346 M. MELLONI ON THE POLARIZATION OF HEAT. 
considerably with respect to different calorific rays, while they remain 
sensibly constant with respect to light, we may class the variable ab- 
sorbent power exercised on the different species of heat by the white 
surfaces of opake bodies which reflect the same proportion of coloured 
rays. Such, then, are the conditions to be satisfied henceforth by every 
theory that will refer the phenomena of light and radiant heat to a 
single principle. 
mistake into which the young and able successor of Leslie has 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 
the special aim of my labour was to raise objections against the undulatory 
theory of heat: “ M. Melloni lately read a paper to the Academy of Sciences 
staling certain objections to the undulatory theory of heat.) Now such was 
most assuredly not my intention; and I have, I think, 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 
the 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 this 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 sources ; arguments which must necessarily be given 
in the language of one or the other of the hypotheses on which calorific phzeno- 
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 /ength 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 interior 
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 systems tremble. I shall rather tremble myself lest, in 
consequence of preadopted notions, I may mistake the truth of the phenomena; 
and I suppose there is no one who will censure a timidity so salutary... but I 
will frankly avow that I consider any other fear scarcely philosophical. 
