362 RADIANT HEAT. 



to the conditions of surface which produce the absorptive powers of 

 bodies for different species of heat; and these, again, with those which 

 mark the differences in conductive power, and perhaps also capacity 

 for heat. 



With regard to the establishment of a theory of the nature of radiant 

 heat, we have seen that the hypothesis of undulations certainly sup- 

 plies a clue to a vast range of phenomena, especially those connected 

 with polarization. 



The question of the identity of the heating and illuminating radia- 

 tions seems clearly negatived by many experiments, if we mean it to 

 apply in the sense of one physical agent. But if we refer to the pos- 

 sibility of accounting for the different effects by sets of undulations of 

 the same etherial medium differing in their wave-lengths, this proba- 

 bly presents fewer difficulties than any hypothesis of peculiar heat. 



We may, perhaps, suppose some other element besides the wave- 

 length to enter into the explanation; or while we find that the heating 

 effect is due to waves of greater length, it may also be true that the 

 intensity or accumulation of waves which is necessary for producing 

 the sensation of light follows a very different and much higher ratio 

 than that requisite for producing heat, and that this latter effect may 

 be produced in the highest intensity by longer waves of the same 

 etherial medium but not sufficiently accumulated to impress our visual 

 organs. 



The difference in the polarizability of heat from different sources is 

 not explained by the slight difference of refrangibility, and Professor 

 Forbes is of opinion that we must, in consequence, look for its solution 

 to a mechanical theory of heat in some respects, at least, different 

 from that of light. It is even a question of some difficulty why any 

 portion of the heat should not be subject to the law of polarization 

 which the rest obeys, unless we suppose the heating effect to be of 

 so complex a nature that some part of it only is properly due to rays 

 analogous to those of light, while the other part of the effect is pro- 

 duced by a mode of action altogether different. 



To any such questions, however, we are hardly yet in a condition 

 to give a satisfactory answer; but among the numerous points open 

 to inquiry I have dwelt more particularly on those which appear to 

 me pre-eminently to require more extended investigation before we 

 can hope to obtain materials for constructing any substantial and 

 unexceptionable theory. 



REPORT FOR 1854. 



Pakt I. 



Having been honored, for the third time, with a request from the 

 general committee to continue my former Reports on the state of our 

 knowledge of Radiant Heat, from the date of my last report (1840) up 

 to the present time, I feel bound to explain that the resolution con- 

 veying that request was passed so long since as the meeting at York 

 in 1851, and that it was complied with on the understanding that the 



