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noticed under Density (page 5). There is thus a contest going on between the binding power of cohesion and the repelling power of heat. At first, with a small amount of heat, the cohesion holds its own; but as the heat increases, the vibrations become more violent, and the atoms are strongly pushed apart. Cohesion, then, has less power, because it has to act at a greater distance ; therefore, as the repulsion of the heat increases, the attraction of cohesion diminishes, till the atoms gain sufficient freedom to be able to slide or roll upon one another. The body is then said to be in a liquid state.

In the liquid state, the power of cohesion has not been altogether conquered; the atoms, although they are movable on one another, still resist being torn asunder. But if the heat be still further increased, the last feeble efforts of cohesion are overcome, and the atoms fly apart in the form of vapour. When a liquid has assumed the gaseous form, it is clear that the space it occupies is very much increased ; thus, water converted into steam occupies a space about 1700 times greater than it did before—that is, a cubic inch of water becomes a cubic foot of steam.

3. Vaporisation.—When sufficient heat has been applied to a liquid to make it assume the form of visible vapour, the first particles fly off from the surface, as is seen in the vapour that rises from all water when it becomes heated at all. Let us see what is going on meantime within the liquid

At the bottom of the liquid, where the heat is generally applied, the particles are being more and more repelled from one another, the liquid becomes lighter than that above it, and rises, while the liquid above it sinks down, as seen in the figure, which represents a vessel of water with a lamp under it. While this is going on, small bubbles of vapour rise from the bottom; but as they rise near the surface, where the temperature is lower, they are condensed again to water. The formation and condensation of these first bubbles give rise to the singing sound heard coming from water just before it boils. When, however, the whole of the water has been raised to a certain temperature, the bubbles of vapour that are formed at the bottom rise to the surface, and the water is then said to boil. Sometimes the bubbles are seen to rest on the surface; that is, there is a small quantity of vapour enclosed in a thin coating of the liquid. The repelling power of the heat in

Fig. 40. the steam of course tends to make it burst the bubble ; but it is prevented for a short time from doing so by the pressure

mass.

[graphic]

of the air, which amounts to 15 pounds on every square inch. But take a bubble when it is first formed at the bottom of the water ; there the pressure on it from without is the weight of all the water above it, as well as that of the air; so that the heat necessary to raise a quantity of water to the boiling-point is exactly the quantity of heat that will introduce among its particles a force of repulsion sufficient to overcome the pressure arising from the weight of the liquid above it and the weight of the atmosphere. Hence, to boil a large quantity of water, its temperature must be higher than in a smaller quantity, because the pressure to be overcome by the steam in the bubbles is greater ; hence too, water will boil at a much lower temperature if the pressure of the atmosphere be diminished, as is the case on high mountains.

4. Latent Heat.-When cold water is placed in a vessel over the fire, heat from the fire is communicated to the water, which gradually becomes hotter till it reaches the boiling-point; but, after the water boils, the temperature of the water does not rise. What, then, becomes of all the heat that continues to be communicated to it? We noticed in Section 1 the manner in which different modes of energy could be converted into heat: this is a case of the reverse process ; here heat is converted into motion. For a certain amount of heat a certain amount of work is done in pulling the particles of the liquid asunder. The heat which is consumed in this way after a liquid has been raised to the boiling-pointthat is, heat which goes to form vapour without raising the temperature of the liquid or of the steam—has been called latent, from the notion, at one time entertained, that heat was a fluid, and, consequently, that the heat which seemed to be lost in this way concealed itself in the vapour. The same thing takes place when a solid is being reduced to a liquid. When heat is applied to a piece of ice, its temperature does not rise above the point at which it began to melt till every bit of it is melted. The heat thus absorbed in the melting of ice, is called the latent heat of water; and that absorbed in converting water into steam, the latent heat of steam. When this latent heat is lost in any way, the repulsion existing among the particles diminishes, cohesion regains the mastery, and the steam returns to the form of water, or water to that of ice. It is on this principle that distillation is accomplished. The still usually consists of a copper boiler, in which the fermented liquor is converted into vapour; of a pipe, which conveys the vapour from the top of the ler; and of the worm, a coiled metal tube packed in a vessel through which there is a constant flow of cold water. The vapour arising from the boiling liquor in the copper is deprived of its heat in passing through the tube in the cold water; in consequence of this, it assumes again the liquid form, and drops or runs in a small stream from the end of the worm into a vesses placed to receive it.

5. Conduction and Radiation.—Just as light is the vibratory motion of the particles of a luminous body, and reveals itself by affecting the nerves of the eye; so it is this vibratory motion of the particles of a hot body, when communicated to the nerves, which causes the sensation of heat. These vibrations pass in all directions, and are hence called radiant heat. (See OPTICS, P. 25.) Again, as sound is transmitted by solids as well as by the air, so heat is transmitted by solids. When the end of a poker is placed in the fire, a vibratory motion is imparted to the atoms of that end; but this motion is communicated from atom to atom, till the other end also becomes heated. Metals shew the greatest facility in the passing of heat in this way; in other words, they are the best conductors of heat. The principle on which heat is transmitted through a fluid, as described at p. 39, and represented in the figure there, is called convection, because the particles of the body change their position, and, as it were, convey the heat; in the conduction of metals, no particle changes its position, the motion is merely passed from one to the other.

