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Macedonio Melloni (11 Apr 1798 - 11 Aug 1854) Italian physicist who was the first to extensively research infrared radiation. Short biography of Macedonio Melloni >>

Macedonio Melloni
Transmission and Refraction of Heat

from A Review of the Progress of Mathematical and Physical Science (1858)
by James David Forbes

§ 8. Melloni.—Recent History of Radiant Heat—Transmission and Refraction of Heat; Properties of Heat analogous to Colour.—Experiments in Great Britain on the Polarization and Double Refraction of Heat.

(707.)Recent Observations on radiant heat.

[p.157] The length to which this chapter has already extended, must be my apology for bringing concisely to a on radiant conclusion what remains to be stated regarding the heat progress of the subject of radiant heat. With the exception of the excellent researches of De la Roche on the immediate transmission of radiant heat through glass ..., but which that ingenious philosopher did not live to extend and complete, little of importance was done between the researches of Leslie and those of Melloni, of which we are now to speak.

(708.) Melloni—the associate of Nobili.

Macedonio Melloni, a native of Parma in Italy, Melloni became associated as an experimenter, probably about the year 1828 or 1829, with Nobili, a skilful and ingenious physicist of Reggio (Modena). Nobili was well known by his experiments on galvanic Electricity and on Electro-Magnetism. He was also the great improver of Schweigger's Multiplier, rendering it an instrument of precision; and to him we owe the happy and ingenious application of Thermo-Electricity to the measurement of minute effects of heat.

(709.) The thermo-multiplier, an instrument of research.

The Thermo-Multiplier, a thermometer of extreme delicacy, though improved by Melloni, was (as has just been stated) the invention of Nobili. It consists of instrument two portions, a sentient part and an indicating part. The first is composed of a number of short thin bars of antimony and bismuth, arranged like a square faggot, pairs of bars being soldered together in consecutive order at the opposite ends of the faggot, so as to form a single bent metallic conductor. If the junctions exposed at one end of the faggot are subjected to heat, and those at the other end kept cool, the effect will be a thermo-electric current of considerable intensity generated by the pile. This current is conveyed by means of two wires from the opposite ends of the system, which are connected with a delicate galvanometer which forms the indicating part of the apparatus. In practice, one end of the pile, armed with a conical reflector for concentrating the rays of heat, is exposed to a calorific source whose radiant effect is to be measured, whilst the other end is carefully screened from external influences. The deviations of the galvanometer needle indicate the heating effect as on the scale of a thermometer. The precautions required in the construction and use of the instrument, and in the interpretation of its results, are too numerous to be mentioned here.

(710.) Nobili, in conjunction with Melloni, applied the thermo-multiplier (amongst other experiments) to the proof of the instantaneous transmission of heat through glass and other solid and liquid bodies.

(711.) Melloni discovers the wonderful transparency of rock-salt for heat.

From 1831, this enquiry was conducted nearly exclusively by Melloni, who about that time settled Melloni first in Geneva and then in Paris, having been compelled, on political grounds, to quit Italy. His first and most important original memoir was presented to the Academy of Sciences early in 1833, and was received with marked coldness, if not incredulity, by that body. A few months later, the writer of these pages had an opportunity of seeing Melloni's experiments in Paris, and he made known their importance at the immediately succeeding meeting of the British Association at Cambridge. The Royal Society of London in no long time awarded their Rumford medal to Melloni, after which mark of foreign approbation, he first obtained a hearing from the Institute of France. The most considerable result at which he had then arrived was this: that rock-salt possesses a power unapproached by any other substance of transmitting heat of any temperature and from whatever source, with extremely little loss; and as a natural consequence of this, that heat wholly devoid of luminosity, such as that from boiling water, or even the heat of the hand, may be refracted by prisms and lenses of rock-salt exactly in the same manner as light is refracted by glass. The reality of these effects (which had excited the persevering scepticism of the Parisian savans) was demonstrated by a great number of most ingenious experiments, in which every possible source of error and confusion was avoided or allowed for.

