ticles 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 aeriform state, the dilatation increasing with the temperature; brit every substance expands according to a law of its own. 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 aérif rm coudrion, the equable dilatation of liquids has n‚¢ so extensive a range. The rate of expansion of wi ita varies at their transition to liquidity, and that of liquids is no longer equable near their change w an aëriform state. There are exceptione, however, to the general laws of expansion; one landa have a maximum density corresponding to a cer tain temperature, and dilate whether that tenvr76 ture be increased or diminished. For exacre,water expands whether it be heated avere se ponied below 40°. The solidification of wne ni and especially their crystallization, adven w companied by an increase of box.

rapidly when converted into ice, co vá a hre sufficient to split the hardest mico

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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 temperature; 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