First Lessons in Natural Philosophy for Beginners
CHAPTER III.
Heat.
Sources of Heat.
What is heat?
Heat is that agent which causes the feeling of warmth.
Can heat be seen?
No; heat cannot be seen, it can only be felt.
When we touch a substance hotter than we are, what takes place?
A part of the heat from that substance comes to us and causes a feeling of warmth.
When we touch a substance colder than we are, what takes place?
A part of the heat from us goes to that substance, and we experience a feeling of cold.
Does the amount of heat in any substance vary?
Yes; water may be heated until it is scalding hot, or the heat may be taken from it until it is frozen into ice.
What causes a substance to become cold?
Taking away heat from a substance causes it to become cold.
Then what is cold?
Cold is only the absence or want of heat.
When we pass from a very hot room to one moderately warm, how do we feel?
We experience a feeling of cold.
When we pass from a very cold room to one moderately warm, how do we feel?
We have a feeling of warmth, although the room is no warmer than in the first instance.
How else can we prove that we judge of heat and cold by our feelings?
If one hand be held in quite warm water, and the other in quite cold water, for a few moments, and then both hands be plunged into tepid water, the tepid water will feel cold to the hand that was in the warm water, and warm to the hand that was in the cold water.
Is there anything without heat?
No; the coldest substance known still contains some heat.
How many kinds of heat are there?
There are two kinds of heat.
What are the two kinds of heat?
Heat accompanied by light, as the heat from the sun, or a lamp; and heat without light, as the heat from boiling water.
From what source does heat mostly come?
Heat mostly comes from the sun, which is also the source of light.
What is said of the heat and light from the sun?
Heat and light from the sun come together in the sunbeam.
From what other source is heat obtained?
Heat is obtained from the burning of any substance.
What else does the burning of any substance produce?
It often produces light.
Can the light and heat from the sun easily be separated?
They cannot easily be separated.
What does a glass window do with the light and heat of the sunbeam?
It permits both the light and the heat to pass into the room, so that we can see the one and feel the other.
Are the light and heat from a fire united, as they are in the sunbeam?
They are not; but they seem to be separated from each other.
What does a glass window do with the light and heat from a fire?
It permits the light to pass through; but it stops the heat.
How may this be proved?
When a pane of glass is held between the face and a fire, it will protect the face from the heat.
Is there any heat without light?
Yes; many substances contain heat, but do not emit light.
Does light change the amount of heat in a substance?
No; boiling water is as hot in the dark as it is in the light; and ice is as cold in the daytime as it is at night.
Is there any other source of heat?
Yes; heat is produced by rubbing or striking substances together.
What will result from rubbing two pieces of wood together?
They may be rubbed until they take fire.
How do Indians kindle fires?
By rubbing two pieces of wood together until they take fire.
How is heat produced by the brakes on railroad cars?
It is produced by the car-wheels rubbing against the brakes.
What may be seen when a horse strikes his shoes against a stone?
Small sparks of fire, which contain heat, may be seen.
If a piece of iron be hammered, will its heat be increased?
Its heat will be increased.
How can this be proved?
A person can, by hammering a piece of cold iron, make it red-hot.
Could we live without heat?
No; our bodies must be kept warm, and this can be done only by heat.
Is this true of other living things?
Yes; all the animals and all the plants would die if heat were taken away.
How should we feel in regard to these things?
We should feel thankful to an All-wise Creator, for having provided us with both heat and light from a never-failing source—the sun.
Expansion by Heat.
How does heat generally affect substances?
Heat _expands_ them or makes them larger.
Does heat expand all metals?
Yes; all metals are expanded by heat.
When a rod of iron is heated, is it longer or shorter than when cold?
It is longer than when it is cold.
How do we know this?
We know it by measuring the rod when it is cold, and again when it is heated.
Will the rod be larger, as well as longer?
The rod will be larger.
How can we prove this?
The rod, when red-hot, will not go through as small a hole as when it is cold.
How do heat and cold affect most substances?
Heat expands most substances, and cold contracts them.
Do all metals expand alike by heat?
No; some metals expand much more than others.
How does heat expand substances?
Heat pushes the parts or atoms of an object farther from each other, and thus its size is increased.
What does a blacksmith do with a tire before putting it on a wheel?
He heats the tire red-hot.
Why does he heat the tire red-hot?
So that it will be increased in size, and will go on the wheel more easily.
What takes place as the tire cools?
It becomes smaller, and thus binds the parts of the wheel tightly together.
When a fire is made in a room, why does the furniture often make a snapping noise?
