RADIATION. 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 4*7 



Iodide of methyl 115 



Benzol 136 



Amylene 321 



Sulphuric ether 440 



Formic ether 548 



Acetic ether 612 



