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In the first place, the heat radiated by a thin plate of any sub- 

 stance at a given temperature, is proportional to the absorptive 

 capacity of that substance for the heat of that temperature ; or, in 

 few words, its radiation is equal to its absorption. 



Rock-salt, for instance, has a small absorptive capacity for heat of 

 212 F., and, in consequence, its radiation when heated to 212 F. is 

 comparatively small. In point of fact, a plate of this substance, 

 0'18 inch thick, only gives out 15 per cent, of the heat which 

 lamp-black radiates at the same temperature. Glass, on the other 

 hand, absorbing nearly all the heat of 212F. which falls upon it, 

 has at this temperature a radiation comparatively great, and nearly 

 equal to that of lamp-black. A similar law holds with regard to 

 radiant light. 



If a piece of perfectly transparent glass be heated in an ordinary 

 fire, removed to the dark, and there viewed, it will be found to emit 

 scarcely any light ; if the glass be slightly coloured, its radiation 

 will be more copious ; the amount of light given out, as far as I 

 have been able to make the comparison, invariably depending upon 

 the depth of colour or absorptive power of the glass for light, pro- 

 vided its colour stands heating. A good way of performing this 

 experiment is to heat a dark glass by the side of a colourless one, by 

 means of a chemical tongs, in some uniform field of heat. When 

 viewed in the dark together, the contrast is very striking between 

 the bright light of the one and the bare visibility of the other. 



A stratum of heated gas may likewise be instanced as a substance 

 which neither absorbs nor emits light to a sensible extent ; and it has 

 similar properties with respect to heat. 



Let us now proceed to another consequence of Prevost's theory. 

 It is well known, from Melloni's experiments, that thin plates of 

 various substances have the property of sifting the heat which falls 

 upon them ; they stop certain rays, and allow others to pass ; the 

 heat stopped being of one description, and the heat passed of 

 another. Now, it may be shown to flow from the theory of ex- 

 changes, that the heat radiated by a thin plate of any substance at a 

 given temperature is precisely that description of heat which the 

 plate absorbs when heat of that temperature is allowed to fall upon 

 it. The heat which it absorbs being that kind of heat which has 

 a difficulty in passing through it, if the heat which it radiates be of 