Because heat expands the wood, and the particles make a snapping noise upon separating.
Why does a stove make a crackling noise as it grows hot?
The particles of the iron make the noise in expanding.
If a piece of glass be held in the hand, will the glass become warm?
The glass will become as warm as the hand.
What part of the glass will be warmed first?
The part touching the hand will be warmed first.
What part of the glass will be warmed last?
The part farthest from the hand will be warmed last.
Which part of the glass will be expanded first?
The part touching the hand, because it is first warmed.
Which part will be expanded last?
The part farthest from the hand, because it is the last part warmed.
If any other warm substance touch the glass, will the glass be heated in the same manner?
The glass will be heated in the same manner.
What often results from this unequal heating?
The glass is broken.
Why are glass plates or tumblers broken when placed in hot water?
They are broken because the part next to the hot water is expanded more than that farther away.
Why will a cloth dipped in hot water and wrapped around the neck of a bottle loosen its stopper?
Because the heat from the cloth expands the neck of the bottle before it does the stopper, so that the stopper is loosened.
Why do stove-plates often break?
Because they are fitted together in the stove so that they have not room to expand without breaking.
What effect has cold upon substances?
Cold causes them to contract or grow smaller.
Things _expand_ unequally; do they also _contract_ unequally?
They do contract unequally, and in this way may be broken.
Why are lamp chimneys often broken while the lamp is burning?
Because the heat of the flame causes them to expand unequally.
In what other way are they broken?
By a draught of cold air, or a drop of cold water, touching them, thus causing them to contract unequally.
Why are they sometimes broken when the lamp is not burning?
Because a current of cold air from a window or a door, blows upon them.
Does quicksilver expand when heated?
Like other metals, quicksilver expands as it grows warmer, and contracts as it grows cooler.
Is quicksilver in the form of a solid or a fluid?
It is a fluid as we usually see it.
For what is quicksilver used?
It is used for filling the tubes of thermometers.
What is a Thermometer?
It is an instrument used for measuring the degree or quantity of heat in any substance.
How does heat affect the quicksilver?
Heat expands the quicksilver, and causes it to rise in the tube.
How does cold affect the quicksilver?
Cold contracts the quicksilver, and causes it to fall in the tube.
Does heat expand liquids more than solids?
Heat does expand liquids more than solids.
Why are liquids expanded more than solids?
Because the particles of which they are formed are more easily separated or pushed apart by the heat.
If we continue to apply heat to a liquid, what becomes of it?
The liquid is changed into a gas or vapor.
What common example have we of vapor?
Water, when heated, is turned into steam.
Mention some gas.
The _air_ we breathe is a gas.
How does heat affect the air?
Heat warms the air and causes it to expand.
Do solids all expand equally when heated?
No; some solids expand more than others; thus, zinc expands more than iron, and iron more than glass.
Do liquids all expand equally when heated?
No; liquids, although more sensitive to heat, do not expand as equally as solids.
Do gases expand equally when heated?
Yes; gases expand uniformly; thus, 491 cubic inches of any gas, if heated one degree, will become 492 cubic inches.
If one cup be filled with lead, and another cup of the same size with water, which will weigh the more, the lead or the water?
The lead will weigh more than the water.
Which is the heavier substance, lead or water?
Lead is heavier than water.
Why do we say lead is heavier than water?
Because a cup full of lead will weigh more than the same cup fall of water, or any bulk of lead will weigh more than an equal bulk of water.
Why is cork lighter than water?
Because any bulk of cork is lighter than an equal bulk of water; a cup filled with cork will weigh less than when it is filled with water.
When lead is put in water, why does it sink to the bottom?
Because the lead is heavier than an equal bulk of water.
When cork is put in water, why does it rise to the surface?
Because the cork is lighter than an equal bulk of water.
Does heat increase the weight of any substance?
Heat does not increase the weight of any substance.
How do we know this?
Because a piece of iron when cold will weigh as much as when heated red-hot.
When a liquid is heated, does it become lighter or heavier?
It becomes lighter, bulk for bulk, when heated.
Why does a liquid become lighter when heated?
Because the same liquid is expanded and takes up more room, although it has no more weight.
Why will a gallon of cold water weigh more than a gallon of hot water?
Because a gallon of cold water, when heated, will make more than a gallon of hot water.
When do we get the most molasses, by buying it in hot or in cold weather?
We get the most by buying it in cold weather, because a gallon bought then will make more than a gallon when the weather becomes warmer.
