when the uncovered intervals will be scorched, but the letters themselves will be untouched and conspicuous.

§ 277. The radiation of heat is one of the most important processes in the economy of nature, and it is one of the means by which equilibrium of temperature is brought about. Not only does heat travel from a hot to a cold body by the processes of conduction and convection, which we have before examined, but it is projected from the one and absorbed by the other, at a rate dependent upon the state of the surfaces of the two. In every assemblage or system of bodies, as for instance the various objects in a room, there is a tendency in each to radiate its heat; which, if met by an equal force or exchange in others, or in the walls of the apartment, is balanced or restrained; but if any inequality exist in the system, the projection takes place towards the weaker point till the balance is restored by absorption.

The same radiation takes place upon a large scale from all the substances upon the surface of the earth towards the regions of space; which, if not met and counteracted by the radiation from the sun, would soon annihilate all organic being by the rigors of an eternal frost.

§ 278. We may easily obtain evidence of this tendency by placing in the focus of a concave metallic mirror the bulb of a thermometer, covered with some good radiating substance, as the short white fibres of wool or cotton. By turning this apparatus towards the clear sky, the thermometer will fall several degrees. It is protected by its position from the radiation of surrounding objects, and its own radiant heat is projected towards the clear space, or falling upon the concave surface of the mirror, is reflected in parallel lines in the same direction. This effect is produced even while the sun is above the horizon, provided the mirror be turned from the direct rays of that luminary; and at night a depression of seventeen degrees below the temperature of the air and surrounding objects may commonly be produced. Perfect stillness of the atmosphere is necessary, and perfect transparency also, for otherwise the balance of temperature is soon restored by convection, and the slightest mist destroys the effect by a counter-radiation.

§ 279. It was upon these principles that Dr. Wells first

explained the formation of dew in one of the most beautiful experimental essays which ever graced the annals of inductive philosophy. He ascertained that the formation of this important phenomenon was always preceded by the cooling of the body, upon which it was deposited, below the temperature and dew-point of the air, by radiation. Hence it is that the best radiating substances, such as the fibrous and filamentous textures of vegetables, collect this moisture most abundantly; and the short-mown grass-plat will be covered with it, while the gravelled walk, or compact road by its side, will remain perfectly dry: and hence it is that dew never forms on a cloudy night, or when there is wind enough to restore the balance of temperature by its circulation.

§ 280. If in one of the foci of the conjugate mirrors before described, a piece of ice, or a glass filled with a freezing mixture, be placed instead of a heated body, the thermometer in the other focus will indicate a depression of temperature; and the experiment has sometimes been referred to as proving cold to have a positive existence, distinct from heat; but the phenomenon is easily explicable upon the principles which we have already laid down, without reference to any new hypothesis. In the new arrangement, the thermometer is the hotter body, and radiating its heat upon the nearest mirror the rays are projected upon the second, and collected in the focus, where they are absorbed by the ice; and as no adequate return is made, the temperature of the thermometer necessarily falls. The effect is exactly the same as that of radiation directed into clear space, just described, which was referred by Sir J. Leslie, to cold pulses shot downwards from the sky.

Radiation, however, was ascertained by Sir H. Davy to proceed with greater energy in vacuo than in the air. He, by means of the voltaic battery, ignited charcoal placed in the focus of a small mirror confined in the exhausted receiver of an air-pump. The receiver being exhausted to th, the effect upon a thermometer placed in the focus of another mirror below, was nearly three times as great as when the air was in its natural state of condensation.

§ 281. The law of the cooling of bodies by radiation, which is approximately stated for low temperatures by saying that the temperature communicated is proportional to the excess

of temperature, was more correctly ascertained by the researches of MM. Dulong and Petit. They found, 1st, that in vacuo the quickness of cooling for a constant excess of temperature increases in geometrical progression, when the temperature of the surrounding space increases in arithmetical progression.

2ndly, that the quickness of cooling, so far as it depends on the excess of temperature of the hot body, increases as the terms of a geometrical progression diminished by a constant number, when the temperature of the hot body increases in arithmetical progression.

§ 282. It has long been known that the heating power of the sun's rays depends upon the colour of the surfaces upon which they fall; and that dark and black bodies are more heated than those which are of light tints, or white. The fact was proved by Dr. Hooke and Dr. Franklin, who placed pieces of cloth, of similar texture and size, upon snow, allowing the sun's rays to fall equally upon them. The dark specimens always absorbed more heat than the light ones, and the snow beneath them melted to a greater extent than under the others; and they remarked that the effect was nearly in proportion to the depth of the shade. With regard to this experiment, the different colours stood in the following order,-black, blue, green, purple, red, yellow, white.

