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from the screen be coated with lamp-black, its heat now passes 

 the glass screen as readily as ordinary heat of that temperature. 

 A similar fact is noticeable with regard to light. A red glass, 

 which, when heated and viewed in the dark, gives out a greenish 

 light, while in the fire scarcely appears to differ in tint from the 

 surrounding coals ; and the same fact holds for all coloured glasses. 

 Ultimately they all appear to lose their colour in the fire as they ap- 

 proach in temperature the coals around them. This may be ex- 

 plained thus : the red glass, for instance, still gives out its greenish 

 light ; but it passes red light from the coals behind it, in such a 

 manner that the light which it radiates precisely makes up for that 

 which it absorbs ; so that we have virtually a coal radiation coming 

 partly from and partly through the glass. 



Let us now consider Prevost's theory with regard to bodies of 

 indefinite thickness. One of its consequences was experimentally 

 discovered by Leslie ; viz. that metals which are good reflectors of 

 heat are very bad radiators. As a variety of this experiment, I have 

 endeavoured to show that a powdered diathermanous body will 

 radiate less than bodies which are opaque for heat, powdered. Thus, 

 if a plate of table-salt have one side blackened, the white side will 

 radiate only 83 per cent, of that which the blackened side radiates 

 at the temperature of 212 F. No such difference is observed in 

 sugar, which, though white for light, is black for heat of 212. 



We have here also similar facts with regard to light. If a pot of 

 red-hot lead or tin be carried to the dark, and the dross scummed 

 aside by means of a red-hot iron ladle, the liquid metal momentarily 

 disclosed will appear less luminous than the surrounding dross ; the 

 difference being more observable in the case of tin, which has a 

 higher reflecting power than lead. Also, if a piece of platinum, 

 partly polished and partly tarnished, be held above a flame in a dark 

 room, the tarnished portion will shine much more brilliantly than 

 the polished. Again, if we take a china cup with a white and black 

 pattern, and heat it to a white heat in the fire, while there we shall 

 not perceive much difference between the white and black of the 

 pattern ; but, if we bring it into a dark room, we shall perceive the 

 black to shine much more brilliantly than the white. This reversal 

 of the pattern presents a very curious appearance. 



Finally, it is a consequence of Prevost's theory and an experi- 



