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solid; when the cohesion and repulsion seem to be equal, the particles are less firmly united, are freely movable among one another, and the body is said to be liquid; when the repulsion is greater than the cohesion, the particles are still more freely movable than in liquids, and such bodies are called gases.

Certain names have been applied to different states of cohesion :

(1) Tenacity is the quality by which a body resists being torn sunder, and depends on the intensity of the cohesive force. An iron wire onetenth of an inch in thickness will sustain a weight of 700 pounds. Fibrous substances, as silk and flax, possess great tenacity. The most tenacious of all substances is steel.

(2) Malleability means the capability possessed by some metals, as gold, silver, copper, &c. of being hammered out into thin plates. This quality depends on the union of softness and tenacity in the bodies possessing it ; being soft, their particles can be made to change their position with regard to one another; being tenacious, the particles will not readily separate.

(3) Ductility, the capability possessed by a metal of being drawn out into wire, is of the same nature as Malleability, both depending on a certain degree of softness and great tenacity. These properties are not, however, identical, for the most malleable metals are not the most ductile. The most malleable metal is gold ; the most ductile, platinum.

(4) Hardness.—Both in malleable and in hard bodies, the force with which the particles stick together is very great. In a malleable body the particles can be made to change their position by sliding or rolling on one another without separating, while in a hard body they resist change of position, and, if forced, separate or break. The relative hardness of two bodies is ascertained by trying which of them will scratch the other. Thus glass will scratch gold, and even platinum. All precious stones are very hard. The diamond is the hardest substance known, and is used in cutting glass.

(5) Brittleness is closely allied to hardness, for most hard bodies are brittle. It would seem to be the opposite of Malleability, for on a very slight attempt to displace the position of the particles, the body flies in pieces. Glass has this property to a remarkable degree.

The five properties enumerated above are due to certain states of the cohesion of the particles of a single solid body; there is also an attraction between two different bodies, which makes them stick to each other by their surfaces. This attraction is called adhesion.

5. Adhesion.—(1) Adhesion between two solids.—If two lead bullets be taken, and a piece be cut off each, leaving perfectly smooth surfaces, the bullets will stick together when these two surfaces are joined; or if two pieces of glass be laid upon each other, they stick together with considerable force. (2) Adhesion between a solid and a liquid is seen whenever a solid which has been put in a liquid comes out wet. This attraction affords an explanation of what often happens when water is poured from a vessel—the water runs down the outside of the vessel, instead of flowing as desired. (3) Adhesion between solids and gases.If a piece of cork be pushed down into water, little air-bubbles are seen sticking to it. When a lump of sugar is dropped into a cup of tea, the atmosphere of air which surrounds the particles does not quit them till they are dissolved ; bubbles are seen rising till all the sugar has disappeared.

6. Porosity.—Looking at a piece of cork or of sponge, we see that it is full of little holes ; these holes are called pores, and the substances having these holes are said to be porous. In ordinary language, it is only such substances as cork and very soft woods, in which these little holes are visible, that are said to have pores; but, in reality, all substances are more or less porous. When a piece of bone is examined with a microscope,

it

appears almost like a pile of empty boxes; and a piece of wood appears like a bundle of pipes. Even the densest solids, as gold and silver, have been proved to be porous. Thus, when a hollow sphere of silver was filled with water, and squeezed with great force, the water oozed through the silver, and stood in drops on the outside of the sphere.

7. Density.—This property is very closely connected with porosity: the two are, in fact, the converse of each other, because the more porous a body is, it is the less dense. If we squeeze a body to half its former size, there is of course no less matter in it, but we say we have doubled its density, while we have reduced it to half its original volume ; so that there are in the body after being compressed more atoms (that is, more matter) in the same space than there were before. In comparing the density of different substances, the density of water (distilled) is taken as a standard, and called 1. Measured by this standard, the density of other bodies is called their Specific Gravity: thus, the specific gravity of a body whose density is double that of water is said to be 2. Density is a very variable property : it is often increased owing to the Compressibility of bodies, and as often lessened by their Dilatability. An iron rod when heated becomes both thicker and longer, and contracts again with cold. So much is this the case, that when measurements are being made with an iron rod or chain, if the chain be exposed to great heat or cold, allowance must be made for difference of length. The iron rims of wheels could not be made to fit so tightly, were it not that they are put on when hot. Being a perfect fit when hot, when the iron cools and contracts, the rim binds the wheel very closely. Gases are also highly dilatable.

If a bladder be filled almost full of cold air, so as to shew the bladder loose and wrinkled, when heated it is seen to swell ont and become quite tight, from the air expanding : on the other hand, many cubic feet of air may be compressed into a single inch.

8. Elasticity.—This property is intimately connected with porosity and density. We have seen that bodies are both compressible and dilatable ; now, some bodies when compressed have a natural tendency, when the pressure is removed, to expand again to their original volume ; such bodies are said to be elastic, while those that retain the form given them by the compressing force are called non-elastic. Elasticity is not confined, however, to bodies that may be compressed like a piece of sponge, but those bodies are also said to be elastic, which return to the state or position in which they were before the force changing them was applied. A steel spring, when it is bent and then let go, immediately springs back to its former position ; so with a piece of india-rubber when stretched. The most elastic of all bodies are gases. The air-gun affords a very good illustration of the elasticity of common air. At the breech of a gun, with an ordinary barrel, is a small hollow sphere, into which air is forced by an instrument for the purpose until it is very much condensed. The opening by which the air is forced in is closed by a valve which opens inward, and when a shot is to be ed, the cock strikes the valve in, allowing a quantity of the condensed air to escape into a chamber behind the ball; and so great is the elasticity of the air, that the ball is projected from the gun with a force almost equal to that of a charge of gunpowder.

