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stantaneously transmitted through solid and liquid substances, there being no appreciable difference in the time they take to pass through layers of any nature or thickness whatever. They pass also with the same facility whether the media be agitated or at rest ; and in these respects the analogy between light and heat is perfect. Radiant heat passes through the gases with the same facility as light; but a remarkable difference obtains in the transmission of light and heat through most solid and liquid substances, the same body being often perfectly permeable to the luminous, and altogether impermeable to the calorific rays. For example, thin and perfectly transparent plates of alum and citric acid sensibly transmit all the rays of light from an argand lamp, but stop eight or nine tenths of the concomitant heat; whilst a large piece of brown rock-crystal gives a free passage to the radiant heat, but intercepts almost all the light. Alum united to green glass is also capable of transmitting the brightest light, but it gives not the slightest indication of heat; while rocksalt covered thickly over with soot, so as to be perfectly opaque to light, transmits a considerable quantity of heat. M. Melloni has established the general law in uncrystallized substances such as glass and liquids, that the property of instantaneously transmitting heat is in proportion to their refractive powers.
The law, however, is entirely at fault in bodies of a crystalline texture. Carbonate of lead, for instance, which is colourless, and possesses a very high refractive power with regard to light, transmits less radiant heat than Iceland spar or rock-crystal, which are very inferior to it in the order of refrangibility ; whilst rock-salt, which has the same transparency and refractive power with alum and citric acid, transmits six or eight times as much heat. This remarkable difference in the transmissive power of substances having the same appearance is attributed by M. Melloni to their crystalline form, and not to the chemical composition of their molecules, as the following experiments prove. A block of common salt cut into plates entirely excludes calorific radiation ; yet, when dissolved in water, it increases the transmissive power of that liquid : moreover, the transmissive power of water is increased in nearly the same degree, whether salt or alum be dissolved in it; yet these two substances transmit very different quantities of heat in their solid state. Notwithstanding the influence of crystallization on the transmissive power of
bodies, no relation has been traced between that power and the crystalline form.
The transmission of radiant heat is analogous to that of light through coloured media. When common white light passes through a red liquid, almost all the more refrangible rays, and a few of the red, are intercepted by the first layer of the fluid; fewer are intercepted by the second, still less by the third, and so on : till at last the losses become very small and invariable, and those rays alone are transmitted which give the red colour to the liquid. In a similar manner, when plates of the same thickness of any substance, such as glass, are exposed to an argand lamp, a considerable portion of the radiant heat is arrested by the first plate, a less portion by the second, still less by the third, and so on, the quantity of lost heat decreasing till at last the loss becomes a constant quantity. The transmission of radiant heat through a solid mass follows the same law. The losses are very considerable on first entering it, but they rapidly diminish in proportion as the heat penetrates deeper, and become constant at a certain depth. Indeed, the only difference between the transmission of radiant heat through a solid mass, or through the same mass when cut into plates of equal thickness, arises from the small quantity of heat that is reflected at the surface of the plates. It is evident, therefore, that the heat gradually lost is not intercepted at the surface, but absorbed in the interior of the substance, and that heat which has passed through one stratum of air experiences a less absorption in each of the succeeding strata, and may therefore be propagated to a greater distance before it is extinguished. The experiments of M. de Laroche show that glass, however thin, totally intercepts the obscure rays of heat when they flow from a body whose temperature is lower than that of boiling water ; that, as the temperature increases, the calorific rays are transmitted more and more abundantly ; and, when the body becomes highly luminous, that they penetrate the glass with perfect ease. The extreme brilliancy of the sun is probably the reason why his heat, when brought to a focus by a lens, is more intense than any that has been produced artificially. It is owing to the same cause that glass screens, which entirely exclude the heat of a common fire, are permeable by the solar heat.
The results obtained by M. de Laroche have been confirmed by the experiments of M. Melloni on heat radiated from sources of different temperatures, whence it appears that the calorific rays pass less abundantly not only through glass, but through rock-crystal, Iceland spar, and other diaphanous bodies, both solid and liquid, according as the temperature of their origin is diminished, and that they are altogether intercepted when the temperature is about that of boiling water.
In fact, he has proved that the heat emanating from the sun or from a bright flame consists of rays which differ from each other as much as the coloured rays do which constitute white light. This explains the reason of the loss of heat as it penetrates deeper and deeper into a solid mass, or in passing through a series of plates ; for, of the different kinds of rays which dart from a vivid flame, all are successively extinguished by the absorbing nature of the substance through which they pass, till those homogeneous rays alone remain which have the greatest facility in passing through that particular substance ; exactly as in a red liquid the violet, blue, green, orange, and yellow rays are extinguished, and the red are transmitted.
