NATURAL PHILOSOPHY. 143 



These results are unquestionably among the most curious and inter- 

 esting of those which experimental research has recently brought 

 before us. When first announced, some ten or twelve years ago, they 

 did not attract the attention which they deserved ; but more recently 

 their importance has been fully recognized by all those who cultivate 

 the department of science to which they belong. Of this Mr. Joule 

 received last year one of the most gratifying proofs, in the award 

 made to him by the Council of the Royal Society of one of the medals 

 placed annually at their disposal. 



This theory is in perfect harmony with the opinions now very gen- 

 erally entertained respecting radiant heat. Formerly light and heat 

 were regarded as consisting of material particles continually radiating 

 from luminous and heated bodies respectively ; but it may now be 

 considered as established beyond controversy that light is propagated 

 through space by the vibrations of an exceedingly refined ethereal 

 medium, in a manner exactly analogous to that in which sound is 

 propagated by the vibration of the air, and it is now supposed that 

 radiant heat is propagated in a similar manner. This theory of radi- 

 ant heat, in accordance with the dynamical theory of which I have 

 been speaking, involves the hypothesis that the particles of a heated 

 body, or a particular set of them, are maintained in a state of vibra- 

 tion, similar to that in which a sonorous body is known to be, and in 

 which a luminous body is believed to be. At the same time, there are 

 remarkable differences between light and heat. We know that light 

 is propagated with enormous velocity, whether in free space or through 

 transparent media ; sound also is propagated with great rapidity, and 

 more rapidly through most media than through air. Heat, on the 

 contrary, whatever may be the velocity with w r hich it may radiate 

 through free space, is usually transmitted with extreme slowness 

 through terrestrial media. There appears to be nothing in light analo- 

 gous to the slow conduction of heat. Again, the vibrations which ren- 

 der a body sonorous have no tendency to expand its dimensions, nor 

 is there reason to suppose that luminous vibrations have any such ten- 

 dency on luminous bodies ; whereas, with the exception of particular 

 cases, heat does produce expansion. It is principally from this prop- 

 erty of heat that it becomes available for the production of motive 

 power, as, for instance, in the expansion of steam. These phenomena 

 of the slow conduction of heat and the expansion of heated bodies, are 

 proofs of differences between light and heat not less curious than the 

 analogies above indicated. They must, of course, be accounted for by 

 any perfect theory of heat. Mr. Eankine has written an ingenious 

 paper on a molecular theory of heat ; but before any such theory can 

 be pronounced upon, it will be necessary, I conceive, to see its bear- 

 ing on other molecular phenomena, with which those of heat are in 

 all probability intimately connected. Prof. W. Thomson has also 

 given a clear and compendious mathematical exposition of the new 

 dynamical theory of heat, founded on Mr. Joule's principle of the 

 exact equivalence of heat and mechanical effect. This is not, like 

 Mr. Rankine's, a molecular theory, but one which must henceforth 

 take the place of Carnot's theory. 