Does heat expand the air and make it lighter?
Heat does expand the air and make it lighter.
What does the air do when heated?
It ascends or rises up, because it is lighter, and the cold or heavier air falls to take its place.
What part of a room, in which there is fire, is the warmest?
The part next to the ceiling is the warmest, because the heated air always ascends.
Why does a soap-bubble ascend?
Because it is filled with heated air from the lungs, and is, therefore, lighter than the air around it.
Why do balloons ascend?
Because they are filled with a gas lighter than the air around them.
Why does a chestnut split open when roasting?
Because the air in it is expanded by the heat, so that it bursts the shell.
Why does the chestnut not split open when a hole is made in the shell?
Because the air can then escape as it expands, without bursting the shell.
What becomes of the air in an apple, when the apple is roasted?
The air, upon being heated, bursts through the peel of the apple, carrying the juice with it.
Why does an apple become soft when roasted?
Because the air in the cells of the apple expands, and breaks those cells, so that the apple becomes soft.
What part of the apple first becomes soft?
The outside, because it is first heated.
Why do all fruits and vegetables become soft when cooked?
Because the heat expands the air in them, and thus breaks up their cells, so that they become soft.
Why does wood make a snapping noise when burning?
Because the air in the cells of the wood bursts them apart, thus making the noise.
Why are sparks of fire thrown out?
Because the heated air bursts the cells with such force as to throw off small pieces of the burning wood.
What kinds of wood snap the most?
The coarse-grained kinds, like chestnut; while the finer-grained kinds, like walnut, seldom snap much.
Why does dry wood snap more than green wood?
Because the sap in the pores of the green wood dries up as the wood becomes dry, and its place is filled with air, so that there is more air in dry wood than in green wood.
Why does smoke rise in a chimney?
Because the air in the chimney is heated by the fire, and as the air rises, it carries the smoke with it.
Why does a chimney smoke when the fire is first kindled?
Because the cold walls of the chimney cool the heated air so rapidly as to prevent its rising to the top, and the smoke then sinks back into the room.
How are houses heated with hot air?
By having the fires made in the cellar, so that the air when heated, may ascend through pipes to the different parts of the building.
Is there any substance that does not contract by cold?
Yes; water, when near freezing, does not contract by cold.
What takes place with the water?
When it is reduced nearly to the freezing point, it begins to expand, and when it freezes, it expands still more.
How much does water expand by freezing?
It expands about one-seventh of its bulk. That is, _seven_ gallons of _water_ will make _eight_ gallons of _ice_.
Does freezing water expand with much force?
Yes; it expands with great force.
Why are vessels containing water often broken during a cold night?
As the water in them freezes, it expands and breaks the vessels.
Why do lead pipes and iron pipes often burst during the winter?
Because the water in them is permitted to freeze, and as there is not room for it to expand, the pipes are burst.
Is ice heavier or lighter than water?
Ice is lighter than the same bulk of water.
How much lighter is ice than water?
Eight gallons of ice weigh no more than seven gallons of water.
Does the ice protect the water beneath it from freezing?
Ice does protect the water from freezing, to a very great extent.
If ice were heavier than water, what would result?
The ice would sink as rapidly as formed, and our streams would, every winter, freeze to the bottom, thus destroying the fish and other creatures living in them. In this, as in many other ways, we may learn the lesson, that in the creation of the world, God wisely made all things to serve some useful purpose.
Conduction of Heat.
Why does iron feel cold to the touch on a cold morning?
Because heat goes from the hand to the iron, thus giving the sensation of cold.
What is the passage of heat from the hand to the iron called?
It is called _conduction_.
When one end of a rod of iron is placed in a fire, does the whole rod become heated?
The whole rod does become heated.
How does this take place?
The heat of the fire passes from one particle to another throughout the rod.
What is this called?
It is called _conduction_.
What then is conduction of heat?
It is the passage of heat from one body to another which it touches, or from one particle of a body to another particle of the same body.
What bodies are good conductors of heat?
Such bodies as gold, platinum, copper, silver, iron, zinc, tin, and lead, in the order named, are the best conductors of heat.
What bodies are poor conductors of heat?
Such bodies as glass, wood, charcoal, wool, hair, and fur.
Fur is the poorest conductor of heat known.
Why does a piece of wood blazing at one end not feel hot at the other end?
Because wood is a poor conductor, and the heat goes slowly through it.
Why does a piece of iron feel cold in winter?
Because the iron is a good conductor, and carries the heat away from our hands very rapidly.
Why does fur feel warmer than iron in the winter?
