« PreviousContinue »
formation of ice is therefore a powerful agent in the disintegration and decomposition of rocks, operating as one of the most efficient causes of local changes in the structure of the crust of the earth, of which we have experience in the tremendous éboulemens of mountains in Switzerland.
Heat is propagated with more or less rapidity through all bodies; air is the worst conductor, and consequently mitigates the severity of cold climates by preserving the heat imparted to the earth by the sun. On the contrary, dense bodies, especially metals, possess the power of conduction in the greatest degree, but the transmission requires time. If a bar of iron, twenty inches long, be heated at one extremity, the caloric takes four minutes in passing to the other. The particle of the metal that is first heated communicates its caloric to the second, and the second to the third; so that the temperature of the intermediate molecule at any instant is increased by the excess of the temperature of the first above its own, and diminished by the excess of its own temperature above that of the third. That, however, will not be the temperature indicated by the thermometer, because, as soon as the particle is more heated than the surrounding atmosphere, it will lose its caloric by radiation, in proportion to the excess of its actual temperature above that of the air.
The velocity of the discharge is directly proportional to the temperature, and inversely as the length of the bar. As there are perpetual variations in the temperature of all terrestrial substances, and of the atmosphere, from the rotation of the earth and its revolution round the sun, from combustion, friction, fermentation, electricity, and an infinity of other causes, the tendency to restore the equability of temperature by the transmission of caloric must maintain all the particles of matter in a state of perpetual oscillation, which will be more or less rapid according to the conducting powers of the substances. From the motion of the heavenly bodies about their axes, and also round the sun, exposing them to perpetual changes of temperature, it may be inferred that similar causes will produce like effects in them too. The revolutions of the double stars show that they are not at rest, and though we are totally ignorant of the changes that may be going on in the nebulæ and millions of other remote bodies, it is more than probable that they are not in absolute repose; so that, as far as our knowledge extends, motion seems to be a law of matter.
Heat applied to the surface of a fluid is propagated downwards very slowly, the warmer, and consequently lighter strata always remaining at the top. This is the reason why the water at the
bottom of lakes fed from alpine chains is so cold; for the heat of the sun is transfused but a little way below the surface. When heat is applied below a liquid, the particles continually rise as they become specifically lighter, in consequence of the caloric, and diffuse it through the mass, their place being perpetually supplied by those that are more dense. The power of conducting heat varies materially in different liquids. Mercury conducts twice as fast as an equal bulk of water, which is the reason why it appears to be so cold. A hot body diffuses its caloric in the air by a double process. The air in contact with it, being heated, and becoming lighter, ascends and scatters its caloric, while at the same time another portion is discharged in straight lines by the radiating powers of the surface. Hence a substance cools more rapidly in air than in vacuo, because in the latter case the process is carried on by radiation alone. It is probable that the earth, having originally been of very high temperature, has become cooler by radiation only. The ethereal medium must be too rare to carry off much caloric.
Besides the degree of heat indicated by the thermometer, caloric pervades bodies in an imperceptible or latent state; and their capacity for heat is so various, that very different quantities of caloric are required to raise different substances
to the same sensible témperature; it is therefore evident that much of the caloric is absorbed, or latent and insensible to the thermometer. The portion of caloric requisite to raise a body to a given temperature is its specific heat; but latent heat is that portion of caloric which is employed in changing the state of bodies from solid to liquid, and from liquid to vapour. When a solid is converted into a liquid, a greater quantity of caloric enters into it than can be detected by the thermometer; this accession of caloric does not make the body warmer, though it converts it into a liquid, and is the principal cause of its fluidity. Ice remains at the temperature of 32° of Fahrenheit till it has combined with or absorbed 140° of caloric, and then it melts, but without raising the temperature of the water above 32°; so that water is a compound of ice and caloric. On the contrary, when a liquid is converted into a solid, a quantity of caloric leaves it without any diminution of its temperature. Water at the temperature of 32° must part with 140° of caloric before it freezes. The slowness with which water freezes, or ice thaws, is a consequence of the time required to give out or absorb 140° of latent heat. A considerable degree of cold is often felt during a thaw, because the ice, in its transition from a solid to a liquid state, absorbs sen
sible heat from the atmosphere and other bodies, and, by rendering it latent, maintains them at the temperature of 32° while melting. According to the same principle, vapour is a combination of caloric with a liquid. About 1000° of latent heat exists in steam without raising its temperature: that is, boiling water, at the temperature of 212°, must absorb about 1000° of caloric before it becomes steam; and steam at 212° must part with the same quantity of latent caloric when condensed into water. The elasticity of steam may be increased to an enormous degree by increasing its temperature under pressure, yet its latent heat remains the same; however, it acquires an additional quantity, if allowed to expand; so that the latent heat of high-pressure steam issuing from a boiler is really two-fold-the latent heat of elastic fluidity and that of expansion. High-pressure steam expands the instant it comes into the air; the latent heat of expansion is increased at the expense of the latent heat of fluidity, in consequence of which, a portion of the steam is instantly condensed, and then the remaining portion, being mixed with air and particles of water, is so much reduced in temperature, that the hand may be plunged, without injury, into high-pressure steam, the instant it issues from the orifice of a boiler.