Royal Society. 535 



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 

 this description also, it follows that the heat given out by a plate of 

 any substance will experience difficulty in passing through a screen 

 of the same substance. This we find to be the case : thus, a plate 

 of rock-salt 0*77 inch thick passes only 30 per cent, of the heat from 

 a thin plate of rock-salt heated to 212° F., whereas it will pass 75 

 per cent, of heat from lamp-black at that temperature. The same 

 thing holds with regard to glass. A thin plate of crown glass will 

 only pass half as much heat from heated crown glass as from heated 

 lamp-black. But this peculiar quality of the radiating plate is de- 

 stroyed if we coat the side of it furthest from the screen with lamp- 

 black ; it then behaves precisely as lamp-black alone would do. The 

 reason of this is that the rays of heat given out by the glass are the 

 equivalent of those which it absorbs from the lamp-black behind ; 

 so that both together give out the same heat in quantity and quality 

 as lamp-black would alone. 



We have here also analogous properties of light. Let us take a 

 number of differently coloured glasses. With respect to the light of 

 an ordinary fire, these may be divided into two groups ; viz. those 

 which redden, and those which whiten the fire when we look through 

 them. The first group comprises red and orange glasses ; the second 

 group green and blue glasses. 



Glasses of the former group absorb the whiter, glasses of the 

 latter group, the redder descriptions of light. We should therefore 

 expect red and orange glasses to give out, when heated, a peculiarly 

 white light, and green and blue glasses a peculiarly red light. A 

 number of red and orange glasses have been found which fulfil this 

 expectation. Among the reds, those coloured by gold, when re- 

 moved from the fire and held in the dark, give out a milky-white, 

 or even greenish light; and the orange glasses used by photo- 

 graphers do the same. Other glasses, of a dingy red tint, give out, 

 when heated, light slightly whiter than the ordinary light of their 

 temperature ; while there are others in which I have not, by this 

 somewhat rude method of experimenting, been able to detect a 

 sensible peculiarity of tint : yet this is not to be wondered at, if it 

 be remembered that the following is the method in which the ex- 

 periment is made. The glass under examination is held in a tongs, 



