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GENERAL PROPERTIES OF BODIES.

Or Avarice, a fiend more fierce than they? Flee to the shade of Academus' grove;

Where Cares molest not, Discord melts away

In harmony, and the pure passions prove

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How sweet the words of Truth breathed from the lips of Love.

What cannot Art and Industry perform,

When Science plans the progress of their toil! They smile at penury, disease, and storm;

And oceans from their mighty mounds recoil.
When tyrants scourge, or demagogues embroil
A land, or when the rabble's headlong rage
Order transforms to anarchy and spoil,

Deep-versed in man, the philosophic sage
Prepares with lenient hand their frenzy to assuage.
"Tis he alone, whose comprehensive mind,
From situation, temper, soil, and clime
Explored, a nation's various powers can bind
And various orders, in one form sublime
Of polity, that midst the wrecks of time,
Secure shall lift its head on high, nor fear

Th' assault of foreign or domestic crime,
While public Faith, and public Love sincere,
And Industry and Law maintain their sway severe.

BEATTIE.

LESSON 15.

General Properties of Bodies.

Symmetrical, proportionate, having parts well adapted to each other.

Cap'illary, a term applied to tubes of a very small bore, scarcely larger than to admit a hair, derived from capillus, the Latin

word for hair.

WHEN We speak of bodies, we mean substances, of whatever nature, whether solid or fluid; and matter is the general term used to denote the substance of which the different bodies are composed. As we do not suppose any body

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GENERAL PROPERTIES OF BODIES.

to exist without certain properties, such as impenetrability, extension, figure, divisibility, inertness, and attraction, these, therefore, are called the general properties of bodies.

By impenetrability, is meant the property which bodies have of occupying a certain space, so that, where one body is, another cannot be, without displacing the former; for two bodies cannot exist in the same place at the same time. A liquid may be more easily removed than a solid body; yet it is not the less substantial, since it is as impossible for a liquid and a solid to occupy the same space at the same time, as for two solid bodies to do so. If some water be put into a tube closed at one end, and a piece of wood be inserted that accurately fits the inside of the tube, it will be impossible to force the wood to the bottom, unless the water is first taken away. The air is a fluid differing in its nature from liquids, but not less impenetrable. If you endeavour to fill a phial by immersing it in water, the air will rush out in bubbles in order to make way for the water; and if you reverse the phial, and plunge it perpendicularly into the water, so that the air will not be able to escape, the water will not fill it, though it will rise a little, because it compresses the air into a smaller space in the upper part of the glass.

A body which occupies a certain space must necessarily have extension; that is to say, length, breadth, and depth. These are called the dimensions of extension, and we cannot form an idea of any body without them. The limits of extension are called figure or shape. A body having length, breadth, and depth, cannot be without form, either symmetrical or irregular; and this property admits of almost an infinite variety. The natural form of mineral substances is that of crystals; many of them are very beautiful, and not less remarkable for their transparency and colour, than for their perfect regularity, as may be seen in the various museums and collections of natural history. The vegetable and animal creation appears less symmetrical, but is still more diversified in figure than the mineral kingdom. Manufactured substances assume the various arbitrary forms which the art of man designs for them.

Divisibility is that property of matter, by which its parts may be divided and separated from each other; and of this division there can be no end. We can never conceive of a particle of matter so small as not to have an upper and under

GENERAL PROPERTIES OF BODIES.

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surface, which might be separated, if we had instruments fine enough for the purpose. A grain of gold may be hammered by the gold-beaters to such a degree of fineness, that the two millionth part of it may be seen by the naked eye; and by the help of a microscope the fifty millionth part will be visible. There are animals, it is said, so small that a single grain of sand is larger than four millions of them. But the natural divisions of matter are still more wonderful. The fragrance of a body is a part of the body itself, and is produced by very minute particles or exhalations which escape from it. How inconceivably small must be the odoriferous particles of a carnation, which diffuse themselves through a whole garden, so that, in every part of it, its fragrance is perceptible!

The word inertness expresses the resistance which inactive matter makes to a change of state. It requires some external force to put a body which is at rest in motion; and an exertion of strength is also requisite to stop a body which is already in motion. If a ball were fired from a cannon with a certain velocity, and there were no resistance from the air, it would circulate round the earth perpetually, and never come to a state of rest. In this manner the moon goes round the earth.

