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salt, being divided into flakes, instantly arrests calorific radiation, and the transmissive power of water is only increased from 11 to 12, by dissolving in it either alum or rock salt, two substances which in their solid state are the opposite extremities of the scale of transmission.

M. Melloni performed all his experiments with an apparatus of extreme delicacy, the construction and application of which constitutes not the least interesting part of his work: but such as have not a thermoscopic apparatus may easily satisfy themselves that rock salt transmits almost all the radiant heat that falls on its surface, by fixing vertically on the same stand a plate of this substance, and a plate of glass or alum of the same dimensions, and by bringing the stand quite close to the fire of a stove. If it be allowed to remain in this state for five or six minutes, the glass will become burning hot, while the rock salt, if applied to the most tender part of the hand, will produce no sensation of warmth. This difference may be rendered visible by placing a piece of wax on each of the plates; that on the glass will soon begin to melt, while that on the salt will remain

quite solid.

The greatest part of the experiments may also be repeated by means of a large air thermometer bent twice at right angles; between the blackened balls of which a stand is placed, for the reception of the source of heat and the intervening plates. The liquid will be depressed on that side on which the heat most

freely radiates.

§ 290. The results above detailed were obtained from the burner of an argand lamp; but M. Melloni found that produced from different calorific sources.

different effects were

The four which he compared together were the flame of oil without the interposition of glass; platina wire kept in a state means of the flame of a spirit lamp, (both

of incandescence, by

of which are luminous); a plate of copper heated to the temperature of 732°, and a vessel of thin copper blackened on the outside filled with boiling water, (both of which were nonluminous); some of the principal results are shown in the

following table:

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§ 291. Thus it appears that rock salt, successively exposed to radiation from different sources, always transmits immediately the same quantity of heat, (and it is the only known substance which thus acts.) 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. Whence it follows, that the calorific rays from different sources are intercepted in a greater or less quantity, not at the surface and in virtue of an absorbent power varying with the temperature of the source, but in the very interior of the plate, and in virtue of an absorbent force similar to that which extinguishes certain species of light in a coloured medium.

§ 292. In extending this observation to the solar rays, M. Melloni found that the quantity of heat intercepted by increasing the thickness of the transparent medium is greater for the less refrangible than for the more refrangible rays: that is, that while the thinnest possible pellicle will permit as much per cent. of the heat accompanying the red ray to pass through it as of that of the violet ray, if the thickness be increased, a much larger per centage of the former will be stopped than of the latter; whence he concluded that the refrangibility of a heating ray a measure of its intensity. The rays of heat which are separated by a common prism do not undergo the same action upon passing through a layer of water: the most refrangible rays, or those towards the violet end of the spectrum, pass undiminished; while the less refrangible, or those in the red ray, are entirely

is

stopped by that liquid. Thus the changes in the maximum of temperature in the solar spectrum, when different liquids were employed by Professor Seebeck, are accounted for.

§ 293. The refraction and constant transmission of the calorific rays from any source through rock salt being established, it is easy to see the use which may be made of this substance in carrying on the investigation with regard to the nature of radiant heat. When formed into lenses and prisms, it will act upon calorific rays in a manner perfectly analogous to that in which optical instruments act upon luminous rays. The feeblest emanations may thus be concentrated to a focus, or propagated to a distance; and in this manner we may, with the aid of a common differential thermometer with small balls, obtain very decided indications of the heat issuing from a vessel filled with tepid water and placed at a great distance. It constitutes, to use M. Melloni's expression, the true glass of radiant heat. All other transparent bodies are but partial and incomplete transmitters of heat, totally intercepting calorific rays of a certain kind, just as coloured media intercept coloured rays of certain kinds. All examinations of the nature of solar heat with common prisms of glass, water, alcohol, &c., are necessarily as fallacious as any attempt would have been to analyse solar light with prisms formed of coloured glass.

§ 294. Amongst the calorific rays of the sun there are some which have a resemblance to terrestrial heat, and amongst the calorific rays of flame some are found similar to the heat of the sun. The differences observed between solar and terrestrial heat, as to their properties of transmission, are therefore to be attributed merely to the mixture, in different proportions, of several kinds of rays.

