Macedonio Melloni

“ The great number of experiments that I have since performed by means of that instrument have produced in my mind a thorough conviction of the truth of that opinion. As there are still many experimental researches to be made not only in that class of phænomena, of the history of which we have scarcely given an outline, but in every branch of the study of radiant heat, it is to be wished, for the interests of science, that every investigator would furnish himself with a thermomultiplier. ”

“ But this curious fact, which constitutes, as it were, the basis of our inquiries, ceases to surprise us as soon as we feel convinced that bodies which are transparent and colourless act upon heat in a manner similar to that in which coloured media act upon light. For, as upon the intensity of the colour depends the degree of transparency, that is, the number of luminous rays that pass through the coloured substances, in like manner upon this species of invisible calorific tint which diaphanous bodies possess will depend whether a greater or a less quantity of heat be transmitted [8] . ”

“ The solar heat which presents itself to the anterior face of the prism of water contains rays of every degree of refrangibility. Now the ray which has the same index of refraction as the red light, suffers in passing through the prism a loss porportionally greater than the ray which possesses the refrangibility of orange light, and less is lost by the latter in the passage than by the heat of the yellow ray. These increasing ratios in the losses of heat sustained by the less refrangible rays have an evident tendency to transfer the maximum to the violet. ”

“ The refraction and constant transmission of the calorific rays through the rock salt being placed beyond the possibility of doubt, we immediately see the use that may be made of this substance in investigating the nature of radiant heat. If, for instance, it is proposed to propagate to great distances the action of a heated body of small dimensions, we are now certain that we have only to place the body at the focus of a lens of rock salt, which will refract the calorific rays and make them form a real pharos of heat by issuing in a direction parallel to the axis. ”

“ If between the flame of a candle and the eye we interpose a plate of glass or any other substance more or less transparent, we find the diminution of the intensity of the light always the same, however the distance between the plate and the candle may vary. The effect produced by distance on the freely transmitted caloric is exactly similar; and if at a certain distance from the active source there be a thermoscopic apparatus sensible to this portion of heat, the apparatus will always give the same indication, whether the screen be laid close to the source or to the thermoscope. ”

“ In the latter case we should say that in the prism the refractive force acts unequally on the different caloric rays, as it does, in a greater or less degree, on the different luminous rays, and thus throws certain quantities of heat on the very spaces occupied by the different colours of the spectrum; but that in the coloured glasses and, generally, in bodies more or less diathermanous, the absorbent force does not act in the same manner as the force of refraction, which sometimes extinguishes more heat than light and at others more light than heat. ”

“ Let us now proceed to consider the transmission of heat through heterogeneous screens. The calorific rays emerging from each plate exposed to the action of the same source produce a particular elevation of temperature when they fall on the thermoscopic body of our apparatus. Whence we have inferred that the quantity of heat which passes through a given screen varies according to the quality and thickness of the substance. ”

“ The colours introduced into a diaphanous medium always diminish its diathermancy in a greater or a less degree, without communicating to it any tendency to arrest certain calorific rays rather than others: they affect the transmission of radiant heat as dusky bodies affect the transmission of light. There is, it is true, an exception to be made in respect to green and opake black, at least in certain kinds of coloured glass. ”

“ As to coloured glasses, their action is more feeble in proportion as their tint is less vivid. In short the greatest decrease in the absorption of the white surface is produced by the interposition of a yellow glass, and the least by the interposition of the red and the violet, and, as to each pair of plates of the same tint, the less effect is invariably derived from that in which the tint is deeper. ”

“ We find, in fact, that in the different sorts of glass and liquids it follows the order of the different degrees of refrangibility; for flint-glass possessing a greater refracting power than crown-glass affords an easier passage to the caloric radiation. Carburet of sulphur is at the same time more refracting and more diathermanous than the essence of turpentine; the same may be said of turpentine as compared with olive oil, and so on until we come to pure water; a liquid which, as it possesses the least power of refraction, possesses also the least power of transmitting heat. ”

“ These differences of temperature exist not merely in appearance, but are as palpable as those that are felt when we touch wood and marble that have been exposed to the sun. To prove this, we need only lay some pieces of wax or suet on the two bodies. Those laid on the glass will melt rapidly, but those laid on the rock salt will continue in their solid state. We may also demonstrate in a direct manner, and without the aid of a thermomultiplier, the great transmissiveness of rock salt as compared with other diaphanous substances. ”

“ The order of the colours considered relatively to their degrees of heat and the numerical relations of those degrees are so altered that the violet light, which in the spectrum possesses a temperature twenty-five or thirty times lower than that of the red light, appears here of a higher temperature. Such a difference is not to be explained by supposing that, in the transmission of the violet glass, there passes a great quantity of red rays; for it should, on this hypothesis, be found to transmit them in a greater proportion than they are transmitted by the red glass ”

