

 may be one cause of this augmented hold. Another, and 

 probably very potent one may be, that the ether itself, con- 

 densed and entangled among the constituent atoms of a 

 compound, virtually increases the magnitude of the group, 

 and hence augments the disturbance. But whatever may 

 be the fate of these attempts to visualize the physics of the 

 process, it will still remain true that, to account for the 

 phenomena of radiation and absorption we must take into 

 consideration the shape, size, and complexity of the mole- 

 cules by which the ether is disturbed. 



1G. 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 com- 

 bination on radiation and absorption were considered and 

 experimentally illustrated. Through the transparent ele- 

 mentary 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 clement 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, 



