170 Mr. B. Stewart on the Radiative Powers of Bodies with 



reference to radiant heat rather than to radiant light-— if we can 

 by any method ascertain whether there are bodies the particles 

 of which give out the same quality of heat at the same tempera- 

 ture. A law which flows from Prevost's theory of exchanges, as 

 proved by the author in a paper published in the last volume of 

 the Transactions of the Royal Society of Edinburgh, affords, it 

 is believed, a method of ascertaining this fact. 



4. This law asserts that when a number of bodies exist in an 

 enclosure at a uniform and constant temperature, the heat ab- 

 sorbed by any particle is equal to that emitted by it ; and that 

 this equality subsists with regard to every individual description 

 of heat which goes to form the heterogeneous radiation of that 

 temperature. 



5. Let us now suppose two plates M and N, composed of two 

 different substances more or less diathermanous, to be hung up 

 side by side in such an enclosure of which the temperature is = t ; 

 and for the sake of simplicity we may conceive the walls of the 

 enclosure to be covered with lampblack, a substance which 

 radiates, but which does not reflect heat. Let us also suppose 

 that the refractive indices of the two plates are the same, or nearly 

 so ; and finally, let us limit our consideration to those rays which 

 enter each plate at right angles to its surface. It may be shown 

 that the same quantity of heat will flow through the substance 

 of each plate. 



6. Before the investigation be proceeded with, one hypothesis 

 requires to be made ; but one which, besides its apparent pro- 

 bability, has been proved by the author to hold good for mica 

 and glass, viz. that the mere heating of a substance through a 

 considerable range of temperature, if unaccompanied by any 

 chemical change, does not alter the absorptive power of its par- 

 ticles for a given description of heat. 



7. Let ttj, a 2) a 3 , &c. denote the absorptive power of the plate 

 M for the different kinds of heat which compose the lampblack 

 radiation of the temperature t, and let b lf # 2 , b 3 , &c. denote the 

 same constants for the plate N. 



(By the absorptive power of a substance is meant the quantity 

 of heat which would be absorbed by a plate of the substance of 

 thickness = unity when traversed by a ray of heat, the intensity 

 of which is kept equal to unity throughout the whole of this 

 thickness.) 



Let Aj, A 2 , A 3 , &c. be the quantities of the different descrip- 

 tions of heat which together compose the whole radiation which 

 flows perpendicularly through M and N at the temperature t. 



Also let MjoY, M 2 6Y, M 3 oY, &c. denote the radiations for these 

 different kinds of heat of a particle or plate of exceedingly small 

 thickness oY of the body M at the above temperature ; and let 



