Falsely luxurious, will not man awake, For is there aught in sleep can charm the wise? The fleeting moments of too short a life; Total extinction of the enlightened soul! 'Wilder'd, and tossing through distemper'd dreams! Who would in such a gloomy state remain Longer than nature craves; when every muse And every blooming pleasure wait without, To bless the wildly-devious morning walk? Thomson. LESSON XVIII. HEAT. 1. Heat, strictly speaking, is the name of a sensation, though it is customary to speak of the heat of the sun, or the heat of fire, just as readily as of the heat which these bodies are capable of exciting. The nature of heat is not yet well understood, it being still doubtful whether it is a material substance or a mere property of matter. 2 3 2. Heat is produced in various ways: by combustion,1 by friction, by percussion, by the mixture of two or more substances, as when sulphuric acid is poured into water, and by electricity. The principal source of heat, however, is the sun. 3. Heat is either latent or free. All bodies are supposed to contain it, but when it is neither perceptible by the senses, nor affects the thermometer, it is termed latent heat. 4. Heat always tends to diffuse itself equally; in other words, when two bodies are of different temperatures, the warmer gradually parts with its heat to the colder, till they are both brought to the same temperature. Thus, when a thermometer is applied to a hot body it receives heat, when to a cold one it gives to it part of its own heat; and this giving and receiving goes on until the thermometer and the body arrive at the same temperature. 5. Cold is merely a diminution of heat. When you lay your hand on a marble table you indeed feel it cold, but the cold you experience consists 4 merely in the loss of heat that your hand sustains whilst its temperature is being brought to an equilibrium with the table. If you lay a piece of ice upon the same table you will find that a contrary effect takes place, the ice will be melted by the heat which it abstracts from the marble. 6. The facility with which heat enters or leaves bodies depends much upon the nature of the body, some species permitting the passage of it through them with ease, and others with much difficulty. Those substances which permit it to pass readily through them are called good conductors; thus metals and liquids are good conductors; but silk, cotton, wool, wood, &c., are bad conductors. 7. For example, if we put one end of a poker into the fire, the other end will soon become hot; but this will not happen with a piece of wood of the same length, and under the same circumstances. A person may stand so near the fire as to make the metal buttons on his coat too hot to touch, while the temperature of the cloth will be apparently scarcely altered. 8. When there is occasion to hold any metallic instrument, we take care that the part by which it is to be held shall not be made of metal, but of wood or bone. Good conductors of heat would evidently form bad clothing. The object of clothing is to intercept the heat, and preserve the body as much as possible at a uniform temperature. 9. In cold weather, the temperature of the atmosphere being lower than that of the body, clothing formed of non-conductors prevents the too rapid escape of heat from the body to the surrounding air; and, in very hot weather, it answers a contrary purpose-preventing the too rapid communication of heat to the body. Animals are clothed in fur, wool, feathers, &c., all non-conductors; and man borrows his clothing, in a great degree, from them. 10. One of the most remarkable properties of heat is the repulsion which exists amongst its particles. Hence it happens that when this principle enters into a body, its first effect is to remove the molecules of the substance to a greater distance from one another. The body, therefore, becomes less compact than before, occupies a greater space, or, in other words, expands. 11. Now this effect of heat is manifestly in opposition to cohesion-that force which tends to make the particles of matter hold together, and which must be overcome before any expansion can ensue. It may be expected, therefore, that a small addition of heat will occasion a small expansion, and a greater addition of it a greater expansion, because in the latter case the cohesion will be more overcome than in the former. 12. It may be anticipated also that whenever heat passes out of a body, the cohesion being then left to act freely, a contraction will necessarily follow; so that expansion is only a transient effect, occasioned solely by the accumulation of heat. 13. It follows, moreover, from this view that heat must produce the greatest expansion in those bodies which have the least cohesive power, and the inference is fully justified by observation. 14. Thus the force of cohesion is greatest in solids, less in liquids, and least of all in gases; while the expansion of solids is trifling, that of liquids is much more considerable, and that of gases far greater. It may be laid down as a rule, the reason of which is now obvious, that all bodies are expanded by heat, and that the expansion of the same body increases with the quantity of heat which enters it. PROGNOSTICS1 OF THE WEATHER, 1. Red clouds in the west, at sunset, especially when they have a tint of purple, portend fine weather; the reason of which is, that the air, when |