(712.) Melloni even believed that the loss observed in passing heat of any temperature, high or low, through polished screens of rock-salt, was precisely the same; and, moreover, that it occurred entirely at the two surfaces by partial reflection, so that the solid medium was absolutely transparent for every kind of heat. It is certain that the loss is in every case small; but this almost paradoxical conclusion has not been completely confirmed by those who have repeated his experiments. The important and unexpected discovery of the nearly complete transparency of rock-salt for heat, enables us to construct complex thermotic apparatus for refracting and concentrating it, analogous to those of glass which are used in optics.

(713.) The specific action of different substances on the rays of heat.—Coloration of heat.

The next point clearly made out by Melloni, was the specific action of different bodies in sifting the [p.158] rays of heat they transmit, stopping altogether certain different kinds or qualities of heat and transmitting others. Substances, in general, transmit most readily the heat on the rays radiated by surfaces having a high temperature; this had already been shown to be true in the case of glass of by De la Roche. That experimenter had also demonstrated ... that successive plates of glass intercept a constantly decreasing percentage 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 respect to other substances, we may imagine (for the sake of illustration) heat to be coloured, and that different media, though equally transparent and colourless 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 Melloni) is absolutely colourless with respect to heat, transmitting 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 :—

(714.) Refrangibility of Heat

Let, however, heat which has been sifted by a plate 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 transmissive qualities, as alum and green glass, the combination is almost absolutely opake, just as a combination 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 method, founded on the Total Reflection of heat within a prism of rock-salt, has obtained the following Indices of Refraction:—

(715.) Explanation of point of maximum heat in the spectrum.

Melloni ingeniously applied the facts previously mentioned to explain the variable position of the hottest part of the spectrum, as observed by 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.

(716.) Death of Melloni.

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, under circumstances of ease and comparative affluence, that he had done in the period of distress and obscurity. Nevertheless, several original papers were written 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. Melloni died of cholera at Portici, in August 1854, aged 53.

(717.) Early attempts to polarize radiant heat—Berard.

The analogy of Radiant Heat to light, strikingly established by Melloni, with respect to the diversified refrangibility and other qualities of the various radiations emitted by one or different sources, suggests an enquiry as to the intimate nature of these two 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 medium. Some years before any of Melloni's papers appeared,—indeed, before he had entered on the investigation 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 1812) 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 myself 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 reflection.3

(718.) Experiments by the present writer [James David Forbes].

I have just referred to my own early experiments on the subject (which were likewise inconclusive), in order to explain that it was natural, on hearing of the application of the thermo-multiplier 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 Melloni [p.159] had announced did not take place)4 next, by transmission through a bundle of very thin mica plates, inclined to the transmitted ray; and afterwards by reflection from the multiplied surfaces of a pile of thin mica plates placed at the polarizing angle.5 I next succeeded in showing that polarized heat is subject to the same modifications which doubly refracting crystallized bodies impress upon light, by suffering a beam of heat (even when quite obscure), after being polarized by transmission, to pass through a depolarizing plate of mica, the heat traversing a second mica bundle before it was received on the pile. As the plate of mica used for depolarization was made to rotate in its own plane, the amount of heat shown by the galvanometer was found to fluctuate just as the amount of light received by the eye under similar circumstances would have done. This experiment which, with the others just mentioned, was soon repeated and confirmed by other observers, still remains the only one proving the double refraction of heat unaccompanied by light; and though somewhat indirect, it will hardly be regarded by competent judges as otherwise than conclusive. Iceland spar and other doubly-refracting substances, absorb invisible heat too rapidly to be used for effecting directly the separation of the rays, which requires a very considerable thickness of the crystal. I also succeeded in repeating Fresnel's experiment of producing circular polarization by two internal reflections. The substance used was of course rock-salt.6

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