158 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



of beat. 



The specific rays of heat they transmit, stopping altogether certain 

 action of ki n( j g or qualities of heat and transmitting others, 

 substances Substances, in general, transmit most readily the heat 

 on the rays radiated by surfaces having a high temperature; this 

 of heat. jjad a l r eady been shown to be true in the case of glass 

 by De la Roche. That experimenter had also demon- 

 strated (as we have seen in Art. 704) that successive 

 plates of glass intercept a constantly decreasing per- 

 centage of the heat incident upon them. This may 

 be explained by supposing radiant heat to be, like 

 the light of the sun or of a flame, heterogeneous, 

 containing rays of different qualities, some of which 

 are easily transmitted and others are wholly stopped 

 by glass. And if we pursue the analogy with re- 

 spect to other substances, we may imagine (for the 

 sake of illustration) heat to be coloured, and that 

 different media, though equally transparent and co- 

 lourless as regards light such as glass, rock-crystal, 

 and ice exercise a specific action on the rays of heat, 

 each transmitting certain portions of the heat and 

 stopping others. Rock-salt alone (according to Mel- 

 loni)is absolutely colourless with respect to heat, trans- 

 mitting all its varieties with uniform facility. Thus 

 equally thick and equally clear plates of salt, glass, 

 and alum, transmit, out of 100 rays of heat from 

 different sources the following proportions : 



Heat at 

 212 



92 

 

 



Heaif 



(714.) Let, however, heat which has been sifted by a plate 

 R .^ ran f i ~ of alum fall on another similar plate, then instead of 

 9 per cent., 90 per cent, will be transmitted. On the 

 other hand, if we unite two plates of opposite trans- 

 missive qualities, as alum and green glass, the com- 

 bination is almost absolutely opake, just as a combi- 

 nation of two coloured glasses giving different pure 

 tints (say red and green) would be opake for light. 

 The working out of this beautiful enquiry is entirely 

 due to Melloni ; and he has published a separate work 

 on the " Coloration of Heat." 1 He also rendered it 

 probable that the rays most easily absorbed by glass 

 and bodies generally are the least refrangible ; and this 

 has been made certain by direct experiments by the 

 author of this Dissertation, who, by a peculiar me- 

 thod, founded on the Total Reflection of heat within 

 a prism of rock-salt, has obtained the following In- 

 dices of Refraction : 



Heat from a lamp, mean refractive index 1-531 



Do. passed through glass 1-547 



Do. do. alum 1-558 



Mean luminous ray 1'562 



Melloni ingeniously applied the facts previously 

 mentioned to explain the variable position of the 

 hottest part of the spectrum, as observed by Sir 



(715.) 

 Explana- 

 tion of 

 point of 

 maximum 

 heat in the 

 spectrum. 



William Herschel and others. It depends on the 

 nature of the prism employed. In a prism of rock- 

 salt, the hottest part of the spectrum is as far beyond 

 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 

 Humboldt, Melloni ultimately obtained permission to 

 return to Italy and to reside at Naples, where he spent 

 his latter years. He ceased, however, to prosecute 

 his researches on radiant heat with the same energy, 

 undercircumstances of ease and comparative affluence, 

 that he had done in the period of distress and obscu- 

 rity. Nevertheless, several original papers were writ- 

 ten by him at this period, as well as the condensed 

 account of his earlier researches on the Coloration of 

 Heat, of which only the first volume appeared. Mel- 

 loni died of cholera at Portici, in August 1854, 

 aged 53. 



(717.) 



The analogy of Radiant Heat to light, strikingly 

 established by Melloni, with respect to the diversified f^rly 



iM'-i-i !<' i T tempts to 



retrangibihty and other qualities or the various radi- polarize ra- 

 ations emitted by one or different sources, suggests diant heat 

 an enquiry as to the intimate nature of these two ^erard. 

 agencies. No answer is likely to be so conclusive 

 as an appeal to the test of Polarization, which, in 

 the case of light, has been so remarkably explained 

 by the theory of the transverse undulations of a me- 

 dium. Some years before any of Melloni's papers 

 appeared, indeed, before he had entered on the in- 

 vestigation just noticed, the writer of the present 

 Dissertation had attempted, by means of common 

 thermometers, to test the polarizability of heat. The 

 trial was not a new one ; but, except in the case of 

 the heat of the solar rays, the results seemed to be 

 inconclusive, or were even wholly negative. Berard 

 had, indeed, not long after the discovery by Malus of 

 luminous polarization by reflection, repeated (in 1 8 1 2) 

 that experiment with sun-heat, and also with the 

 heat emanating from terrestrial sources ; and as he 

 believed with success. 2 I have ventured to call his 

 experiments inconclusive, because others besides my- 

 self vainly endeavoured to repeat them. Professor 

 Powell failed with ordinary thermometers, and at a 

 later period Nobili announced a decidedly negative 

 result, obtained with the thermo-multiplier. Simple 

 radiant heat, he affirmed, is not polarizable by re- 

 flection. 8 



I have just referred to my own early experiments (718.) 

 on the subject (which were likewise inconclusive), Experi- 

 in order to explain that it was natural, on hear- *" 

 ing of the application of the thermo-multiplier writer, 

 to measure radiant heat, that I should wish to 

 repeat them with the new instrument. This I 

 did in 1834. I first succeeded in proving the 

 polarization of heat by tourmaline (which Mel- 



1 La Thermochrose, ou la Coloration Calorifique. Naples, 1850. 



2 Memoires cCArceuil, vol. iii., p. 5. 



3 Bibliotheque Universelle, Sept. ,1834, 



