SECTION XXIV.

HEAT. CALORIFIC RAYS OF THE SOLAR SPECTRUM.CHEMICAL RAYS OF THE SOLAR SPECTRUM.-EXPERIMENTS OF MM. DE LAROCHE AND MELLONI ON THE TRANSMISSION OF HEAT. — THE POINT OF GREATEST HEAT IN THE SOLAR SPECTRUM VARIES WITH THE SUBSTANCE OF THE PRISM. POLARIZATION OF HEAT. CIRCULAR POLARIZATION OF HEAT. TRANSMISSION OF THE CHEMICAL RAYS. — ABSORPTION OF HEAT. RADIATION OF heat. -DEW. HOAR FROST.-RAIN. HAIL.-COMBUSTION. DILATATION OF BODIES BY HEAT. PROPAGATION OF HEAT. LATENT HEAT. HEAT PRESUMED TO CONSIST OF THE UNDULATIONS OF AN ELASTIC MEDIUM.

It is not by vision alone that a knowledge of the sun's rays is acquired, touch proves that they have the power of raising the temperature of substances exposed to their action; and experience likewise teaches that remarkable changes are effected by their chemical agency. Sir William Herschel discovered that rays of caloric which produce the sensation of heat, exist in the solar spectrum independently of those of light; when he used a prism of flint glass, he found the warm rays most abundant in the dark space a little beyond the red extremity of the spectrum, that from thence they decrease towards the violet, beyond which they are insensible. It may therefore be concluded, that the calorific rays vary in refrangibility, and that those beyond the extreme red are less refrangible than any rays of light. Dr. Wollaston and MM. Ritter and Beckman discovered simultaneously, that invisible rays, known only by their chemical action, exist in the dark space beyond the extreme violet where there is no sensible heat. These are more refrangible than any of the rays of light or heat, and gradually decrease in refrangibility towards the other end of

the spectrum where they cease.

Thus the solar spectrum

is proved to consist of five superposed spectra, only three of which are visible -the red, yellow, and blue; each of the five varies in refrangibility and intensity throughout the whole extent, the visible part being overlapped at one extremity by the chemical, and at the other by the calorific rays. The action of the chemical rays blackens the salts of silver, and their influence is daily seen in the fading of vegetable colours. What object they are destined to accomplish in the economy of nature remains unknown; but certain it is, that the very existence of the animal and vegetable creation depends upon the calorific rays. That the heat-producing rays exist independently of light, is a matter of constant experience in the abundant emission of them from boiling water. Yet there is every reason to believe that both the calorific and chemical rays are modifications of the same agent which produces the sensation of light. Rays of heat dart in diverging straight lines from flame, and from each point in the surfaces of hot bodies, in the same manner as diverging rays of light proceed from every point of the surfaces of such as are luminous. According to the experiments of Sir John Leslie, radiation proceeds not only from the surfaces of substances, but also from the particles at a minute depth below it. He found that the emission is most abundant in a direction perpendicular to the radiating surface, and that it is more rapid from a rough than from a polished surface; radiation, however, can only take place in air and in vacuo; it is altogether imperceptible when the hot body is inclosed in a solid or liquid. Heated substances when exposed to the open air, continue to radiate caloric till they become nearly of the temperature of the surrounding medium. The radiation is very rapid at first, but diminishes according to a known law with the temperature of the heated body. It

inversely as its reflecting power; and bodies that are most impermeable to heat radiate least.

Rays of heat, whether they proceed from the sun, from flame, or other terrestrial sources luminous or non-luminous, are instantaneously 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 rockcrystal gives a free passage to the radiant heat, but intercepts almost all the light. M. Melloni has established the general law in uncrystallised 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 rockcrystal, 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 caloric. 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 crystallisation 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, consisting of blue, yellow, and red rays, passes through a red liquid, almost all the blue and yellow 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 dif ference 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 caloric 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 caloric.

The results obtained by M. de Laroche have been confirmed by the recent experiments of M. Melloni on caloric 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 red, yellow, and blue rays do which constitute white light. This explains the reason of the losses 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