Because the fur is a poor conductor, and carries the heat away from our hands very slowly.
Why does a wooden pump-handle seem less cold than one of iron?
Because wood is not so good a conductor as iron; hence, it does not carry the heat of our hands away so rapidly.
Why does carpet seem warmer than the bare floor?
Because the carpet is a poorer conductor than the wood of the floor, and does not carry away the heat of our bodies so rapidly.
Why does a stone pavement make our feet cold in winter?
Because the stone is a good conductor, and rapidly carries the heat away from our feet.
Why does heated iron feel hot to us?
Because the iron is a good conductor, and gives off its heat rapidly to our bodies.
Why does a piece of cloth, when heated, not feel hot?
Because the cloth is a poor conductor, and gives off its heat very slowly.
Why will a block of wood, when heated, remain hot longer than a heated brick?
Because wood is a poorer conductor than brick, and does not give off its heat so rapidly.
Why is a block of wood better than iron or brick, to keep our feet warm while on a journey?
Because it does not give off its heat so rapidly, and therefore remains warm longer than iron or brick.
Why should the wood or brick be wrapped in cloth?
Because the cloth, being a poor conductor, helps to keep the heat from passing off so rapidly.
Why does iron feel colder than water when both are at the same temperature?
Because iron is a better conductor than water, and carries away the heat from our hands more rapidly.
Are liquids good conductors of heat?
No; liquids are poor conductors of heat.
Is air a good conductor of heat?
No; air is a poor conductor of heat.
Are gases as good conductors as liquids?
No; gases are poorer conductors than liquids.
Why is water a better conductor than air?
Because the particles of water are closer together than the particles of air.
Why is iron a better conductor than water?
Because the particles of iron are closer together than the particles of water.
How do we know that water is a poor conductor of heat?
Because water may be made to boil at its surface without melting ice a short distance below the surface.
How do we know that air is a poor conductor of heat?
Because the air at the ceiling of a room may be made very warm without melting ice near the floor.
Why does a linen shirt feel cool?
Because linen is a good conductor, and there is but little air among its fibres.
Why is a cotton shirt warmer than one made of linen?
Because the cotton contains more air among its fibres, and air is a poor conductor of heat.
Why is a woollen shirt warmer than one made of cotton?
Because wool contains more air among its fibres, and is, therefore, a poorer conductor of heat.
Why is fur warmer than wool?
Because there is more air among the hairs of the fur, and it is, therefore, a poorer conductor of heat.
What fur is the warmest?
Fur with very fine hairs, like that of the rabbit, because it contains the most air.
Why is fur warmer when the hair is next to our bodies?
Because the fur and the air in it both help to keep the heat of our bodies from passing off rapidly.
Is the earth a good conductor of heat?
No; the earth is a poor conductor of heat.
How do we know that the earth is a poor conductor of heat?
Because the heat of summer warms the earth only a few inches below the surface.
How is it with the earth in winter?
In this latitude, the frost of winter reaches only a few inches below the surface of the earth.
In all parts of the earth, except in the Frigid Zones, the heat of the summer takes away the frost and ice of the winter, and the earth is warmed so that plants may grow.
Why is spring-water generally cool, even in summer?
Because the springs are mostly so far below the surface of the earth, that they are not warmed by the sun’s rays.
Why is snow a warm covering for the earth?
Because the air among the particles of snow prevents the heat of the earth from passing off rapidly.
Why does wrapping straw around a pump in winter, prevent the water in it from freezing?
Because the stalks of straw are hollow, and the air in the stalks makes them poor conductors of heat.
Will straw, wrapped around shrubbery, prevent it from freezing?
It will; because it keeps the heat of the shrubbery from passing off into the air.
Why are rooms warmer from having double windows?
Because the air which is confined between the double windows, being a poor conductor, keeps the warmth of the room from escaping.
Why is the space between the double walls of ice-coolers filled with charcoal?
Because charcoal is a poor conductor, and keeps the heat of the air from the ice.
Why do ice-houses have double walls, with saw-dust or straw between them?
Because the saw-dust and the straw are both poor conductors, and keep the heat of the air from the ice.
Why does ice melt more slowly when wrapped in flannel?
Because the flannel is a poor conductor, and keeps the heat of the air from the ice.
Why are woollen holders used about the stove?
Because wool is a poor conductor, and thus keeps the heat of the iron from burning the hand.
Convection of Heat.
When air near the floor of a room is heated, what becomes of it?
It ascends to the ceiling of the room.
