The absorptive power of many vapours for rays of obscure heat is still more remarkable; as will be seen by the following table, which shows a few of Tyndall's results at low pressures, compared with that of air at the normal pressure, taken as I.

The absorbent action of the perfumes of many flowers for these obscure rays was also shown to be singularly high. A few drops of an essential oil placed in a tube, and exposed to a current of dry air, gave a scented atmosphere of which the absorptive power varied greatly; that of patchouli being 30, that of lavender 60, that of cassia 109, while that of aniseed was as high as 372; so that the perfume proceeding from a flower-bed absorbs a large proportion of the radiant heat of low refrangibility re-radiated from it.

The experiments of Tyndall seemed to have completely established the fact that aqueous vapour has a powerful absorbent action upon heat of low refrangibility, although Magnus arrived at a different conclusion. (Pogg. Annal. 1867, cxxx. 207.)

The results obtained by Tyndall on diluting boracic ether with air are so singular and anomalous, that there is reason to suspect some undiscovered source of error, boracic ether being a substance liable to decomposition even by a trace of moisture.

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VARIATIONS OF DIATHERMANCY.

[163.

It must not be forgotten that hitherto Tyndall's experiments upon the various gases and vapours have been confined for the most part to radiant heat of low refrangibility. No doubt other and very different results will be furnished when heat of high refrangibility is made the subject of inquiry.

The radiation of heat by gases has also been clearly established by the experiments of Tyndall, and conformably with what we know of radiation and absorption in solids, he has proved that amongst gases the most powerful absorbents are likewise the best radiators.

(164) Influence of Structure on Diathermancy.-It by no means necessarily follows that a body which is transparent to light is also able to allow the passage of heat, and vice versá; crystallized cupric sulphate, which permits the passage of blue light abundantly, arrests the rays of heat entirely. Again, the opaque black glass, used for the construction of polarizing mirrors, transmits a considerable portion of the thermic rays. Smoked rock salt and black mica also exhibit the same power.

Mechanical arrangement appears to have even more influence upon diathermancy than chemical composition. Common table salt is perfectly adiathermic. A solution of rock salt is scarcely

superior to pure water in diathermancy, and a solution of alum is equally diathermic with a solution of rock salt. This is perfectly consistent with the effect which alteration of structure produces on the action of bodies on light. Common loaf-sugar is opaque and of dazzling whiteness, but pure sugar-candy (the same body only in larger crystals) is colourless and transparent: the most transparent glass, by pulverization, may be reduced to a white opaque powder.

As already mentioned, pure colourless rock salt is the only solid substance the diathermancy of which approaches perfection; and, according to the researches of Knoblauch, which have been confirmed by those of Stewart and others, even rock salt absorbs certain of the rays of heat, somewhat more freely than others; the rays thus absorbed are those of greatest wave length, or lowest refrangibility. All other bodies upon which Melloni made experiments, transmit a quantity of heat which varies greatly with the nature of the source, from a second cause, which has been termed thermochrosis, or calorific, tint, which is analogous to a difference in colour for objects transparent to light; to this cause must be attributed the remarkable differences in the amount of absorption (161) according to the source from which the heat emanates. Before quitting this subject, it may be observed that B. Stewart

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REFRACTION OF HEAT.

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(Proc. Roy. Soc. 1860, x. 386) has shown that highly diathermic bodies are bad radiators, while adiathermic bodies are good radiators. He has proved that the radiation from a plate of rock salt goes on from a considerable depth below the surface: but the kind of heat which rock salt emits is the same which it absorbs, a thick plate of cold rock salt having been found to arrest at least three-fourths of the heat radiated from a thin plate of heated rock salt.

The

(165) Refraction.-Radiant heat, like light, is susceptible of refraction a large convex lens, placed in the sun's rays, not only gives a focus of intense light, but, as is well known, constitutes a powerful burning-glass. Inflammable objects are easily ignited by this means, and the focus of heat is found to correspond nearly with that of the greatest light. Further, if a solar beam be subjected to the action of a prism of transparent rock salt, and the coloured spectrum so obtained be examined by means of a small but sensitive thermometer, it is found that the rays of heat, like those of light, possess unequal degrees of refrangibility; hence the rays of heat are not all accumulated in one spot, but are dis. tributed over the entire spectrum. There are, in fact, differences in the rays of heat corresponding to those of colour in the rays of light. The greater portion of the rays of solar heat are even less refrangible than the red rays, for the maximum of temperature in the solar spectrum is found at a distance below the extreme red rays as great as the brightest yellow is above them. length of the wave of these heat rays is consequently considerably greater than that of the red rays, and the frequency of undulation is proportionately less than that of the least refrangible luminous rays. As by the employment of different sources of light, spectra are obtained in which the intensity of the light varies in different parts according to the prevailing colour of the luminous rays, the yellow light of common salt giving a spectrum most intense in the yellow rays, and the red light of strontic nitrate giving a spectrum in which the red rays possess the greatest intensity;-so in like manner, by varying the source of heat which is employed, the position of maximum temperature in the refracted beam is found to vary the less intense the source of heat, the smaller is the refrangibility of the heat radiated. The flame of a naked lamp, for example, emits rays of heat of all degrees of refrangibility, its maximum of intensity being about the middle of the spectrum; from the ignited platinum, the maximum of heat falls nearer to the red; from copper at 400° C. nearer still; and the heat radiated from a surface at 100° C. con

342

DIFFERENT QUALITIES OF REPRACTED HEAT RAYS.

