RADIATION 61 



vibrations the progeny of the slower? IB 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, what- 

 ever 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 transmitting 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 certain amplitude, the mutual jostling of the 

 atoms produces quicker tremors, and the light-giving 

 waves follow as the necessary product of the heat-giv- 

 ing 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 vola- 

 tile liquids. Here, as in the case of the gases, vast 

 differences have been proved to exist between various 

 kinds of molecules, as regards their power of intercept- 

 ing the calorific waves. While some vapors allow the 

 waves a comparatively free passage, the faintest mix- 

 ture of other vapors causes a deflection of the magnetic 

 needle. Assuming the absorption effected by air, at a 

 pressure of one atmosphere, to be unity, the following 