What is done with the cold air at the ceiling?
It descends to the floor, to take the place of the warm air.
When the air ascends to the ceiling, does it carry the heat with it?
Yes; it carries the heat with it.
What is this mode of carrying heat called?
It is called _Convection_.
What is needed so as to heat a substance by convection?
Its particles must be able to move about freely.
Why is the air easily heated by convection?
Because the particles of the air move about freely.
Why is a liquid easily heated by convection?
Because its particles move about freely.
Why cannot a solid be heated by convection?
Because its particles cannot move.
When water near the bottom of a vessel is heated, what becomes of it?
It rises to the surface.
What becomes of the cold water at the surface?
It falls to the bottom of the vessel, to take the place of the warm water.
How long will this circulation continue?
It will continue so long as heat is applied to the bottom of the vessel.
When heat is applied to the surface of the water, what is the result?
The water near the surface only is heated.
Why does it not heat all the water in the vessel?
Because the water when heated, becomes lighter, and stays at the surface, while the cold water is heavier and remains at the bottom of the vessel.
Where must heat be applied to warm a liquid or a gas?
It must be applied to the bottom of the vessel containing the liquid or the gas.
Where must a fire be, so as to warm a room?
The fire must be near the floor of the room.
Why does a hot substance cool quickly in the air?
Because the air touching a substance ascends as soon as heated, and cold air takes its place; as this process goes on rapidly, the substance is quickly cooled.
How is water cooled by convection?
The water at the surface is cooled by giving off its heat to the air; it then falls to the bottom, and the warm water rises to take its place at the surface.
Why does stirring hot coffee cool it quickly?
Because the hot coffee at the bottom of the vessel, is brought more rapidly in contact with the air, and, therefore, gives off its heat more rapidly.
Why does blowing hot coffee cool it quickly?
Because the hot air over the coffee is removed by blowing, and the cold air is brought more rapidly in contact with the coffee.
Liquefaction.
Is ice a solid or a liquid?
Ice is a solid.
When heat is applied to ice, what becomes of the ice?
The ice is changed into water.
Is water a liquid?
Water is a liquid.
How then can a _solid_ be changed to a _liquid_?
By applying _heat_ to a solid, it may be changed to a liquid.
What is meant by liquefaction?
_Liquefaction_ is the changing of a solid to a liquid, by heat.
How may tallow be melted?
By heating the tallow, it may be melted.
Will the tallow become hard again when cold?
It will be hard again when cold.
Can lead and other metals be melted?
They can be melted or changed to a liquid state by heat.
Can all solids be changed to liquids?
No; there are many solids, such as wood, coal, paper, and leather, that cannot be changed to liquids.
How can a liquid be changed to a solid?
By taking heat away from the liquid, it may be changed to a solid.
Why is a solid melted by heat?
Because heat forces the particles of the solid apart, until it is changed to a liquid.
Latent Heat.
Why does warm iron feel warm to the hand?
Because it gives off some of its heat to the hand.
Is the hand sensible of the heat coming from the iron?
The hand is sensible of the heat.
What then may this heat be called?
It may be called _sensible_ heat, because it can be felt.
How do we measure heat?
We measure heat by degrees.
How many degrees of heat in ice when at the melting point?
Thirty-two degrees of heat, as measured by the thermometer.
How can we change ice into water?
By applying heat to the ice it will be changed into water.
How many degrees of heat are necessary to change the ice into water?
One hundred and forty degrees of heat are necessary.
How many degrees of heat are in the water when the ice is all melted?
Only thirty-two degrees of heat, by the thermometer.
Does the water feel any warmer than the ice to the hand?
No; the water does not feel any warmer than the ice.
What has become of the one hundred and forty degrees of heat?
They have been hidden in the water.
What is this hidden heat called?
It is called _latent_ heat.
What is latent heat?
It is heat that is not sensible to the touch, or to the thermometer.
When water is made to boil, what change takes place?
The water is changed into steam.
How many degrees of heat are necessary to change water into steam?
More than nine hundred degrees of heat are necessary.
How many degrees of latent heat in steam?
More than nine hundred degrees of latent heat.
When steam is changed back again to water, what becomes of the latent heat?
It passes off to the air and the surrounding objects, and makes them warmer.
When water is changed into ice, what becomes of the latent heat?
It passes off to the surrounding objects and makes them warmer.
Is freezing a warming process?
Yes; because the water gives off its latent heat while freezing; hence, the surrounding objects are made warmer.
Is thawing a cooling process?
