RADIANT HEAT. 333 



In liquids it has been disputed whether there can bo radiation: and 

 they are worse conductors than solids. 



In elastic media radiation can commence and continue; but they 

 are still worse conductors. 



In vacuo it might be presumed by analogy tkat a yet more free 

 radiation might take place; yet some experiments (as we have seen) 

 show the contrary; and here there is no conduction. 



With regard to that portion cf the heat which accompanies or be- 

 longs to light, the theory which I originally suggested, (merely as an 

 hypothesis representing the facts,) viz: that it was simply the latent 

 heat of light, developed, of course, when the light ivas absorbed, is con- 

 nected with the hypothesis of the materiality of light; but it may be 

 worth inquiry whether it does not apply even better to the elastic 

 cether, in whose undulations light is now proved to consist. 



REPORT FOR 1840. 



Having been one of those who at the first institution of the British 

 Association were applied to to prepare reports on the state and pro- 

 gress of the different branches of science, and having in consequence 

 laid before the Association at the Oxford meeting in 1832 such a 

 review of the subject of Radiant Heat, I have felt peculiar satisfac- 

 tion in being again honored by a request from the council to furnish 

 a second report supplementary to the former, embracing the progress 

 of knowledge in that department from the period to which the first 

 report extends, up to the present time. 



Such a supplementary account has been rendered peculiarly neces- 

 sary, from the great number and high importance of the results which 

 have been arrived at by several eminent experimenters in the interval 

 which has elapsed; and though much is still required to be done before 

 we attain complete and satisfactory grounds for an unexceptionable 

 theory of radiant heat, yet the discoveries recently made have at 

 least tended greatly to modify all our previous conceptions, and to 

 enable us to refer large classes of the phenomena to something like 

 a simple and common principle. 



In my former report I divided the subject under various heads, de- 

 rived from what appeared, in the existing state of our knowledge, 

 well-marked distinctions between several kinds of effects ascribed to 

 radiant heat. The more recent discoveries have in a great degree so 

 changed our views of the subject that these divisions cannot with any 

 advantage or convenience be adhered to. One grand principle of 

 arrangement, however, has been newly supplied in the capital dis- 

 covery of the polarization of heat; so that all the researches we have 

 to describe will be conveniently classed under. two heads, as they 

 relate— first, to radiant heat in its ordinary or unpolarized state; and 

 secondly, to its polarized condition. 