[165.

tains scarcely any of the more refrangible rays. Now it is obvious, that a mixed pencil of heat, if it falls upon a diathermic medium which absorbs certain of the rays of heat and not others, will be altered in a manner similar to that in which a ray of light is affected in traversing a coloured glass.

With a knowledge of these facts, there is no difficulty in understanding how it is that the sun's rays can traverse a plate of glass and experience but little absorption, and can be brought to a point by a convex lens, or by a glass concave mirror, either of which remains cool, while intense heat is developed at its focus; whereas, if the same lens or concave mirror be held opposite to a common fire, a bright spot of light will be obtained at the focus, but little or no heat; whilst the glass of which the lens or mirror is composed will become strongly heated. The rays which glass transmits most readily are those which abound in solar light, but these are precisely the rays which are least abundant in incandescent bodies. Advantage has long been taken of this fact by those who have occasion to inspect the progress of operations carried on in furnaces; they are able by the use of a glass screen to protect the face from the scorching rays which the glass absorbs, although it offers no impediment to the transmission of light.

This absorption of radiant heat by glass is easily demonstrated by placing a canister of hot water in the focus of one of the conjugate mirrors (fig. 131), and a thermoscope in the focus of the other: the air in the acting ball of this instrument ceases to expand the instant that a glass screen is interposed anywhere between the two mirrors, in which case the glass absorbs the rays, and becomes itself heated.

The foregoing observations show that in the analysis of radiant heat, prisms and lenses of glass should not be used, since they lead to results as incorrect as those which would be furnished by studying the phenomena of light by means of coloured prisms and lenses. Rock salt furnishes the only known material of which such apparatus can properly be constructed, and by its means, rays proceding even from the human body may readily be concentrated and made to act upon a thermoscope.

(166) Separation of Radiant Heat from Light.-A consideration of the preceding facts led Melloni to expect that by a combination of screens which allow light of a given colour to pass, radiant heat may be arrested; and in fact he thus effected an apparent separation of light from heat. By transmitting the solar rays, first through a glass vessel filled with water, which arrests the less refrangible rays, and then through a plate of a peculiar green glass tinged by means of oxide of copper, which stops the

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SEPARATION OF RADIANT HEAT FROM LIGHT.

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more refrangible rays, a greenish beam was obtained, which was concentrated by lenses, and furnished a greenish light of great intensity, but yet produced no perceptible heating action when it was allowed to fall upon the face of a sensitive thermoscope. A similar separation of light and heat seems to be effected in nature, in the light reflected by the moon. Melloni concentrated the rays of the moon by means of an excellent lens of a metre in diameter, and obtained a brilliant focus of light of I centimetre in diameter, the intensity of which consequently was nearly 10,000 times greater than that of the diffused light of the moon; upon directing this focus of light upon the face of a very sensitive thermo-multiplier, only an extremely feeble indication of heat was obtained.* (Melloni, Thermochrose, Part I. note, 251.)

Lord Rosse has obtained evidence of the emission of heat from the moon by causing the image of the moon produced by

* Notwithstanding these results, Melloni maintained, during the latter years of his life, the identity of the agent that produces light and heat. Traces of heat, he says, are found in every luminous ray; he supposes that the rays of heat may be invisible, just as the chemical rays beyond the violet end of the spectrum are invisible, because the structure of the retina is not susceptible of undulations the frequency of which exceeds or falls short of a certain amount. No doubt there exists an average limit to the power of the retina to receive luminous impressions from solar radiations; the boundary between light and darkness being almost imperceptible. In certain individuals the retina is insensible to the extreme rays at the red end of the spectrum, which are plainly discerned by others. A parallel case occurs in the audibility of sounds: in some individuals the ear is unable to perceive notes in which, as in the chirp of a cricket, the vibrations exceed a certain number per second, though such sounds are distinctly audible to the majority of persons. Whether light can be obtained absolutely free from heat may still be doubtful, but there is no doubt of the existence of radiant heat of great intensity unaccompanied by light. According to Tyndall's observations upon his own eyes, the retina is quite insensitive to these obscure rays of heat, even when concentrated by a lens and thrown directly into the eye, though the focus of heat was sufficiently intense to kindle paper. It appears, therefore, from these experiments, that the intensity of the sensation of light which is experienced is by no means a measure of the intensity of the force by which the undulations which excite the sensation are produced, but that it is rather a measure of the exquisite sensitiveness of the retina to vibrations of certain degrees of frequency, and that above and below these points the retina becomes less and less sensitive, until at last a very much greater intensity of the force expended in producing the undulations fails to produce any sensation of light. Indeed, it is now assumed by those who adopt the undulatory theory of heat, that whenever light is absorbed, whatever be its source or intensity, it is converted into heat. Rigid experimental proof upon this point is, however, still wanting, though it is rendered probable, upon the principle of the conservation of energy, that the differences between the heating, the luminous, and the chemical rays are due to differences in the relative wave lengths; the rays of greatest wave length being the least refrangible.