incandescent platinum wire. Melloni (p. 201) filled a copper canister with water, and kept it at the boiling-point, and by means of a very delicate instrument, called the thermo-multiplier, obtained the following relative absorptive powers, as shown in column 1. If, however, the heat is derived from an incandescent platinum wire, as in column 2, the figures are different; and white lead is found to absorb a less quantity of the rays of heat when they are luminous, and Indian ink more. Leslie's principle does apply to clothing, and it appears that if we imitate nature, and, like the Polar bear, wear white, we shall be warmer in winter and cooler in summer. In running streams, and even in the Rhine, what is called "ground ice" is frequently found. This is no contradiction of the laws already explained with reference to the cooling of water. The ice is formed at the bottom of the stream, because the stones and other earthy matters forming the bed of the river emit or radiate heat when the sky is very clear; and as the water of the stream is mixed by the current, and the temperature of the bed of the river is lowered by radiation, the ice forms in spongy masses, which may rise to the surface, carrying stones and even the anchors of ships with them. The rays of heat are more readily absorbed when they fall upon bodies at angles near the perpendicular; hence the rays of the sun are hotter in summer than in winter, when they are more oblique. If the bulb of an air-thermometer be brought near a burning hydrogen flame, its radiating power is found to be very low, although, as is well known, the heat of the flame is so great that it will quickly ignite a spiral of platinum wire; when the heat waves are set in motion, emission or radiation takes place, which will promptly affect the thermometer. Tyndall has investigated the radiating and absorbing powers of gases and vapours, and, although they are feeble, he has been able to discover that vapours and compound gases have a much greater absorbing and emitting power than any simple or elementary gas, such as oxygen or nitrogen, or when they are mechanically mixed, as in atmospheric air. Had our globe been surrounded with a gas like olefiant gas, the absorbent power would have been 240 times greater than that of oxygen. Amongst gases, those which absorb heat the most also radiate it freely. As might be expected from the analogy between light and heat waves, the latter may be reflected, refracted, may undergo double refraction, be absorbed, and even polarized; the latter fact being proved by the use of tourmaline plates or bundles of plates of mica. TRANSMISSION OF HEAT. Melloni's name will ever be associated with all the more important experiments in which the course of heat-waves is traced through various media. As with light there are bodies called transparent, diaphanous, translucent or transparent, opalescent, and opaque, so with reference to the power of transmitting heat, bodies generally are divided into two classes: I.-Diathermanous or diathermic bodies (dia, through, and Oepμos, heat), permitting heat-waves to travel through their substance. Examplesrock salt and certain elementary gases. II.-Athermanous or adiathermic bodies, which arrest or stop the progress of the heat-undulations. Examples-all liquids in variable proportions; alum in crystal and solution. Mr. B. Stewart has shown that bodies of the first class are bad radiators of heat, but that those of the second or adiathermic class are good radiators. It does not follow, because substances like the diamond, glass, ice, &c., ermit light-rays to pass through them, that they will also allow the heatrays to travel through in the same proportion. Glass permits the light to pass freely through its substance, but stops a considerable number of the heat-undulations; and alum, nearly all. Rock salt is the only substance which is entitled to be placed in the first or true diathermanous class, and although it does, according to Krupland and Stewart, absorb certain of the heat-rays more than others, still at present it stands first, and is therefore used in the form of plates, prisms, and lenses for these delicate experiments. Melloni found that certain solids, cut into plates one-tenth of an inch in thickness, allowed the following percentage of heat waves from an Argand lamp to With liquids, when the source of heat was an Argand oil lamp, and the fluids enclosed in a glass cell, the results given in Table I. were obtained. Table II. shows the results obtained by Tyndall from liquids enclosed in a rock-salt box, the source of heat being an ignited platinum wire : Rock salt stands in the same relation to heat, so far as transparency to Argand oil lamp without a glass; spirit-lamp and platinum wire; the copper box, blackened, to contain water at 2126 F.; stand, to place the objects upon; screen, with apertures of various sizes; the thermo-multiplier current, with the galvanometer needle. heat-rays is concerned, as colourless glass does to the light-rays. When a hot metallic ball is placed between the bulbs of a differential thermometer, the liquid remains stationary, because both are equally heated; if, however, a plate of rock salt is interposed as a screen on one side of the ball, and a plate of glass on the other, the thermometer is immediately affected, as more rays pass through the rock salt than through the glass. Melloni's apparatus for these investigations may be regarded as the model of perfection. It includes the various sources of heat, such as a naked flame, an ignited platinum wire, a blackened copper vessel containing water at 100° C. (212° F.), or a copper plate heated to 400° C. (752° F.), and is plainly shown in Fig. 186. The delicacy of the thermo-multiplier as an indicator or measurer of heat is most remarkable, and it will be fully explained in another part of this work. The minute electrical currents set up in the thermo-multiplier are recorded by the galvanometer needle. It has already been shown that in bodies which arrest partially or wholly the heat-waves, the nature of the heat, or rather the particular source from which it is obtained, has