REPORT ON RADIANT HEAT. 35 
conception we can form of such radiation. Is this, we may ask, a 
real change of constitution, or is it a separation or analysis of 
the ray into its components ? 
I have elsewhere remarked, that the terms ‘* luminous” and 
* dark’’ heat are of somewhat barbarous appearance; and the 
objection is more than etymological, especially 'as' we now find 
the luminosity of the source is not the essential characteristic 
of the qualities of the rays. And again, in the compound radia- 
tion from luminous sources, there is included a considerable por- 
tion of “dark” heat as disclosed by its relation to surfaces in 
absorption. 
The relations of heat to swrfaces in absorption, and in the cor- 
responding inverse effects of radiation, are among the most im- 
portant portions of the subject ; and I have in consequence been 
desirous to draw particular attention to the very valuable inves- 
tigations of President Bache. 
The properties which characterize the different species of heat 
(as we have seen) have been most remarkably developed, and 
principally studied, in the phenomena of transmission. A wide 
field is open to the experimenter in connecting these properties 
with those belonging 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 con- 
ductive 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 supplies a clue to a vast range of phenomena, especially 
those connected with polarization. 
The question of the identity of the heating and illuminating 
radiations 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 possibility of accounting for the different effects by 
sets of undulations of the same etherial medium differing in 
their wave-lengths, this probably 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 
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