gree, by the colour of the objects upon which the rays may fall; but, generally speaking, the absorption and radiation of terrestrial heat are dependant more upon the texture than the colour of the surfaces upon which it impinges or from which it emanates. 190. 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. 191. In experimenting with colourless and perfectly-polished 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 airthermometer 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. 192. 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. 193. The difference between radiant heat or light, and heat in its other condition, 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 the flame undergoes no chymical 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. 194. 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 imitation of those who are engaged in similar physical investigations. He has ascertained that the power in bodies of transmitting rays of heat is by no means proportioned to their transparency or their power of transmitting rays of light; and among crystallized bodies in particular, he has found that some which are highly transparent intercept nearly the whole of the calorific rays, while others act in a manner directly contrary. These properties are invariably manifested, whatever be the temperature of the source, and become yet more singular at low temperatures; for in the latter case it is found that the ordinary heat of the hand will pass through a solid body of several inches in thickness. Liquid chloride of sulphur, of a deep red-brown colour, will, out of 100 rays, allow 63 to pass; while an equal thickness of colourless spirit of turpentine will only transmit thirtyone; of sulphuric ether, twenty-one; sulphuric acid, seventeen; and of distilled water, only eleven: the case is the same with solid bodies. Different kinds of glass vary in their powers of transmission, from sixty-seven per cent. in flint glass, to forty-nine per cent. in crown glass: while perfectly diaphanous rock-salt will transmit 92 out of every 100 rays; and equally diaphanous alum only twelve. To distinguish those bodies which possess a capacity for calorific transmission, from those which possess a capacity for luminous transmission, he has proposed the term diathermanous for the former, as analogous in form to the epithet diaphanous applied to the latter. In one experiment he employed a plate of alum, well polished and perfectly transparent, only .06 inches in thickness, and compared it with a plate of smoky quartz 3.38 inches thick, which was of so decided a brown colour that the large letters of a printed page, placed in the fullest light, could not be traced through it; and found that the former allowed only six per cent. of the rays to pass through it, while the latter afforded a passage to nineteen per cent. He also found that the perfectly opaque glass employed in the construction of mirrors, for experiments upon the polarization of light, was diathermanous enough to transmit a considerable quantity of calorific rays: on the other hand, sulphate of copper, which is of a blue colour, and strongly diaphanous, is perfectly athermanous. 195. The results above detailed were obtained from the burner of an argand lamp; but M. Melloni found that different effects were produced from different calorific sources. The four which he compared together were, the flame of oil, without the interposition of glass; platina wire, kept in a state of incandescence by means of the flame of a spirit lamp (both of which are luminous); a plate of cop per, heated to the temperature of 7320; 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: TABLE XXIII.-Of Diathermancy. Names of the interposed substances Naked Incandescent Copper at Copper at flame. Platinum. 212°. 92 common thickness, 0.102 in. 732°. 0 196. 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. ELECTRICITY. 197. An intensity of light, equal to that given by the ignition of platinum in the oxy-hydrogen flame, and attended with great heat, may be pro |