100 REPORT 1S61. 



ception had been formed of the character, with respect to intensity and 

 polarization, of the heat emanating from the surface of a body in different 

 directions, necessary in order that the condition of equilibrium of tempe- 

 rature be fulfilled. No attempt, however, seems to have been made to split 

 up this body of heat into its constituent wave-lengths, with the view of ascer- 

 tainino- whether the same laws of equilibrium hold for each of these which 

 hold for the body of heat taken as a whole. Internal radiation, as a subject 

 for experiment, seems also to have been overlooked, and its essential con- 

 nexion with the theory of exchanges does not appear to have been recognized. 

 In March 1858, I communicated to the Royal Society of Edinburgh the 

 results of an experimental research having reference to the two points just 

 mentioned. By means of a thermo-electric pile and galvanometer the fol- 

 lowing facts were established : — 



1. The radiating power of thin polished plates of different substances was 

 found to vary as their absorptive power; so that the radiation of a plate of 

 rock-salt was only 15 per cent, of the total lampblack radiation for the same 

 temperature. 



2. It was shown that the radiation from thick plates of diathermanous 

 substance is greater than that from thin plates, no such difference being 

 manifested when the substances are athermanous. 



3. It was found that heat radiated by a thin diathermanous plate is less 

 transmissible through a screen of the same material as the heated plate than 

 ordinary or lampblack heat, this difference being very marked in the case of 

 rock-salt. 



4. Lastly, heat from a thick diathermanous plate is more easily transmitted 

 through a screen of the same nature as the source of heat than that from a 

 thin plate. 



All these facts are easily explained by means of the theory of exchanges. 

 Let us recur to the hypothetical chamber before introduced, the sides of which 

 are covered with lampblack and kept at a constant temperature, and let us 

 hang up in this chamber two slices of polished rock-salt, of which the one is 

 twice as thick as the other ; these plates will ultimately attain the temperature 

 of the sides of the chamber, when their radiation will exactly equal their 

 absorption. Now, since the thick plate will absorb more than the thin one of 

 the heat which falls upon them from the walls, it will therefore also have a 

 greater radiation than the latter; as, however, both plates, being diathermanous, 

 absorb only a small portion of the heat which falls upon them, the radiation 

 of both will be comparatively small. We have thus an explanation of the 

 experimental fact that diathermanous bodies radiate very little heat, and that 

 their radiation increases with their thickness. We see also why in an ather- 

 manous body an increase of thickness does not augment the radiation, — the 

 reason being that, since it is already athermanous, this increase cannot pos- 

 sibly make it absorb more heat, and therefore cannot make it radiate more. 



We are therefore brought to recognize internal radiation as a consequence 

 of the theory of exchanges ; but the question now arises, Is the radiation of a 

 particle independent of its distance from the surface ? A little reflection will 

 enable us to answer this question in the affirmative ; for it is evident (neg- 

 lecting the surface reflexion, which does not really alter the result arrived at) 

 that the amount of heat absorbed by two plates of any substance placed 

 loosely together is not different from that absorbed by a plate equal in thick- 

 ness to the two, and hence the radiation is the same also in both these cases. 

 I have likewise shown experimentally that the heat from two plates of rock- 

 salt placed the one behind the other, is the same as that from a single plate 

 equal iu thickness to the two. 



