CHAPTER VIII.

Attraction.

Cohesion and Adhesion.

Why can a cup be lifted by taking hold of the handle?

Because the particles of the cup do not separate from each other.

What is the power which holds these particles together called?

It is called _Cohesion_.

In what bodies is cohesion the strongest?

Cohesion is strongest in solid bodies.

Is the cohesion alike in all solids?

No; the cohesion in iron is much greater than the cohesion in wood.

Upon what does the strength of a substance depend?

The strength of a substance depends upon the amount of cohesion among its particles.

What effect does heat have upon the cohesion of bodies?

Heat tends to destroy the cohesion of bodies by separating their particles.

How does heat affect ice?

Heat destroys the cohesion between the particles of ice, and thus it is changed into water.

How does heat affect water?

When still more heat is applied, the cohesion in the water is destroyed, and thus it is changed into steam.

Is there any cohesion in steam?

No; there is not any cohesion in steam.

Is there any cohesion in air?

No; because the particles of air are always trying to separate from each other.

Is there any cohesion in liquids?

Yes; there is some cohesion in liquids, but it is only strong enough to hold them together in drops.

Does the cohesion in liquids vary?

Yes; the cohesion in mercury is stronger than it is in water; therefore, a drop of mercury is larger than a drop of water.

How is it in chloroform?

The cohesion in chloroform is weaker than it is in water; therefore, a drop of chloroform is smaller than a drop of water.

How is shot made?

When lead is melted and poured through a sieve at the top of a high tower, the attraction of cohesion forms it into round drops or globules; these cool, while falling, thus forming the grains of shot.

How high must a shot-tower be?

A shot-tower must be not less than two hundred feet high, so that the lead may harden while falling.

Why does putty stick to a pane of glass?

Because there is an attraction between the putty and the glass.

What is this attraction called?

It is called _Adhesion_.

Upon what does adhesion depend?

Adhesion depends upon the attraction which substances have for each other.

Has water any attraction for oil?

No; therefore, there is no adhesion between oil and water.

Has glass any attraction for water?

Yes; therefore, the water collects on the glass in drops.

What other examples of adhesion may be given?

Paint adheres to wood, dust to furniture, and tin and mercury to the back of a looking-glass.

Will you name a few of the most adhesive substances?

Glue, mortar, paste, and putty.

Of what use is glue?

Glue fastens pieces of wood together.

Of what use is mortar?

Mortar fastens together the bricks and stones used in the construction of houses and other buildings.

Of what use is paste?

Paste fastens pieces of paper together, and it fastens paper to the walls and ceilings of rooms.

Of what use is putty?

Putty fastens the glass to the wood-work of our windows.

Capillary Attraction.

Why does a sponge suck up water?

Because the particles of the sponge have an attraction for the water.

What is this attraction called?

It is called _Capillary_ attraction.

In what bodies do we find the most capillary attraction?

In those that are of a porous or spongy nature.

Why does a lump of sugar or salt, when partly dipped into water, soon become entirely wet?

Because sugar and salt are porous substances, and the water rises in them by capillary attraction.

Why does a heap of dry sand soon become damp when one edge touches the water?

Because the water is drawn through the sand by capillary attraction.

Why does water poured into the saucer of a flower-pot soon wet the earth in the flower-pot?

Because the earth is porous, and the water is drawn up through it by capillary attraction.

How does the oil in a lamp rise so as to be burned?

The oil is drawn up through the wick by capillary attraction.

Where may examples of capillary attraction be noticed?

In blotting-paper when it takes up the ink, and in bread when it soaks up the juice of meats.

What other examples may be given?

When one end of a towel is placed in a basin of water, and the other end is left hanging outside the basin, the water will pass through the towel by capillary attraction, until the basin is entirely dry.

Why is water in a glass tumbler a little elevated at the edges?

Because the glass has a slight attraction for the water near it, and thus lifts it up a little.

Why will water rise in a small glass tube higher than in the surrounding vessel?

Because the glass has an attraction for the water, and elevates it at the edges, as is shown in Fig. 26.

