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 lignt 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 sh.arpness, 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 fire. Combustible bodies were 
* See " Physical Considerations," Art. iv., p. 102. 
