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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 unfavourable 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-S0 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 cohe
sive 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. sion 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 par
tieles 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 dilatation increasing with the temperature; but every substance expands according to a law of its
Metals dilate uniformly from the freezing to the boiling points of the thermometer; the uniform expansion of the gases extends between still wider limits ; but as liquidity is a state of transition from the solid to the aëriform condition, the equable dilatation of liquids has not so extensive a range. The rate of expansion of solids varies at their transition to liquidity, and that of liquids is no longer equable near their change to an aëriform state. There are exceptions, however, to the general laws of expansion; some liquids have a maximum density corresponding to a certain temperature, and dilate whether that temperature be increased or diminished. For example, water expands whether it be heated above or cooled below 40°. The solidification of some liquids, and especially their crystallization, is always accompanied by an increase of bulk. Water dilates rapidly when converted into ice, and with a force sufficient to split the hardest substances. The