a great influence upon the result. Thus fluor-spar permits 33 per cent. of the heat-waves derived from boiling water to pass through its substance, whilst the power rises to 78 per cent. when the source of heat is a burning lamp. Heat-waves which have passed through one plate of glass will also pierce another, with a small amount of loss; the same waves are nearly all stopped by alum. Tyndall's discovery, that the vapour of water absorbs thirteen times more obscure heat than air, is a most important fact, and shows why the air containing vapour nearer the earth is warmer than that which is dry and found on the summit of lofty mountains. The dry air allows the obscure heatwaves to travel through, and is too diathermanous, whilst air charged with moisture has considerable athermaneity for obscure rays, which are produced when the rays of the sun have passed through our atmosphere and fallen upon the earth. When the rays of the sun fall upon the earth to warm it, they are radiated and then diffused; a change in their quality takes place, and they become obscure rays of heat. It is these obscure rays which melt snow, and perform other useful offices. THE CONVERSION OF LIGHT RAYS INTO HEAT RAYS, AND VICE VERSA, BY CHANGE OF REFRANGIBILITY. At the meeting of the British Association, held at Newcastle, in 1863, Dr. Akin proposed three experiments for the conversion of rays of light into heatrays; of these one is deserving of notice, viz., the proposal to collect the rays of the sun in a concave mirror, and then to cut off the light with "proper absorbents,” and to bring platinum foil into the focus of invisible rays. Although Dr. Akin was the first to propose definitively to change the refrangibility of the ultra-red rays of the spectrum by causing them to raise platinum foil to incandescence, yet the chief merit, in connection with this branch of heat, is due to Dr. Tyndall, because, in the spirit of Lord Bacon, he was not content with a theory which merely suggested that a certain result might be obtained, but industriously worked out the crude idea, and proved that it was substantially true, by devising a number of clever and original experiments, which had never been shown before. In the article on Light (p. 92), the change of refrangibility of certain rays at the violet end of the spectrum, and the beautiful experiments with "fluorescence," by Professor Stokes, have already been specially considered. And just as he obtained a large proportion of these rays, existing in and beyond the violet, by using prisms of quartz, so Melloni, by using a prism of rock-salt, was enabled to prove that the ultra-red rays discovered by Sir W. Herschel formed an invisible heat spectrum as long as the visible one. Other experimentalists continued the investigation, especially Professor Müller, of Freiberg, who worked out a curve expressing the heating power of the whole spectrum; but it was left for Tyndall to complete the investigation, and directly isolate the invisible or obscure rays of heat; and as Stokes, by lowering the refrangibility of the invisible ultra-violet rays, rendered them visible, so Tyndall, by raising the refrangibility of the ultra-red rays, rendered them also visible. The instruments he used, to quote his own words,* "consisted of the electric lamp of Duboscq and the linear thermo-electric pile of Melloni. "The spectrum was formed by means of lenses and prisms of rock-salt; it was equal in width to the length of the row of elements forming the pile; and the latter being caused to pass through its various colours in succession, and also to search the space right and left of the visible spectrum, the heat falling upon it at every portion of its march was determined by the deflection of an extremely sensitive galvanometer. * "Proceedings of the Royal Institution of Great Britain," vol. iv., part 5. Frofessor Tyndall, "On Combustion by Invisible Rays." "As in the case of the solar spectrum, the heat was found to augment from the violet to the red, while in the dark space beyond the red it rose to a maximum. The position of the maximum was about as distant from the extreme red in the one direction as the green of the spectrum in the opposite one. "The augmentation of temperature beyond the red in the spectrum of the electric light is sudden and enormous. Representing the thermal intensities by lines of proportional lengths, and erecting these lines as perpendiculars at the places to which they correspond, when we pass beyond the red these perpendiculars suddenly and greatly increase in length, reach a maximum, and then fall somewhat more suddenly on the opposite side of the maximum. When the ends of the perpendiculars are united, the curve beyond the red, representing the obscure radiation, rises in a steep and massive peak, which quite dwarfs by its magnitude the radiation of the luminous portion of the spectrum. "Interposing suitable substances in the path of the beam, this peak may be in part cut away. Water, in certain thicknesses, does this very effectually. "The vapour of water would do the same; and this fact enables us to account for the difference between the distribution of heat in the solar and in the electric spectrum. The comparative height and steepness of the ultra-red peak in the case of the electric light are much greater than in the case of the sun, as shown by the diagram of Professor Müller. No doubt the reason is, that the eminence corresponding to the position of maximum heat in the solar spectrum has been cut down by the aqueous vapour of our atmosphere. Could a solar spectrum be produced beyond the limits of the atmosphere, it would probably show as steep a mountain of invisible rays as that exhibited by the electric light, which is practically uninfluenced by atmospheric absorp tion. "Having thus demonstrated that a powerful flux of dark rays accompanies the bright ones of the electric light, the question arises, 'Can we not detach the former, and experiment on them alone?' "In the author's first experiments on the invisible radiation of the electric light, black glass was the substance made use of. The specimens, however, |