Why do salt and other substances dissolve in water?

Because water overcomes the force of cohesion in the salt, and thus separates its particles. The capillary attraction between the salt and the water is greater than the cohesion between the particles of salt; hence, the salt is dissolved by the water.

Gravitation.

When a ball is thrown into the air, why does it fall back to the earth?

Because the earth attracts the ball.

Why do rain and snow fall to the earth?

Because the earth attracts them.

What is this kind of attraction called?

It is called attraction of _Gravitation_.

Does the attraction of gravitation exist in all things?

Yes; every substance attracts every other substance.

Upon what does the attraction of gravitation in a body depend?

It depends upon the size and weight of that body.

Do larger bodies attract with more force than smaller ones?

Yes; larger bodies do attract more than smaller ones.

Do heavy bodies attract with more force than light ones?

Yes; heavy bodies do attract more than light ones, although they may be of the same size.

Where are all substances drawn by the attraction of gravitation?

They are all drawn towards the centre of the earth.

Why are substances drawn towards the centre of the earth?

Because the earth is larger than any substance on or near its surface.

In what direction do all objects fall through the air?

They all fall towards the centre of the earth.

Which way then do we call “down”?

_Down_ is always towards the centre of the earth.

Which way do we call “up”?

_Up_ is always away from the centre of the earth.

Where must all plumb lines or perpendicular lines point?

They must all point with one end towards the centre of the earth.

When the earth turns over, can any substance fall away from its surface?

No; all things are drawn to the earth with so much force that they cannot fall away.

Upon what does the weight of a body depend?

The weight of a body depends upon the force with which it is drawn towards the earth.

What kind of substances are attracted with the most force?

Those that are the most dense, like the metals; hence, they are the heaviest.

What causes a liquid to form into a drop at the mouth of a bottle?

The attraction of cohesion causes the drop to be formed.

What causes the drop to fall to the earth?

The attraction of gravitation causes the drop to fall to the earth.

When water is poured from a tumbler, why does it run down the side of the tumbler?

Because the attraction of cohesion draws the water to the side of the tumbler, as is shown in Fig. 27.

Why does a spout prevent the water from running down the side of the vessel?

Because the spout is curved so that its lowest point is where the water falls from it; hence, gravitation causes the water to fall directly to the earth, as is shown in Fig. 28.

Does the air obstruct bodies when falling through it?

Yes; air obstructs all bodies when falling through it; but it obstructs light bodies more than heavy ones.

Do all bodies fall with equal rapidity in a vacuum?

Yes; all bodies fall with equal rapidity in a vacuum.

What bodies fall through the air most rapidly?

Those that are the heaviest or most dense; because they are the smallest in bulk.

Do heavy bodies increase in velocity as they descend?

They do increase rapidly in velocity as they descend.

How far does a heavy body like lead fall in one second?

It falls sixteen feet in one second.

A falling body increases thirty-two feet in velocity for every second of time it is in falling. During the first second it falls sixteen feet. During the second second, it falls sixteen feet plus thirty-two feet, or forty-eight feet. During the third second, it falls forty-eight feet plus thirty-two feet, or eighty feet, etc.

A rising body decreases thirty-two feet in velocity for every second of time it is in rising. If it be three seconds in rising, its velocity will be eighty feet the first second, forty-eight feet the second second, and sixteen feet the third second, being just the reverse of the same body when falling, as is shown in Fig. 29.

How may the distance a body falls in a given time be found?

It may be found by multiplying the number of seconds it is in falling, by the same number, and this product by sixteen feet. Thus, when it is three seconds in falling, multiply three by three, and this product by sixteen feet, which gives one hundred and forty-four feet, the distance it falls in three seconds.

What familiar example have we of the force of falling bodies?

We have an example in the large hail-stones which fall to the earth in summer-time, breaking the glass in our windows and doing much injury to the crops.

Centre of Gravity.

What is the centre of gravity in a body?

The _centre of gravity_ in a body, is the point on which the body may be balanced, as is shown in Fig. 30.

