Radiation of Heat by Gases and Vapours, 285 



blends harmoniously with the perfectly independent conception 

 of an ether, and enables us to reduce the phenomena of radia- 

 tion and absorption to the simplest mechanical principles. 



Considerations similar to the above may, I think, be applied 

 to the phenomena of conduction. In the Philosophical Magazine 

 for August 1853, 1 have described an instrument used in exami- 

 ning the transmission of heat through cubes of wood and other 

 substances. When engaged with this instrument, I had also 

 cubes of various crystals prepared, and determined with it their 

 powers of conduction. With one exception, I found that the 

 conductivity augmented with the diathermancy. The exception 

 was furnished by a cube of very perfect rock-crystal, which con- 

 ducted slightly better than my cube of rock-salt. The latter, 

 however, had a very high conductive power ; in fact rock-salt, 

 calcareous spar, glass, selenite, and alum stood in my experi- 

 ments, as regards conductivity, exactly in their order of diather- 

 mancy in the experiments of Melloni. I have already adduced 

 considerations which show that the molecules of rock-salt glide 

 with facility through the ether; but the ease of motion which 

 these molecules enjoy must facilitate their mutual collision. 

 Their motion, instead of being expended on the ether which 

 exists between them, and communicated by it to the external ether, 

 is in great part transferred directly from particle to particle, or 

 in other words, is freely conducted. When a molecule of alum, 

 on the contrary, approaches a neighbour molecule, it produces a 

 swell in the intervening ether, which swell is in part transmitted, 

 not to the molecules, but to the general ether of space, and thus 

 lost as regards conduction. This lateral waste prevents the 

 motion from penetrating the alum to any great extent, and the 

 substance is what we call a bad conductor*. 



Such considerations as these could hardly *occur without 

 carrying the mind to the kindred question of electric conduc- 

 tion ; but the speculations have been pursued sufficiently far for 

 the present, and must now abide the judgment of those com- 

 petent to decide whether they are the mere emanations of fancy, 

 or a fair application of principles which are acknowledged to be 

 secure. 



The present paper, I may remark, embraces only the first 

 section of these researches. 



* In the above considerations regarding conduction, I have limited 

 myself to the illustration furnished by two compound bodies ; but the ele- 

 mentary atoms also differ among tbemselves as regards tbeir powers of 

 accepting motion from the ether and of communicating motion to it. I 

 should infer, for example, tbat the atoms of platinum encounter more 

 resistance in moving tlirough the ether than the atoms of silver. It is 

 needless to say that the physical texture of a substance also has a great 

 influence. 



