HADIATION. 41 
attained a certain width of swing. In solid and molten 
bodies a certain amplitude cannot be surpassed without the 
introduction of periods of vibration, which provoke the 
sense of vision. How are we to figure this? If permitted 
to speculate, we might ask, are not these more rapid vibra- 
tions the progeny of the slower? Is it not really the mutual 
action of the atoms, when they swing through very wide 
spaces, and thus encroach upon each other, that causes 
them to tremble in quicker periods? If so, whatever be the 
agency by which the large swinging space is obtained, we 
shall have light-giving vibrations associated with it. It 
matters not whether the large amplitudes be produced by 
the strokes of a hammer, or by the blows of the molecules 
of a non-luminous gas, like air at some height above a gas- 
flame; or by the shock of the ether particles when trans- 
mitting radiant heat. The result in all cases will be 
incandescence. Thus, the invisible waves of our filtered 
electric beam may be regarded as generating synchronous 
vibrations among the atoms of the platinum on which they 
impinge; but, once these vibrations have attained a cer- 
tain amplitude, the mutual jostling of the atoms produces 
quicker tremors, and the light-giving waves follow as the 
necessary product of the heat-giving ones. 
11. Absorption of Radiant Heat by Vapors and Odors. 
We commenced the demonstrations brought forward in 
this lecture by experiments on permanent gases, and we 
have now to turn our attention to the vapors of volatile 
liquids. Here, as in the case of the gases, vast differences 
have been proved to exist between various kinds of mole- 
cules, as regards their power of intercepting the calorific 
waves. While some vapors allow the waves a comparatively 
free passage, the faintest mixture of other vapors causes a 
deflection of the magnetic needle. Assuming the absorp- 
tion effected by air, at a pressure of one atmosphere, to be 
unity, the following are the absorptions effected by a series 
of vapors at a pressure of one-sixtieth of an atmosphere: 
Name of vapor. Absorption. 
Bisulphide of carbon 47 
Iodide of methyl 115 
Benzol 136 
Amylene 321 
Sulphuric ether 440 
Formic ether 648 
Aceticether 613 