Where is the centre of gravity in a circle?

It is the centre of the circle.

Where is the centre of gravity in a ball?

It is the centre of the ball.

What influence does the earth have on the centre of gravity?

The earth draws the centre of gravity in a body as near to itself as possible.

Why will an egg, when placed on either end, fall over on its side?

Because the centre of gravity is nearer the earth when the egg is placed upon its side than when upon either end, as is shown in Fig. 31 and Fig. 32.

Why will a ball remain in any position when placed on the earth?

Because its centre of gravity is always the same distance from the earth, whatever may be its position.

When will a body change its position if left unsupported?

A body will change its position whenever by so doing its centre of gravity will be lowered.

When may a body be readily moved about?

When its centre of gravity always remains at the same distance from the earth while being moved.

Why is a round body, like a ball, easily rolled about the floor?

Because its centre of gravity remains at the same distance from the floor.

When is the position of a body difficult to change?

Whenever the centre of gravity in it has to be lifted up in order to make the change, as is shown in Fig. 35.

The centre of gravity, in Fig. 33, may be lowered; hence, it will change its position if unsupported. The centre of gravity, in Fig. 34, can neither be lowered nor lifted up; hence, it may be readily rolled about. The centre of gravity, in Fig. 35, must be lifted up; hence, it remains firm in its place.

When does an object stand firm?

It stands firm when it has a broad base, and its centre of gravity is near the earth.

What effect is produced by elevating the centre of gravity?

The higher the centre of gravity in any body is raised above the earth, the greater is its tendency to fall.

Why is a load of hay more easily overturned than a load of iron, when upon the same wagon?

Because the centre of gravity in the load of hay is much higher than it is in the load of iron.

Why do passengers on top of a stage cause it to overturn more easily?

Because their weight on the top raises the centre of gravity in the stage, and it is more readily overturned.

Why is a person more likely to fall when standing up than when sitting down?

Because the centre of gravity is higher in a person when standing than when sitting.

Why is a boy more likely to fall when raised on stilts?

Because the centre of gravity in the boy is farther from the earth when he is raised on stilts.

When will a load of hay be overturned?

It will be overturned whenever the line from the centre of gravity to the earth falls outside of the wheels, as is shown in Fig. 36.

Why should lamps have large bottoms?

Lamps should have large bottoms so that the line of gravity may not fall outside of the bottom.

Where must the line of gravity in a body always fall?

It must always fall within the base of that body, or the body will be overturned.

How must a leaning tower be built so as not to fall?

It must be built so that the line of gravity will fall within its base.

Why do we lean back when carrying a load in front of us?

We lean back so that the line of gravity may be kept directly over our feet.

Why does a person standing up in a wagon fall when the wagon is suddenly moved?

Because the feet, being in contact with the wagon, move with it before the motion extends to the rest of the body, thus throwing the line of gravity behind the feet; hence, the person must fall, unless supported.

Why does a person standing up in a wagon fall when the wagon is suddenly stopped?

Because the feet, being in contact with the wagon, are suddenly stopped, while the body still moves forward, thus throwing the line of gravity ahead of the feet; hence, the person must fall, unless supported.

Why do animals with _four_ feet stand firmer than those with only _two_ feet?

Because the four feet afford a larger base to stand upon than the two feet do, and, therefore, support the centre of gravity better.

Why can a horse or a cow remain standing for many hours without seeming to be weary?

Because their four feet afford a good support to the centre of gravity.

Why does a person who is standing soon become weary?

Because it requires a constant effort to keep the line of gravity over the feet, and this soon produces weariness.

Why can a person sit longer than he can stand?

Because the centre of gravity is lower, when he is sitting, than when he is standing; hence, it requires less strength to support the body in a sitting position.

Why can a person lie down longer than he can sit or stand?

Because the centre of gravity is nearest the earth when he is lying down, and it requires no effort to remain in that position.

How do we measure time?

We measure time by means of a _pendulum_, such as is used in a common clock.

How does the pendulum move?

