CHAPTER XV, 

 THEORY OF EXCHANGES. 



Theory of Exchanges Uniform Temperature Enclosures Full Radiation Pro- 

 positions regarding Uniform Temperature Enclosures Bodies exchanging 

 Kadiation at different Temperatures Bodies in the same Physical State con- 

 tinue to absorb the same kind of Rays independently of Change of Temperature 

 Eadiation of every kind emitted by a Body increases as the Temperature 

 rises Application to Special Cases. 



Theory Of Exchanges. If an enclosure is maintained at a constant 

 temperature, and we put another body say, a thermometer inside it, 

 the thermometer, if at a lower temperature, will receive energy from 

 the surrounding walls by radiation, while, if at a higher temperature, it 

 will radiate out energy, till, in either case, it arrives at the temperature 

 of the enclosure, after which everything remains steady. But according 

 to the kinetic theory of matter, the molecules of the enclosure and the 

 body are still in vibration, still in a condition to send out radiation. We 

 are led, therefore, to suppose that the radiation is going on just as before, 

 and that the apparent cessation of transfer, when equilibrium of tem- 

 perature is reached, is really due to a balance of exchange, the ther- 

 mometer radiating out just as much as it absorbs. The space between 

 the two bodies is not to be regarded as no longer affected, but as the 

 medium of two equal and opposite streams of radiation. Before the 

 balance is arrived at, the thermometer, if hotter, is sending out more 

 than it receives ; if colder, it is receiving more than it sends out. 

 We may suppose, in fact, that there is, in a given transparent medium, 

 (we say " given " since, as we shall see later, the medium has an effect) 

 a total radiation from a body depending on its own temperature alone, 

 and not on the surrounding bodies. When a body ceases to lose or gain 

 energy, it is receiving an amount equal to its own radiation. We 

 imagine a process more complex than that for which we have direct 

 experimental evidence. 



Perhaps we may illustrate the nature of the theory by considering 

 the money payments between two commercial firms, each supplying the 

 other with goods. Leaving out of consideration altogether the goods 

 passing, at each settling-day a certain balance would generally have to 

 be paid over by one of the firms to the other. But, examining the 

 books, we should find each transaction entered separately, and the effect 

 is, therefore, the same as if each purchase were paid for separately 

 as if each firm paid for the whole of the goods it purchased, and as if it 

 received the whole sum due on the goods it supplied. The book-keeping 

 takes account of the separate items, but the money passing only concerns 

 the balance. Now, in the case of radiation, experimentally we only 



