3i6 M. MELLONI OS THE POl.ARIZATro>) OF HEAT. 



considerably ^ith respect to different calorific rays, wliile they remain 

 sensibly constant with respect to light, we may class the variable a1)- 

 sorbent power exercised on the different species of heat by the white 

 surfaces of opake bodies %vhich reflect the same pi'oportion of coloured 

 rays. Such, then, are the conditions to be satisfied henceforth by every 

 theory that will refer the phfenomena of light and radiant heat to a 

 single principle. 



mistake into which the young and able successor of Leslie lias fallen with re- 

 gard to these experiments. I find in a letter of his addressed to the Editors of 

 the London and Edinburgh Philosophical Magazine (March 1836, p. 245), that 

 tlie special aim of my labour was to raise objections against the undulatory 

 theory of heat: " M. Melloni lateli/ read a paper to the Academy of Scieitces 

 staling certain ohjections to the itiidulatory theory of heat.") Now such was 

 most assuredly not my intention; and I have, I tliink, expressed myself to that 

 effect with sufficient clearness in a note at the end of the memoir. In publishing 

 those facts my only object was to show that which I announced at the head of 

 tlie memoir presented to the Academy, namely, the nonidentity of light and 

 radiant heat, a proposition which is evidently independent of every theory. 

 Thus, whether the system adopted be that of emanation or that of undulations, 

 I do not think it possible to maintain at tliis day that the same molecule or the 

 same wave that gives, for instance, yellow light, at a determinate part of the 

 solar spectrum, produces the concomitant heat also. Such is the only conclu- 

 sion that I have drawn from the experiments contained in my memoir. The 

 author of the letter has been probably misled by the enunciation of the propo- 

 sition, (which I made in terms applicable only to one of the two systems,) as well 

 as by the arguments, which should in my opinion be brought, on the supposition 

 of their identity, to answer the objections derived from the previously known 

 differences between the action of diaphanous and diathermanous media on the 

 light and heat of terrestrial soiu'ces ; arguments which must necessarily be given 

 in the language of one or the other of the hypotheses on which calorific phseno- 

 mena are explained. I chose the language of the undulatory system ; but I 

 might as well have employed that of the system of emanation. It is besides 

 so true that the arguments contained in the memoir do not apply particularly 

 to the theory of undulations, that by suppressing certain expressions proper to 

 this theory, and changing the words wave and length into molecule and species, the 

 arguments are still good, and we thus arrive at the same general conclusion ex- 

 pressed in the language of the system of emission ; that is, that in the intei'ior 

 of the solar spectrum the same molecules cannot produce the two effects of light 

 and heat simultaneously. In short, my only aim in pursuing my researches 

 with respect to radiant heat is to study the laws and properties of this agent. I 

 have not the vanity to think that by any new discovery I shall make the parti- 

 sans of either of the s^'stems tremble. I shall rather tremble myself lest, in 

 consequence of preadopted notions, I may mistake the truth of the phaenomena ; 

 and I suppose thei-e is no one who will censure a timidity so salutary . . .but I 

 will frankly avow that I consider any other fear scarcely philosophical. 



