ON RADIANT HEAT. 
339 
now proved it to be the fact. This constitutes the peculiar value of Prof. 
Forbes’s researches. In the instance of lummous bodies, then, all the 
combined heterogeneous species of rays undergo these modifications; though 
possibly in different degrees, and liable to modifications from the different 
nature of the media employed. 
Yet many recent researches seem altogether to ignore these distinctions; 
and the “radiant heat” from different sources is commonly spoken of as if 
all of one kind, and of which a certain per-centage is stopped, or transmitted, 
in particular cases, by the interposition of certain substances ; whereas the 
body of rays is heterogeneous , and certain integrant rays only are totally 
stopped or totally transmitted in the respective cases, showing not only 
quantitative but qualitative differences. 
In many researches of this kind the “diathermancy " of bodies is spoken 
of, and experiments made to measure it, as if it applied indifferently to all 
species of heating rays, and without any reference to the consideration that 
for certain species of rays bodies are (Uothermanous simply in proportion as 
they are diaphanous to the particular luminous ray in question; while for 
other species of rays their diathermaneity follows some totally different, and 
yet unknown law. The coufusion of ideas introduced by the adoption of 
that term, unqualified by any reference tii this distinction, and applied to 
general conclusions respecting “radiant heat,”ought to be sufficiently manifest, 
yet has been but little considered. And in cases where the rays transmitted 
by a partially diathermanous body are in fact different in kind (agreeably to 
'he above distinction) from those stopped, there is an ambiguity in the mode 
of expression; in fact it is not merely a percentage of rays, but an analysis of 
. e whole heterogeneous body into more homogeneous or simple elements, 
*■ e ' ra y 8 different wave-lengths, or combinations of several such rays. 
In my former Report (1832, p. 299), it is mentioned that I had then re¬ 
peated my original experiment (Phil. Trans. 1825) with u pair of thermo- 
eters of a peculiar construction made on purpose, in which very small dif- 
erenccs were appreciable; but I did not there state any of the results. As 
tention has now been recalled to my original experiment, both by the re¬ 
petition of it by Mellon! with the thermo-multiplier, and the more recent 
, . atory remarks of Knoblauch, and as results obtained in this way 
\ notwithstanding the superior claims of the thermal pile) may still possess some 
i crest, it may not be irrelevant here briefly to state the results of a set of 
serrations which have been lying by me since that period. The instrument 
on lh Unlortunatel y soon afterwards. These observations were made 
dish e . ra * V p an ^ r £ anf * l flni P ; the thermometers, fixed on one frame at the 
a sor I CC r a ^ oul ^ inches apart, were exposed first with, and then without, 
• 0 PMe glass one-eighth of an inch thick. The mode of observation 
wh>M,°\ Qte ^ e . r ^* e l l ,e thermometer, blackened with China ink, in the time 
1 it occupied the white-washed thermometer to rise 1° Fahr. 
Screen. 
No screen. 
Number 
of obser¬ 
vations. 
White. 
Black. 
White. 
Black. 
0 
1 
3 
I o 
1 
O 
J-6 
20 
Another set gave the first momentary rise— 
0-5 
2 
1 
1*5 
5 
z 2 
