240 HEAT. 



If any bodies are suspended in the enclosure, they too will be indis- 

 tinguishable. A piece of coloured glass will absorb special rays, but will 

 also emit an equal amount of those rays. A plate of tourmaline will 

 polarise the light transmitted through it, but it will emit light polarised 

 in a perpendicular plane, making up the issuing stream to the full radia- 

 tion without polarisation. 



A difficulty in accepting these statements arises, perhaps, from our 

 want of experience of true constant-temperature enclosures. A room 

 without a fire in daylight may possibly be at a nearly constant and 

 uniform temperature, yet our experience shows that it is far from 

 answering to the above description, since we can see the various objects 

 in it quite plainly. But it is to be marked that we see them not by 

 their own radiation, but by reflected daylight which is originally sun- 

 light. In fact, the room is not an enclosure. The window being tran- 

 sparent, the sun forms part of the boundary from which radiations are 

 received, and it is at an enormously higher temperature than the rest of 

 the room, while the sky or clouds probably form another part of the 

 boundary far colder than the room. In order, then, to have a uniform- 

 temperature enclosure, the radiations received at any point within it 

 must all proceed from bodies at the same temperature. Hence it would 

 be impossible to make glass or rock-salt enclosures, unless, indeed, the 

 thickness were so great that the impurities and the natural absorption 

 rendered the walls opaque to external radiation. 



A room darkened by thick shutters, or a cellar without windows, 

 gives a very near approach to a uniform-temperature enclosure, but our 

 eyes are not sensitive to the radiations given out at ordinary tempera- 

 tures. A very hot coal-fire, however, frequently has cavities forming 

 nearly complete enclosures, into which we may look, and so verify some 

 of the properties stated above. The boundaries between the different 

 pieces of coal are very indistinct. Pieces of glass of different colours 

 put into the cavity soon become nearly indistinguishable, each restoring 

 to the stream of radiation what it takes from it. Similar observations 

 may be made on heating an ordinary clay crucible, or even a clay 

 tobacco-pipe, to red heat and putting small pieces of glass or of any 

 other non-combustible material at the bottom. On looking in, the same 

 indistinctness of outline will be marked. 



Effect of the Medium on Radiation. We have seen that in all 

 enclosures at the same temperature, containing the same medium, the 

 stream of radiation is the same, our proof depending on the assumption 

 that the radiation from an enclosed body is independent of the nature 

 of the radiating enclosure. 



But we have no ground for supposing it to be independent of the 

 surrounding medium. A body surrounded by rock-salt may be able to 

 send out more or less energy in a given time than the same body 

 surrounded by air. 



Let us now investigate the effect of a change of refractive index on 

 the stream of radiation. The following proof is due to Balfour Stewart 

 (Treatise on Heat} : 



Let AC A' (Fig. 139) represent a spherical constant-temperature 

 enclosure, the lower half being filled with a substance of refractive 

 index //., for some given radiation. Let B be a very small area on the 



