RADIATION. 51 



acceptable as those of the atoms in a molecule of olefiant 

 gas; that the vibrating oxygen then stands so far below the 

 olefiant gas in radiant power must be referred not to 

 period, but to some other peculiarity. The atomic group 

 which constitutes the molecule of olefiant gas, produces 

 many thousand times the disturbance caused by the oxygen, 

 it may be, because the group is able to lay a vastly more 

 powerful hold upon the ether than the single atoms can. 

 Another and probably very potent cause of the difference 

 may be that the vibrations, oeing those of the constituent 

 atoms of the molecule,* are generated in highly condensed 

 ether, which acts like condensed air upon sound. 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 condi- 

 tion of the ether within the molecules, by which the ex- 

 ternal ether is disturbed. 



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 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 puss 

 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 lire. Combustible bodies were 



* See " Physical Considerations," Art. iv., p. 102. 