“ When the screens are liquid, the influence of the conducting power of the layers may always be destroyed if we incessantly renew the matter of the screen by means analogous to the strip of water employed by M. Prevost. But it would be always very difficult, and often impossible, to apply this artifice to solid bodies and even to such liquids as can be obtained only in small quantities. In order therefore to attain the same end in a general manner, and to render the experiments in some degree independent of conduction, other means must be employed. ”

“ Let the direct rays from an unvarying source of heat be received on the thermoelectric pile; let them be first made to pass through any diaphanous screen of one hundred millimetres in thickness: the index of the galvanometer sets itself in motion from the instant when the communications are established, and stops after having described an arc of greater or less extent in an unvarying interval, which, with my apparatus, I find to be ninety seconds. ”

“ The resistance of diaphanous media to the immediate transmission of the rays of heat is therefore of a nature altogether different from that which is presented by the same media to the propagation of light. ”

“ I adopt this term merely for convenience, without attaching to it a definite meaning; for, as there is no body which, if reduced to an extremely thin plate, may not become in some degree transparent, I think also that some rays of heat may pass through all substances in a state of great tenuity. ”

“ One is surprised at first to see caloric rays capable of giving a deviation of 30° fail to produce any effect when they are absorbed by the screen, which must necessarily send its acquired heat upon the apparatus. But our surprise ceases when we reflect that this heat is sent equally in all directions by every point of the heated screen, and therefore that the portion of total radiation which reaches the apparatus is but a very small fraction. ”

“ Rock salt reduced to a plate and successively exposed to radiations of the same force from different sources always transmits immediately the same quantity of heat. A plate of any other diathermanous substance will, under the same circumstances, transmit quantities less considerable in proportion as the temperature of the source is less elevated: but the differences between one transmission and another decrease as the plate on which we operate is more attenuated. ”

“ The quantity of radiant heat which passes through polarizing plates of tourmaline is not affected by any change made in the angle at which their axes of crystallization are made to cross one another. Rays of heat are therefore not polarized in this mode of transmission and are in this respect entirely different from rays of light [17] . But they resemble them in the property of refrangibility. This is completely proved by means of the rock salt, the only diathermanous body that is capable of transmitting the calorific rays emanating from every source. ”

“ Now these homogeneous rays must suffer a loss greater or less in its amount, but constant in respect to layers of equal thickness, as is the case, in the transmission of light, with red rays passing through a medium of the same colour, and with white rays passing through a medium diaphanous and colourless. What we have said of glass is equally true of every other partially diathermanous substance. ”

“ Those kinds of metal, wood, and marble which totally obstruct the passage of light obstruct that of heat also. Some other bodies, such as carburet of sulphur, rock salt, and Iceland spar, allow both kinds of rays to pass at the same time. It is therefore probable that calorific transmission cannot take place without a certain degree of transparency [18] ; but it cannot take place abundantly without the cooperation of another quality, which varies as the bodies happen to be crystallized or without crystallization. ”

“ Does not this constancy in their manner of acting, notwithstanding such great differences in their physical and chemical constitutions, indicate that this law of decrement belongs to the very nature of the heat? We should not however infer from this that there are not bodies which afford a passage equally free to calorific rays of every kind. For we see by the table that a flake of rock salt, whether exposed to the radiations of flame, of incandescent platina, of copper heated to 390°, or of boiling water, always transmits 92 of every hundred incident rays. ”

“ It is easy to show by calculation that we always gain; that is, that we always weaken the conductible caloric more and more by removing the screen from the thermoscope, until we have placed it midway between the thermoscope and the source [8] . Let us, therefore, put the screen in this position (which is the most favourable of all) , and we shall see that its heat has then no appretiable influence on the results obtained by means of the thermomultiplier [9] , and a source whose radiation is much weakened by distance. ”

“ As to lenses and common prisms they refract a certain portion only of the radiant heat; for the glass intercepts several sorts of calorific rays issuing from sources at a high temperature, and absorbs nearly the whole of the heat given out by bodies whose temperature is below incandescence. To this circumstance it is that we must attribute the doubt hitherto entertained as to the refrangibility of nonluminous heat. ”

“ We see, in fact, that carbonate of lead, a highly refractive and colourless substance, transmits less heat than Iceland spar and rock crystal, which are much inferior to it in refracting power; while rock salt, possessing the same transparency and the same index of refraction as citric acid and alum, gives six times their amount of calorific transmission. ”

“ The index of the galvanometer will remain stationary, whatever may be the difference of temperature between the plates and the thermoscopic body, and we may therefore immediately proceed with the experiments. Hence, if we would save time, we should always have a pair of screens of each sort; and, as we have just observed, put both sides of the pile in the same state.
In order to ascertain the influence exercised on free transmission, by the different circumstances relating to the surface, the volume, and the composition of the screens, we must procure a constant source of heat. ”

“ When the quantity of radiant heat that passes through coloured glass is measured, it is always found to be less than that which passes through white glass of the same thickness. The difference indeed is sometimes considerable, though having no apparent relation to the prismatic order or intensity of the colour. ”