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found to be upon the yellow, upon the orange, on the red, or at the dark limit of the red, according as the prism consists of water, sulphuric acid, crown or flint glass. If it be recollected that, in the spectrum from crown glass, the maximum heat is in the red part, and that the solar rays, in traversing a mass of water, suffer losses inversely as their refrangibility, it will be easy to understand the reason of the phenomenon in question. The solar heat
which comes to the anterior face of the prism of water consists of rays of all degrees of refrangibility. Now, the rays possessing the same index of refraction with the red light suffer a greater loss in passing through the prism than the rays possessing the refrangibility of the orange light, and the latter lose less in their passage than the heat of the yellow. Thus, the losses, being inversely proportional to the degree of refrangibility of each ray, cause the point of maximum heat to tend from the red towards the violet, and therefore it rests upon the yellow part. The prism of sulphuric acid, acting similarly, but with less energy than that of water, throws the point of greatest heat on the orange; for the same reason the crown and flint glass prisms transfer that point respectively to the red and to its limit. M. Melloni, observing that the maximum point of heat is transferred farther and farther towards the red end of the spectrum, according as the substance of the prism is more and more permeable to heat, inferred that a prism of rock-salt, which possesses a greater power of transmitting the calorific rays than any known body, ought to throw the point of greatest heat to a considerable distance beyond the visible part of the spectrum-an anticipaticn which experiment fully confirmed, by placing it as much beyond the dark limit of the red
rays as the red part is distant from the bluish-green band of the spectrum.
When radiant heat falls upon a surface, part of it is reflected and part of it is absorbed, consequently the best reflectors possess the least absorbing powers. The absorption of the sun's rays is the cause both of the color and temperature of solid bodies. A black substance absorbs all the rays of light, and reflects none; and since it absorbs at the same time all the calorific rays, it becomes sooner warm, and rises to a higher temperature, than bodies of any other color. Blue bodies come next to black in their power of absorption. Of all the colors of the solar spectrum, the blue possesses least of the heating power and since substances of a blue tint absorb all the other colors of the spectrum, they absorb by far the greatest part of the calorific rays, and reflect the blue where they are least abundant. Next in order come the green, yellow, red, and, last of all, white bodies, which reflect nearly all the rays both of light and heat. The temperature of very transparent fluids is not raised by the passage of the sun's rays, because they do not absorb any of them, and as his heat is very intense, transparent solids arrest a very small portion of it.
Rays of heat proceed in diverging straight lines from each point in the surfaces of hot bodies, in the same manner as diverging rays of light dart from every point of the surfaces of those that are luminous. 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 appears also that the
radiating power of a surface is inversely as its reflecting power; and bodies that are most impermeable to heat radiate least. 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 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 insensible when the hot body is inclosed in a solid or liquid. All substances may be considered to radiate caloric, whatever their temperature may be, though with different intensities, according to their nature, the state of their surfaces, and the temperature of the medium into which they are brought. But every surface absorbs, as well as radiates, caloric; and the power of absorption is always equal to that of radiation, for it is found that, under the same circumstances, matter which becomes soon warm also cools rapidly. There is a constant tendency to an equal diffusion of caloric, since every body in nature is giving and receiving it at the same instant; each will be of uniform temperature when the quantities of caloric given and received during the same time are equal, that is, when a perfect compensation takes place between each and all the rest. Our sensations only measure comparative degrees of heat: when a body, such as ice, appears cold, it imparts fewer calorific rays than it receives; and when a substance seems to be warm,-for example, a fire, it gives more caloric than it takes. The phenomena of dew and hoar-frost are owing to this inequality of exchange, for the caloric radiated during the night by substances on the surface of the earth into a clear expanse
of sky is lost, and no return is made from the blue vault, so that their temperature sinks below that of the air, from whence they abstract a part of that caloric which holds the atmospheric humidity in solution, and a deposition of dew takes place. If the radiation be great, the dew is frozen, and becomes hoar-frost, which is the ice of dew. Cloudy weather is unfavorable to the formation of dew, by preventing the free radiation of caloric, and actual contact is requisite for its deposition, since it is never suspended in the air, like fog. Plants derive a great part of their nourishment from this source; and as each possesses a power of radiation peculiar to itself they are capable of procuring a sufficient supply for their wants.
Rain is formed by the mixing of two masses of air of different temperatures; the colder part, by abstracting from the other the heat which holds it in solution, occasions the particles to approach each other and form drops of water, which, becoming too heavy to be sustained by the atmosphere, sink to the earth by gravitation in the form of rain. The contact of two strata of air of different temperatures, moving rapidly in opposite directions, occasions an abundant precipitation of rain.
An accumulation of caloric invariably produces light: with the exception of the gases, all bodies which can endure the requisite degree of heat without decomposition begin to emit light at the same temperature; but when the quantity of caloric is so great as to render the affinity of their component particles less than their affinity for the oxygen of the atmosphere, a chemical combination takes place with the oxygen, light and heat are evolved, and fire is produced. Combustion-so essential for our comfort, and even existence-takes place very easily from the small
affinity between the component parts of atmospheric air, the oxygen being nearly in a free state; but as the cohesive force of the particles of different substances is very variable, different degrees of heat are requisite to produce. their combustion. The tendency of heat to a state of equal diffusion or equilibrium, either by radiation or contact, makes it necessary that the chemical combination which occasions combustion should take place instantaneously; for if the heat were developed progressively, it would be dissipated by degrees, and would never accumulate sufficiently to produce a temperature high enough for the evolution of flame.
Though it is a general law that all bodies expand by heat and contract by cold, yet the absolute change depends upon the nature of the substance. Gases expand more than liquids, and liquids more than solids. The expansion of air is more than eight times that of water, and the increase in the bulk of water is at least forty-five times greater than that of iron. The expansion of solids and liquids increases uniformly with the temperature, between certain limits, this change of bulk, corresponding to the variation of heat, is one of the most important of its effects, since it furnishes the means of measuring relative temperature by the thermometer and pyrometer. The expansive force of caloric has a constant tendency to overcome the attraction of cohesion, and to separate the constituent particles of solids and fluids; by this separation the attraction of aggregation is more and more weakened, till at last it is entirely overcome, or even changed into repulsion. By the continual addition of caloric, solids may be made to pass into liquids, and from liquids to the aëriform state, the dilation increasing with the tempera