The small quantity of heat which emerges from alum is abundantly transmitted by all diaphanous colourless plates, and suffers no appreciable loss when the thickness of the plates is varied within certain limits; with regard to transmission, these rays, in fact, bear a close resemblance to those of the solar heat. They also resemble the latter in the decided influence which colour has upon their absorption. When the rays of a naked. flame were made to fall upon similar thermoscopic surfaces covered with lamp-black, and Spanish white mixed with gumwater, the effect of the white, as compared with the black, was

as 80.5 to 100; and it was the same when a screen of rock salt was interposed; but when rays which had been previously passed through a screen of alum were employed, the effects were as 42.9 to 100. Professor Powell has ascertained that colourless glass acts in the same way, though with somewhat less energy.

§ 295. From these facts a strong analogy results between luminiferous and calorific rays in their passage through different media. A glass of a very pure red colour transmits only light of that colour and intercepts all other coloured rays; therefore if we examine through such a glass flames of different colours, but of nearly the same intensity, we shall perceive them tinged with red, and the more feeble in proportion to the smallness of the quantity of red light mingled with their principal tint; so that any radiation, wholly free from this colour, will be invisible. In the same manner, radiant heat will pass in greater or less quantity, or be totally intercepted by, a colourless plate of glass, successively exposed to the radiations of flame, incandescent metal, and the sides of a vessel filled with boiling-water. In the first two cases, we find amongst the different kinds of heat which compose the pencil, some rays more or less numerous, which are endued with what we may call the calorific coloration of glass; whilst there does not exist a single ray of this kind in the pencil which proceeds from the last source. The theory then supposes that there exists in glass, and generally in all perfectly limpid and colourless substances, an invisible property quite analogous to coloration. Now, experiment has both proved that the quantity of heat of the calorific coloration of glass, increases with the intensity of the source of heat, and also, that the mean refrangibility of radiant heat increases from the same cause. The rays of heat emitted from a series of different sources disposed in the order of their temperatures, are therefore analogous to the different coloured rays which constitute the solar spectrum; the most refrangible part, that is to say, the violet, the indigo, and the blue, representing the radiation from flame or incandescent bodies, and the opposite part, viz., the red, the orange, and the yellow, resembling the radiation of warm non-luminous bodies. Upon this analogy, therefore, glass which transmits only rays emitted from sources of a high temperature possesses calorific coloration, which corresponds to the most refrangible colours of the spectrum. Now

all diathermanous substances, though more or less permeable to radiant heat, on being exposed to the radiation of different sources behave like glass: they are all subject to the same law, namely, that the calorific emanations which resemble the upper part of the solar spectrum, experience in their interior a less absorption than emanations from sources of lower intensity. But the analogy with light would fail if we could not find media which were capable of transmitting this kind of radiation and absorbing the opposite; for we are well acquainted with substances which are diaphanous to every colour. M. Melloni has more recently discovered the facts which supply this deficiency.

It can scarcely be said that we know anything of the action of those minute particles of foreign bodies which determine the absorption of different rays of light in differently coloured glasses; but with regard to the different rays of heat they all act indifferently, and only diminish the general diathermancy of the glass. There is one exception however to this, in the green oxide of copper with which some species of green glass is coloured; which, whilst it admits of the transmission of most of the rays which are transmissible by glass, absorbs those which are capable of passing through alum, or those which are most refrangible. M. Melloni found also that the rays which are transmitted by some opaque glasses, had all the properties of those proceeding from this green glass, and hence he inferred that in both cases, the substance added to the glass would stop the passage of the highly refrangible heat, but allow of the passage of the rays of lower refrangibility.

The proof of this would have been decisive, if it had been possible to tinge a portion of rock salt, the only perfectly diathermanous body with which we are acquainted, with oxide of copper or with carbonaceous matter, as is the case with glass, but neither by solution nor heat can this be effected without destroying its texture. But M. Melloni at length found that by superficially covering it with lamp-black, by carefully smoking it over a flame, the same end might be attained. He thus formed a combination which was permeable to radiant heat in exact proportion to the lowness of the intensity of the source from which it was emitted, or (which is the same thing,) to the least refrangible rays.

He took three exactly-similar plates of rock salt; the first he left in its natural state; the second, he covered with a film of any diathermanous substance, such as mica, glass, or varnish; and

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