THEORY OF EXCHANGES. 239 



silver radiates very little, but then it reflects nearly all that falls on it. 

 The rock-salt neither reflects nor radiates much, but transmits freely most 

 of the radiation from the surface against which it is placed. 



There is, therefore, a stream of radiation going in all directions in a 

 constant-temperature enclosure, the amount and quality of the stream depend- 

 ing only on the temperature. Since a lampblacked surface absorbs nearly all 

 the radiation falling on it, it must send out a stream nearly equal to this 

 total radiation, and hence the radiation in a constant - temperature 

 enclosure is sometimes termed the lampblack radiation. But we know 

 that even lampblack does not entirely absorb the radiation falling on it, 

 diffusing and transmitting small fractions. Hence it does not radiate 

 out quite as much radiation as falls on it in the enclosure. It is better, 

 therefore, to term the stream in a constant, uniform temperature en- 

 closure, the full radiation for the given temperature. 



From the foregoing we may immediately deduce the following 

 results : 



(a) If a body absorbs any kind of radiation it must also emit the same 

 kind of radiation at the same temperature, and if it is placed in a uniform- 

 temperature enclosure the emission equals the absorption. 



This follows at once from the fact that, in order to make the stream 

 of issuing radiation full, it must restore to the incident stream just what 

 it took from it. 



(b) A body at a given temperature cannot emit more of a given kind of 

 radiation than exists in the full radiation for that temperature. 



For, if it could, and if it were placed in a constant-temperature en- 

 closure, the issuing stream would contain more of the radiation than the 

 incident stream or, the stream would not be uniform in character.* 



(c) A body opaque to any kind of radiation must emit that kind of 

 radiation. 



For, construct a uniform-temperature enclosxire entirely lined with 

 the substance, and let the temperature be such that the given radiation 

 is a constituent of the full radiation at that temperature. The given 

 radiation now existing in the enclosure must have come from the lining 

 substance, for it could not get through from behind. Hence, opacity is 

 always accompanied by the power of emission, and, therefore, of 

 absorption. 



To enable us to think a little more definitely of such an enclosure, 

 let us suppose a hollow iron ball, coated inside partly with lampblack, 

 partly with silver, partly with rock-salt. Let it then be heated red hot 

 and kept at a constant temperature. If we can imagine ourselves placed 

 within, but protected in some way from the high temperature without 

 affecting the full radiation, then, on looking round, everything will 

 appear of the same colour. The lampblack radiates out most but it 

 reflects little, the silver radiates little but reflects much. The rock-salt 

 is nearly transparent to the radiation from the iron behind it, and the 

 uncovered iron partly radiates and partly reflects enough to make up the 

 full radiation. We shall, therefore, have one uniform glare, and shall be 

 quite unable to see the boundary lines between the different substances. 



* This does not necessarily hold if the body is phosphorescent, or if it is under- 

 going chemical or molecular change. In these cases molecular energy may be 

 transformed into radiant energy without a corresponding rise of temperature. 