9. Inertia.—Inertia is the property which bodies have of always remaining in the same state, till that state be changed by external causes. Bodies at rest will remain always at rest, and bodies in motion will remain always in motion, as far as they themselves are concerned ; that is, till they be set in motion or stopped by external causes. No proof is required of the fact that when a body is at rest, it will not begin to move of itself; but it is not so clear that when a body is in motion, it will not stop of itself: indeed, many phenomena seem to prove the contrary. However powerfully a stone may be thrown or rolled along the ground, it will at last come to rest. It would thus seem as if it ceased to move of itself; but the truth is, that two very powerful agents are at work to stop it. These are the resistance of the air and friction. The resistance of the air is far greater than is generally supposed. On putting one's head out of the window of a railway train, the effect experienced is exactly the same as if a gale of wind were blowing It has been calculated that a 24-pound cannon-ball, when fired with a velocity of 2000 feet per second, experiences a resistance of 800 pounds. The retarding force of friction is greater even than this. We best see its effect by considering what takes place when it is removed. It is well known that a ball will roll much farther, with the same force, on a smooth floor than on a rough piece of ground, and much farther on a sheet of ice than on either. The smoother the surface, then, on which a body moves, the longer it continues to move. We also know that when a body, a pendulum for example, is set in motion in a place from which the air has been removed, it continues to move for a very long time. It may therefore be inferred, that if friction and the resistance of the air could be removed altogether, a body would, if once set in motion, continue to move for ever.

Forces and Motion. It has been shewn in the definition of Inertia that a body, whether at rest or in motion, cannot itself alter that state. In order to do so, some external cause is necessary. Such a cause is called a Force. Force is thus intimately connected with motion ; for, if a body be at rest, a force is necessary to set it in motion ; and, if it be in motion, a force is necessary to bring it to rest. The principles connecting force and motion have been expressed in three laws, called the LAWS OF MOTION.

First Law of Motion.—The first law of motion is simply a more precise definition of Inertia. A body will remain either in a state of rest, or in a state of uniform motion in a straight line, unless compelled to change that state by some external force.

The next consideration that naturally occurs is: How does this change of state depend on the force that produces it? The answer to this question is a statement of the second law of motion, which is as follows:

Second Law of Motion.—When a body is in motion under the influence of any number of forces, each force produces the same effect as it would if the other forces were not acting. Suppose a number of forces, P, Q, R, and S, to be acting on a body B; each of these forces, if it were acting by itself, would move the body a certain distance, proportioned to its strength, and in its own direction. Let the lines BA, BC, BD, and BE represent the forces P, Q, R, and S in direction and magnitude. If P alone were acting on the body, it would be moved to A; if, then, Q were to act on it, it would move along AF, parallel to BC, and to a distance from A equal to BC. Let AF be equal to BC; and if, when the body was at F, it were acted upon by R, it would move from F to G, FG being equal and parallel to BD; and so from G to H, GH being equal and parallel to BE. If acted on by the single forces in succession, then, the body would arrive

S

Q

B

H

D

Fig. 1.

at H, moving in the manner described ; if they all acted at once, the body would still arrive at H, but only by passing from B to H. A boat being rowed across a river affords a good illustration of this law. If the head of the boat be kept always pointing right across the stream, the passage will be made exactly in the same time as if it had been across a pond of the same breadth ; but it will be found that the boat has floated down the stream just as far as it would have done had it been simply floating on the stream for the same time.

In describing the second law of motion, we have spoken only of the effect produced on a body by a force acting upon it; but there is also another effect to be noticed in such cases. If a stone rolled along the ground come in contact with another, the latter will of course be dashed onward, but a change will also take place in the motion of the former ; it will either be stopped entirely, or be dashed to one side, or will continue to move in the same direction as before, but with less force. The effect of the rolling stone on the stone at rest is called the action, and that of the stone at rest on the rolling one, the reaction.

Third Law of Motion.To every action there is always an equal and contrary reaction; or, the mutual actions of any two bodies are always equal, and oppositely directed in the same straight line. If a person in a boat push another boat lying alongside, both boats are moved almost equally from where they were floating, thus shewing that the pushing reacts on the one boat as much as it acts on the other.

The force with which a body moves is called its momentum, and depends on the weight of the body, and the velocity with which it is moving. A stone when rolled or thrown has greater force than a ball of wood of the same size would have, because the one is heavier than the other; similarly, a large stone has greater force than a smaller one. Instead of weight, it is more proper to speak of the mass of a body; and in order to understand the meaning of mass, it is only necessary to remember the definition of density ;' for to say that a stone is denser than a piece of wood is the same thing as to say that it has more mass than a piece of wood of the same size. If a single atom of matter were moving at the rate of one foot per second, and if we take this as the measure of momentum, then, in a mass of many atoms, the momentum would be measured by the number of atoms; and if this mass were to move at the rate of 100 feet per second, its velocity would be 100 times the number of atoms greater than that of the single atom. The momentum of a body is therefore measured by the mass multiplied by the velocity.

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