538 



Royal Society s — 



3rd. And, consequently, that when a body is kept at a uniform 

 temperature, it receives back just as much heat as it gives out. 



From these three assumptions may be deduced all the facts that 

 have been stated with regard to radiant heat ; but in the argument 

 it is essential that the rays under consideration shall have the pro- 

 perty of heating the bodies on which they fall, and by which they 

 are absorbed. If this be not granted, the argument fails. Now 

 radiant light, or those rays only that affect the retina, have been 

 found to possess properties analogous to those which radiant heat 

 thus possesses in virtue of its departure lowering the temperature of 

 the body which it leaves, and its absorption raising that of the body 

 on which it falls. If, therefore, we suppose all kinds of radiant 

 light to have the property of raising (however little) the temperature 

 of the body by which they are absorbed, the facts that have been 

 stated in this paper regarding light may be shown to be a natural 

 consequence of Prevost's theory of exchanges ; but if, on the other 

 hand, we do not admit that all the kinds of radiant light given out 

 by heated bodies possess this property, then in that case those facts 

 cannot be explained by Prevost's theory, but they will require a new 

 theory to account for them. 



This circumstance induces me to think that all the descriptions of 

 light radiated by heated bodies have the power of heating, more or 

 less, those bodies by which they are absorbed. Viewing the matter 

 in this light, I have constructed the following Table, in which the 

 logical consequences of Prevost's theory are stated in the first 

 column, while opposite these in the second column are detailed the 

 different experiments which they serve to explain. 

 Table of the consequences of Prevosfs theory, and the facts which 

 they explain. 

 Consequences of Prevost's theory. Facts which these consequences explain. 



r Rock-salt which absorbs little heat 

 of 212°, F., gives out little ; while glass, 

 which absorbs much, gives out much. 



The heat radiated .by rock-salt has 

 great difficulty in passing through a 

 screen of rock-salt — the heat radiated 

 by glass in passing through a screen of 

 glass. 



The radiation of a thin plate or par- Colourless glass, when heated, gives 

 tide is equal to its absorption, and that out little light, opaque glass a great deal, 

 for every description of heat— that is to.} R ed g i ass> w hi c h absorbs the greenish 

 say, in quality as well as in quantity. rays> g j ve8 out greenish rays ; while green 



glass, which absorbs the red rays, gives 

 out red rays. 



When a plate of glass is coated on its 

 further side with lamp-black, its heat is 

 the same as lamp-black heat. 



All coloured glasses appear to lose 

 their colour in the fire. 



Metals radiate little, both of heat and 

 Those opaque bodies which reflect *&*• ^abte-salt, which is white for 

 most, radiate least. Opaque bodies ge- I h « at . of H*! «£? te » ^ ***.*"&*' 

 nerally give out the same kind of rays 

 at the same temperature : these words 

 also express the known fact. 



■4 which is black. When a black and white 

 china cup is heated in the fire and held 

 in the dark, the black of the pattern is 

 more luminous than the white. 