It is probable that the absorption of some kinds of terrestrial heat may also be affected, in some degree, by the colour of the objects upon which the rays may fall; but such a distinction of kinds we must proceed to established.

§ 283. We have now to inquire how radiant heat is affected by its passage through such bodies as it can penetrate and traverse. All the ordinary phenomena which we observe, take place through the medium of the atmosphere, and radiation will proceed through any gaseous medium. The experiment of Sir H. Davy has however just been referred to, to prove that it proceeds with least obstruction in vacuo. Observation has also proved that the intensity of the solar rays is greater upon the summit of a lofty mountain than at its base; and it has been calculated that about one-fifth of the solar heat is absorbed in passing through a column of 6000 feet of the purest air.

As it is by the passage of light through different transparent media that we distinguish different kinds of luminous rays, so

by the same means we are enabled to detect different kinds of calorific rays.

§ 284. Sir Wm. Herschel, in examining the solar ray by means of a prism of flint-glass, found the greatest heat in the red ray, or even in the dark space a little beyond it; and concluded that radiant heat was less refrangible than the least refrangible of the rays of light. Professor Seebeck subsequently ascertained that the place of the maximum of temperature in the solar spectrum depends upon the chemical composition of the substance of which the prism is made. With a hollow prism filled with sulphuric acid, it fell within the limits of the orange space, and with water within those of the yellow.

§ 285. In experimenting with colourless and perfectlypolished and transparent glass, one striking difference immediately occurs between solar heat and the radiant heat of terrestrial bodies; it allows the rays of the former, like the rays of light, to pass through it with little obstruction, while it almost wholly arrests the progress of the latter. The rays of heat, as well as of light, are concentrated in the focus of a concave metallic mirror, and the greatest heat which has ever been produced by art has thus been accumulated. The effect is little lessened when the mirror is formed of silvered glass, in which case the rays which are reflected from the bright metallic surface pass through the interposed substance of the glass before they are collected. On the other hand, if the metallic mirror be held before a common fire, a burning focus will be easily found; but with a glass mirror the light of the fire will be reflected, but not its heat. In a similar way glass lenses refract both the light and heat of the sun, and hence are familiarly distinguished as burning-glasses; but when held before any source of terrestrial heat, the most delicate air thermometer will scarcely be affected.

The principal effects which we have previously described of the reflexion of dark heat from the conjugate mirrors are immediately arrested by the interposition of the thinnest glass plate. This property of glass is sometimes usefully employed where it is desirable to see the light of a fire without being incommoded by the heat; and glass screens are used to protect the eyes when it is necessary to inspect the action of a hot furnace.

The radiant force, however, is not lost by this absorption of the glass: it receives a new direction; the glass itself becomes hot, and begins to throw off heat by secondary radiation.

§ 286. This distinction between solar and terrestrial heat is far from being absolute as was at one time supposed; for by delicate experiments it has been found that glass will arrest some of the former rays, while, on the other hand, it will allow some of the latter to pass. It has also been ascertained that the quantity of terrestrial heat which may be transmitted varies with the nature of its source: from a good radiating surface of the temperature of boiling water it is scarcely appreciable, while from the flame of a gas lamp it may be measured by the air thermometer. M. De Laroche also made the discovery that the heat which has passed through one plate of glass is less subject to absorption when passing through a second.

§ 287. The difference between radiant heat and light, and heat in a lower condition of intensity, is strikingly illustrated by the combustion of a flame of hydrogen and oxygen gases in which no solid matter is concerned. The result is the vapour of water, and the disengagement of the greatest heat which art can command; but it is accompanied by very little light, and if a convex lens be held before it, the radiant heat. which will pass through it will scarcely affect the most delicate air thermometer. If a piece of solid matter, capable of resisting its action, such as a wire of platinum, be held in it, radiation will immediately take place. A piece of lime thus presented to flame undergoes no chemical change, but emits a light which almost rivals that of the sun; and radiant heat is at the same time projected of sufficient intensity to penetrate the lens, and to inflame phosphorus at its focus. This light was used by Lieutenant Drummond for the purpose of signals, and when placed in the focus of a parabolic reflector was visible at a distance of sixty-nine miles.

$288. We are indebted to M. Melloni for almost all that we know, with accuracy, of the passage of radiant heat through different translucent substances. The memoirs in which he has recorded his experiments and deductions have been most justly honoured with the Rumford medal of the Royal Society, and they present a model well worthy of the