Having hitherto treated of the heating of a body, we will now consider the

process of cooling. The motion going on among the atoms of a heated body is communicated to the ether, and the heat is said to radiate; thus the hot body expends energy, the motion of its own atoms gradually diminishes, and it is said to cool. Suppose, then, it were desirable to keep it from cooling, what could be done to prevent it? If the hot body were covered with another, the heat must first be conducted through this covering before it can be radiated. Now, different bodies have different powers of conduction, so that if a hot body be covered with a bad conductor, it will be kept hot for a long time. This is the object of wearing clothes-not to warm one's body, but to keep its heat from being radiated. When a piece of red-hot metal is exposed to the air, the heat radiates from the outside, and the outer coating of cooled metal becomes a conductor to the heat in the interior. When the body cooling is a bad conductor, which is quite a different thing from the radiation of heat from the outside, the internal heat is preserved for a long time. The lava that runs as a red-hot liquid from volcanoes, and spreads out in great sheets, after cooling on the surface, so that people may walk over it, retains its heat under this crust for years, because it is a bad conductor.

6. Evaporation and Dew.—One of the most interesting phenomena connected with heat is Dew, the nature of which will be perfectly intelligible after the above explanations. When treating of the heating of liquids, we saw that, with a degree of heat much less than what would raise a liquid to the boiling-point, vapour is formed at the surface of the liquid. The formation of vapour in this way is called evaporation. The heat of the sun is continually causing evaporation from all bodies of water, or from everything wet; hence it is that anything wet by and by becomes dry, and even water in an open vessel will dry up. There is always more or less of this vapour in the air, even when the sky is clearest. It is only when the vapour, from being cooled in colder air, becomes partially liquefied, that it appears as fog, mist, or cloud. Dew, however, is not fog or mist deposited on the ground. After the sun has set on the evening of a hot summer day, the heat of the ground radiates into the air, or the grass, say, becomes cool, while the heat from the interior is not conducted quickly enough to keep up the temperature. The vapour in the air, coming in contact with this cooled surface, is now condensed into the watery particles of dew. One of the most remarkable things about dew is, that it is not deposited, at least to the same extent, on a cloudy night as under a clear, cloudless sky. This, at first sight, seems a contradiction, but only on the supposition that the moisture falls from the clouds, not when we remember how it is really formed. For the clouds radiate back to the earth the heat which has been radiated from it; so that the surface of the earth does not become colder than the air above it, and therefore the vapour is not condensed. Heat is always transmitted from one body to another which is colder. As was seen above, a certain amount of heat or of motion communicated to ice expands it into water, and a further amount expands the water into vapour. When the surface of the earth is colder than the air containing vapour, the heat of that vapour is transmitted into the ground, and the vapour becomes water or dew; and if the ground is extremely cold, the heat in the water keeping it in the liquid state, is further transmitted into the ground, the watery particles become solid and receive the name of hoar-frost.

PHYSIOLOGY OF THE HUMAN BODY.

THE HUMAN BODY is a most skilfully contrived machine, composed of a great number of different parts, or organs, all admirably adapted for the work they have to do, or, as it is technically expressed, the functions they have to perform. Thus, the limbs, the eyes, the ears, and the nose are organs which respectively perform the functions of motion, seeing, hearing, and smelling.

The functions performed by the organs of the body are of two kinds : (1) those that have to do with the building up and keeping in repair of the body itself; (2) those that bring the individual into connection with surrounding objects. The former have been called the functions of NUTRITION, or of ORGANIC LIFE, and include the digestion of the food, with the absorption of the nutritive materials contained in it, the circulation of the blood, and respiration ; the latter are called the functions of RELATION, or of ANIMAL LIFE, and include all the forms of motion and of sensation. In the following lessons, however, we propose to describe first the bony framework, with its covering of muscles and skin; then, the apparatus for keeping the whole fit for use ; and lastly, the nervous system, with the different forms of sensation which are the true links of connection between a human being and the outer world.

The Bony Skeleton.

BONE is a hard substance, composed of two kinds of material, an animal matter, called gelatine,2 and a mineral or earthy matter, consisting principally of lime. To the former it owes its elasticity and toughness, and to the latter its hardness. The general appearance of bone is that of a network of minute canals, usually running lengthwise, and connected here and there by cross branches. Towards the outside, the substance of the bone is harder and more compact ; and the whole is covered with

1 From Greek organon, an instrument.

? From Latin gelo, to freeze, because the liquid gelatine takes the consistence of jelly when cold,

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