By attraction is meant the tendency that bodies have to approach each other, whatever be the cause of such tendency. All bodies are composed of infinitely small particles of matter, each of which possesses the power of attracting or drawing towards itself any other particle, and of uniting with it, when sufficiently near to be within the influence of its attraction; but in minute particles this power extends to so very small a distance around them that its effect is not sensible, unless they are, or at least appear to be, in contact. It then makes them adhere together, and is hence called the attraction of cohesion. It is by this principle that bodies preserve their forms, and are prevented from falling to pieces. The cohesive attraction of solids is much greater than that of fluids; and in elastic fluids, such as air, there is no cohesive attraction among the particles, and the utmost efforts of human art have proved ineffectual in the attempt to compress them, so as to bring them within the sphere of each other's attraction, and make them cohere. If two polished plates of marble, or of brass be but together with a

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ATTRACTION OF GRAVITATION.

little oil between them to fill up the pores in their surfaces, they will cohere so powerfully as to require a very considerable force to separate them. Two globules of quicksil ver, placed very near to each other, will run together, and drops of water will do the same. The ascent of water and other liquids in sugar, sponge, and all porous bodies is a species of this attraction, and is called capillary attraction.

Some bodies appear to possess a power which is the reverse of the attraction of cohesion. It is called repulsion, and is supposed to extend to a small distance around bodies, so as to prevent them from coming into actual contact. Water repels most bodies till they are wet. A small needle carefully placed on water will float. The drops of dew which appear in the morning on plants assume a globular form, from the mutual attraction between the particles of water; and upon examination it will be found that the drops do not touch the leaves, for they roll off in compact bodies, which would not be the case if there existed any degree of attraction between the water and the leaf. The repelling force between water and oil is so great that it is impossible to mix them in such a manner that they shall not separate again.

QUESTIONS.-1. What is matter? 2. What are the general properties of bodies? 3. What is impenetrability? 4. By what experiments is this property of matter illustrated? 5. Define extension and figure. 6. What is divisibility, and how illustrated? 7. Define inertness? 8. What is meant by attraction? 9. Attraction of cohesion? 10. What is said of the attraction of solids and fluids? 11. What experiments illustrate cohesive attraction? 12. What is capillary attraction? 13. What is repulsion, and by what experiments illustrated.

LESSON 16.

Attraction of Gravitation.

Rectilinear, consisting of right or straight lines.
Curvilinear, consisting of crooked, or curved lines.
Projec'tile, a body put in motion.

Evaga'tion, a wandering deviation.

Phenom'enon, (pl. phenomena) appearance, commonly expressive of some remarkable appearance in nature.

The attraction of gravitation is only a modification of the

ATTRACTION OF GRAVITATION.

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attraction of cohesion. The latter is not perceptible but in very minute particles, and at very small distances, the other acts on the largest bodies, and extends to immense distances.

That very law which moulds a tear,
And bids it trickle from its source,
That law preserves the earth a sphere,

And guides the planets in their course.— -ROGERS.

The tendency which bodies have to fall is produced entirely by the attraction of the earth; for the earth is so much larger than any body, on its surface, that it forces every body, which is not supported, to fall upon it. The following simple incident led to the most extensive and complicated calculations, and was productive of the most noble and wonderful discoveries. Newton happening one day, in the year 1666, when only twenty-five years of age, to be sitting under an apple-tree, and an apple falling upon his head, it suggested a variety of reflections. The phenomena of falling bodies in particular engaged his attention; and, extending his researches to the heavens, he began to investigate the nature of motion in general. Because there is motion, he reasoned, there must be a force that produces it. But what is this force? That a body when left to itself, will fall to the ground, is known to the most ignorant; but if you ask them the reason of its thus falling, they will think you either an idiot or a madman. The circumstance is too common to excite their wonder, although it is so embarrassing to philosophers, that they think it almost inexplicable. It is the mark of a superior genius to find matter for wonder, observation, and research, in circumstances which to the ordinary mind appear trivial, because they are common, and with which they are satisfied, because they are natural, without reflecting that nature is our grand field of observation, that within it is contained our whole store of knowledge; in a word, that to study the works of nature, is to learn to appreciate and admire the wisdom of God.

In applying his reflections on the nature of falling bodies to the celestial motions, Newton soon perceived that the force of gravity was not confined to the surface of our globe;' it being found to act alike at the bottom of the lowest valleys, and at the summit of the most lofty mountains. This led

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