M. Melloni employed four sources of heat, two of which were luminous and two obscure ; namely, an oil-lamp without a glass, incandescent platina, copper heated to 696°, and a copper vessel filled with water at the temperature of 1781° of Fahrenheit. Rock-salt transmitted heat in the proportion of 92 rays out of 100 from each of these sources; but all other substances pervious to radiant heat, whether solid or liquid, transmitted more heat from sources of high temperature than from such as are low. For instance, limpid and colourless fluate of lime transmitted in the proportion of 78 rays out of 100 from the lamp, 69 from the platina, 42 from the copper, and 33 from the hot water; while transparent rock-crystal transmitted 38 rays in 100 from the lamp, 28 from the platina, 6 from the copper, and 9 from the hot water. Pure ice transmitted only in the proportion of 6 rays in the 100 from the lamp, and entirely excluded those from the other three sources. Out of 39 different substances, 34 were pervious to the calorific rays from hot water, 14 excluded those from the hot copper, and 4 did not transmit those from the platinum.
Thus it appears that heat proceeding from these four sources is of different kinds: this difference in the nature of the calorific
rays is also proved by another experiment, which will be more easily understood from the analogy of light. Red light, emanating from red glass, will pass in abundance through another piece of red glass, but it will be absorbed by green glass ; green rays will more readily pass through a green medium than through one of any other colour. This holds with regard to all colours ; so in heat. Rays of heat of the same intensity, which have passed through different substances, are transmitted in different quantities by the same piece of alum, and are sometimes stopped altogether ; showing that rays which emanate from different substances possess different qualities. It appears that a bright flame furnishes rays of heat of all kinds, in the same manner as it gives light of all colours; and, as coloured media transmit some coloured rays and absorb the rest, so bodies transmit some rays of heat and exclude the others. Rock-salt alone resembles colourless transparent media in transmitting all kinds of heat, even that of the hand, just as they transmit white light, consisting of rays of all colours. Radiant heat is unequally refracted by a prism of rock-salt like light, and the rays of heat thus dispersed are found to possess properties analogous to the rays of the coloured spectrum.
The property of transmitting the calorific rays diminishes to a certain degree with the thickness of the body they have to traverse, but not so much as might be expected. A piece of very transparent alum transmitted three or four times less radiant heat from the flame of a lamp than a piece of nearly opaque quartz about a hundred times as thick. However, the influence of thickness upon the phenomena of transmission increases with the decrease of temperature in the origin of the rays, and becomes very great when that temperature is low. This is a circumstance intimately connected with the law established by M. de Laroche ; for M. Melloni observed that the difference between the quantities of heat transmitted by the same plate of glass, exposed successively to several sources of heat, diminished with the thinness of the plate, and vanished altogether at a certain limit; and that a film of mica transmitted the same quantity of heat, whether it was exposed to incandescent platinum or to a mass of iron heated to 360°.
Coloured glasses transmit rays of light of certain degrees of refrangibility, and absorb those of other degrees. For example, red glass absorbs the more refrangible rays, and transmits the red, which are the least refrangible. On the contrary, violet glass absorbs the least refrangible, and transmits the violet, which are the most refrangible. Now M. Melloni has found, that, although the colouring matter of glass diminishes its power of transmitting heat, yet red, orange, yellow, blue, violet, and white glass transmit calorific rays of all degrees of refrangibility; whereas green glass possesses the peculiar property of transmitting the least refrangible calorific rays, and stopping those that are most refrangible. It has therefore the same elective action for heat that coloured glass has for light, and its action on heat is analogous to that of red glass on light. Alum and sulphate of lime are exactly opposed to green glass in their action on heat, by transmitting the most refrangible rays with the greatest facility.
The heat which has already passed through green or opaque black glass will not pass through alum, whilst that which has been transmitted through glasses of other colours traverses it readily.
By reversing the experiment, and exposing different substances to heat that had already passed through alum, M. Melloni found that the heat emerging from alum is almost totally intercepted by opaque substances, and is abundantly transmitted by all such as are transparent and colourless, and that it suffers no appreciable loss when the thickness of the plate is varied within certain limits. The properties of the heat therefore which issues from alum nearly approach to those of light and solar heat.
Radiant heat in traversing various media is not only rendered more or less capable of being transmitted a second time, but, according to the experiments of Professor Powell, it becomes more or less susceptible of being absorbed in different quantities by black or white surfaces.
M. Melloni has proved that solar heat contains rays which are affected by different substances in the same way as if the heat proceeded from a terrestrial source ; whence he concludes that the difference observed between the transmission of terrestrial and solar heat arises from the circumstance of solar heat containing all kinds of heat, whilst in other sources some of the kinds are wanting.
Radiant heat, from sources of any temperature whatever, is