The pendulum moves regularly to and fro, from the line of gravity.

Does the pendulum vary any in its vibrations, or the time of moving to and fro?

The pendulum does not vary any in its vibrations.

Why does the pendulum keep moving?

When the pendulum is at A, in Fig. 37, gravity causes it to descend to C, and the motion it gains while falling to C carries it up to B; gravity then causes it to descend to C, and the motion it gains while falling to C carries it back to A again.

What helps to keep the pendulum in motion?

The works inside of the clock help to keep the pendulum in motion.

How long must the pendulum be, so as to tick once in every second?

The pendulum must be thirty-nine inches long.

How does it tick when the pendulum is _shortened_?

It ticks more _rapidly_.

How does it tick when the pendulum is _lengthened_?

It ticks more _slowly_.

How then does _cold_ affect a clock?

Cold makes the clock go faster by shortening the pendulum.

How does _heat_ affect a clock?

Heat makes the clock go more slowly by lengthening the pendulum.

How may the pendulum always be kept of the same length?

By having the weight at the bottom of the pendulum, so that it may be raised or lowered by means of a screw.

Specific Gravity.

Upon what does the weight of a body, when in the air, depend?

The weight of a body depends upon the force with which it is drawn towards the earth.

What is this attractive force called?

It is called gravitation or gravity.

What is gravity?

_Gravity_ is the weight of any body.

What is specific gravity?

_Specific Gravity_ is the weight of any body, compared with an equal bulk of water.

Do substances weigh as much in water as they do in air?

No; they do not weigh so much.

When we drop lead in a vessel full of water, what becomes of the water?

The water is forced over the sides of the vessel.

How much water is thus forced over the sides of the vessel?

A quantity equal in bulk to the lead dropped into the vessel.

When the water thus displaced weighs just one pound, how much less will the lead weigh while in the water than it did in the air?

It will weigh one pound less.

Then how much less is the weight of a body in water than it is in air?

Its weight in the water is as much less as the weight of the water which it displaces.

When a body has greater specific gravity than water, what will it do?

It will sink in the water.

When it has less specific gravity than water, what will it do?

It will float on the surface of the water.

Will all parts of the body be above the surface of the water?

No; a part of it will sink into the water.

How much of it will sink into the water?

It will sink so as to displace water enough to balance its own weight.

Why does a boat float on the water?

Because it weighs less than a bulk of water of the same size as the boat.

When a cargo is placed on the boat, why does it still float?

Because it still weighs less than a bulk of water of the same size as the boat.

How do we make use of this principle?

We make use of it in the construction of vessels, so as to carry on trade with other parts of the world.

Why can ducks and geese float on the water?

Because their light, downy feathers make their specific gravity less than that of the water; hence, they must float on its surface.

What may be said of the specific gravity of the human body?

When the lungs are filled with air, the specific gravity of the human body is rather less than that of water.

Can a person float in the water?

Yes; if a person, who cannot swim, will throw his head back so that his mouth and nose only remain above the water, he may float in this position for some time.

What is the specific gravity of fish?

The specific gravity of fish is so nearly equal to that of water, that they can move about anywhere beneath its surface.

How do fish rise and sink in the water?

Fish rise and sink in the water by means of an air-bladder in their bodies.

How does the air-bladder help the fish to rise or sink?

When the bladder is filled with air, the specific gravity of the fish becomes lighter, and it rises towards the surface; and when the air is forced out, the specific gravity becomes heavier, and the fish sinks towards the bottom.

Why does cream rise to the surface of milk?

Because the specific gravity of the cream is less than that of the milk.

Why does oil float on water?

Because the specific gravity of the oil is less than that of the water.

Why can a fat person float more easily in the water than a lean person?

Because the fat reduces the specific gravity of the body, so that it floats more easily.

Why can a person float more easily in salt water than in fresh water?

Because salt increases the weight of the water, and, therefore, it buoys up with more force.

What familiar example have we of this?

An egg will sink when placed in fresh water; but if salt be dissolved in the water, the egg will float near the surface.