RADIATION. 55 



formed by the condensation of incandescent vapours. 

 In such cases when the curve representing the radiant 

 energy of the body is constructed, the obscure radia- 

 tion towers upwards like a mountain, the luminous radi- 

 ation resembling a mere ' spur ' at its base. From the 

 very brightness of the light of some of the fixed stars 

 we may infer the intensity of that dark radiation, which 

 is the precursor and inseparable associate of their lu- 

 minous rays. 



We thus find the luminous radiation appearing 

 when the radiant body has attained a certain temper- 

 ature; or, in other words, when the vibrating atoms of 

 the body have attained a certain width of swing. In 

 solid and molten bodies a certain amplitude cannot be 

 surpassed without the introduction of periods of vibra- 

 tion, which provoke the sense of vision. How are we 

 to figure this? If permitted to speculate, we might 

 ask, are not these more rapid vibrations 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 pro- 

 duced 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 mu- 

 tual jostling of the atoms produces quicker tremors, and 



