956 MATHEMATICAL AND 
The specific rays of heat they transmit, stopping altogether certain 
PHYSICAL SCIENCE. [Diss. VI. 
William Herschel and others. It depends on the 
pected kinds or qualities of heat and transmitting others. nature of the prism employed. In a prism of rock- 
substances Substances, in general, transmit most readily the heat salt, the hottest part of the spectrum is as far beyond 
on the rays radiated by surfaces having a high temperature; this 
of heat.— 
Coloration 
had already been shown to be true in the case of glass 
by Dela Roche. That experimenter had also demon- 
the extreme visible red, as the interval between that 
red and the yellow ray in an opposite direction. 
Through the intervention of Arago and of Baron ies 
og strated (as we have seen in Art. 704) that successive Humboldt, Melloni ultimately obtained permission to yy.) 
plates of glass intercept a constantly decreasing per- return to Italy and to reside at Naples, where he spent 
centage of the heat incident upon them. This may his latter years, He eased, however, to prosecute 
be explained by supposing radiant heat to be, like ‘his researches on radiant heat with the same energy, 
the light of the sun or of a flame, heterogeneous, undercircumstances of ease and comparative affluence, 
containing rays of different qualities, some of which that he had done in the period of distress and obscu- 
are easily transmitted and others are wholly stopped rity. Nevertheless, several original papers were writ- 
by glass. And if we pursue the analogy with re- ten by him at this period, as well as the condensed 
spect to other substances, we may imagine (for the account of his earlier researches on the Coloration of 
sake of illustration) heat to be colowred, and that Heat, of which only the first volume appeared. Mel- 
different media, though equally transparent and co- loni died of cholera at Portici, in August 1854, 
lourless as regards light—such as glass, rock-crystal, aged 53. 
and ice—exercise a specific action on the rays of heat, 
each transmitting certain portions of the heat and = The analogy of Radiant Heat to light, strikingly (717.) 
stopping others. Rock-salt alone (according to Mel- established by Melloni, with respect to the diversified pts Be A} 
loni) is absolutely colourless with respect to heat, trans- refrangibility and other qualities of the various radi- olaciae Sas 
mitting all its varieties with uniform facility. Thus ations emitted by one or different sources, suggests diant heat 
equally thick and equally clear plates of salt, glass, an enquiry as to the intimate nature of these two —Berard- 
and alum, transmit, out of 100 rays of heat from agencies. - No answer is likely to be so conclusive 
different sources the following proportions :— as an appeal to the test of Polarization, which, in 
Heat from Heatfrom Heat from tthe case of light, has been so remarkably explained 
abright cent Pla- at 700° 212° by the theory of the transverse undulations of a me- 
Boek welt 92 Da. ron 92 dium. Some years before any of Melloni’s papers 
Plate-glass ..,.,.39 24 6 0 appeared,—indeed, before he had entered on the in- 
Alum wees 9 2 0 0 vestigation just noticed,—the writer of the present 
(714.) Let, however, heat which has been sifted by a plate Dissertation had attempted, by means of common 
ae of alum fall on another similar plate, then instead of thermometers, to test the polarizability of heat. The 
Head 9 per cent., 90 per cent. will be transmitted. Onthe trial was not a new one; but, except in the case of 
other hand, if we unite two plates of opposite trans- the heat of the solar rays, the results seemed to be 
missive qualities, as alum and green glass, the com- inconclusive, or were even wholly negative. Berard 
bination is almost absolutely opake, just as a combi- had, indeed, not long after the discovery by Malus of 
nation of two coloured glasses giving different pure luminous polarization by reflection, repeated (in 1812) 
tints (say red and green) would be opake for light. that experiment with sun-heat, and also with the 
The working out of this beautiful enquiry is entirely heat emanating from terrestrial sources ; and as he 
due to Melloni ; and he has published a separate work believed with success.?_ I have ventured to call his 
on the “ Coloration of Heat.”! He also rendered it experiments inconclusive, because others besides my- 
probable that the rays most easily absorbed by glass self vainly endeavoured to repeat them. Professor 
and bodies generally are the least refrangible; andthis Powell failed with ordinary thermometers, and at a 
has been made certain by direct experiments by the later period Nobili announced a decidedly negative 
author of this Dissertation, who, by a peculiar me- result, obtained with the thermo-multiplier. Simple 
thod, founded on the Total Reflection of heat within radiant heat, he affirmed, is not polarizable by re- 
a prism of rock-salt, has obtained the following In-  flection.® 
dices of Refraction ;— I have just referred to my own early experiments (718,) 
Heat from a lamp, mean refractive index .........1°531 on the subject (which WATS likewise inconclusive), oe 
Do. passed through glass ............sssesesssseses 1-547 in order to explain that it was natural, on hear- j)" presi 
Do. _ do, LGU eeivesess soy onset antad RODS ing of the application of the thermo-multiplier writer, 
iar Mean luminous a Sapna 30 pe. d4nse Gy Wess eb phebereeees 1562 to measure radiant heat, that I should wish to 
Explana-  Melloni ingeniously applied the facts previously repeat them with the new instrument. This I 
tion of | mentioned to explain the variable position of the did in 1834, I first succeeded in proving the 
ee 5 hottest part of the spectrum, as observed by Sir polarization of heat by tourmaline (which Mel- 
heat in the 
spectrum, = 1 La Thermochrose, ou la Coloration Calorifique. Naples, 1850. 
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2 Memoires d’ Arceuil, vol. iii., p. 5. 3 Bibliotheque Universelle, Sept. 1834. 
x 
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