RADIATION. 55 



luminous fogs formed by the condensation of incan- 

 descent vapours. In such cases when the curve repre- 

 senting the radiant energy of the body is constructed, 

 the obscure radiation towers upwards like a mountain, 

 the luminous radiation 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 asso- 

 ciate of their luminous rays. 



We thus find the luminous radiation appearing 

 when the radiant body has attained a certain tem- 

 perature ; 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 

 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 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 aether 

 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. 