Yes; because the ice absorbs so much heat while thawing that it cools the surrounding objects.
Where does the ice obtain its heat, while thawing?
The ice obtains its heat from the surrounding objects.
Are these objects made cooler by the melting of the ice?
They are made cooler by the melting of the ice.
What effect does salt have upon ice?
Salt makes the ice melt more rapidly.
When salt and ice are placed around a vessel of cream, what takes place?
The salt causes the ice to melt quickly, and the ice in melting takes so much heat from the cream as to freeze it, thus making it ice-cream.
Ebullition.
What is meant by ebullition?
_Ebullition_ means boiling.
When is a liquid in a state of ebullition?
When it has been heated so as to boil.
Can solids be made to boil?
No; liquids only can be made to boil.
What change takes place when a liquid boils?
The liquid is changed into a vapor or gas.
What becomes of water when it is boiled?
Water when boiled passes off into steam.
What is necessary to make water boil?
Heat is necessary to boil water or any other liquid.
How much heat is contained in boiling water?
Two hundred and twelve degrees of heat.
When water passes into steam, how much is its bulk increased?
About seventeen hundred times.
How much steam will a pint of water make?
A pint of water will make about seventeen hundred pints of steam.
What do bubbles of boiling water contain?
They contain steam.
Why are these bubbles formed?
Because the water at the bottom of the vessel is heated and changed into steam, which expands and forms the bubbles.
Why do these bubbles rise to the surface?
Because steam is lighter than water.
When does water simmer?
When water is heated, the steam at first forms in very small bubbles at the bottom of the vessel, and as these bubbles rise through the colder water, the steam in them is condensed to water, so that they break before reaching the surface, thus producing what is called “simmering.”
When does water boil?
Water boils when it becomes heated so that the bubbles rise to the surface of the water before breaking.
Why will a vessel full of water run over when heated?
Because the water is expanded by heat, and some of it must run over.
Why will the water boil over when the vessel is not full?
Because the steam, in trying to escape, throws some of the water over the edge of the vessel.
Vaporization.
When water is boiled, what change does it undergo?
It is changed into a vapor, called steam.
What is this process called?
It is called _Vaporization_.
What is meant by vaporization?
Vaporization is the changing of a liquid into a vapor, by boiling.
What is necessary to change a liquid into a vapor?
Heat is necessary to change a liquid into a vapor.
At what temperature is water changed into steam?
At the temperature of two hundred and twelve degrees.
Are all liquids changed to vapor at the same temperature?
No; they vary very much in the amount of heat required.
Ether boils at 96 degrees, alcohol at 176 degrees, and quicksilver at a temperature of 662 degrees.
When water is changed into steam, what becomes of the steam?
The steam passes off into the air.
What does the air do with the steam?
The air condenses or changes it into very small particles of water, so that it is seen like a fog or a cloud.
Can steam be seen?
No; steam is like the air in this respect, and cannot be seen.
How can we prove this?
When water is boiled in a glass vessel, no steam is visible, although it is there.
Cannot we see the steam coming from the spout of a tea-kettle?
No; we do not see anything for half an inch or more from the spout.
Why does it become visible farther away?
Because the air condenses it, and then we see it in its cloud-like state.
How much steam will one gallon of water make?
One gallon of water will make seventeen hundred gallons of steam.
When this steam is confined in a vessel, what does it try to do?
It tries to expand or spread out in every direction.
Does it expand with much force?
Yes; it expands with so much force as frequently to burst the vessel containing it.
Why is the lid of the tea-kettle lifted up when the water boils?
Because the steam expands and raises the lid, so that it can escape.
If the lid were fastened down, what would result?
The steam, in trying to escape, would burst the tea-kettle.
Why does water run out of the spout of the tea-kettle?
Because the steam in the tea-kettle presses on the water with force enough to cause it to run out.
Why does steam burst vessels in which it is confined?
Because the vessels are not strong enough to keep the steam from expanding.
How is the boiler of a steam-engine kept from bursting?
By means of a safety-valve.
What is a safety-valve?
It is a valve or opening through which steam may escape.
What prevents all of the steam from escaping through the valve?
The valve is kept closed by a weight, which prevents the escape of the steam until a certain quantity of it is formed in the boiler; it then lifts up the valve, and a portion of it escapes, the same as it does with the lid of the tea-kettle.
Evaporation.
Does a wet blanket, when hung in the air, remain wet?
No; it soon becomes dry.
What becomes of the water in the blanket?
The water passes off into the air.
What is this process called?
It is called _Evaporation_.
What is meant by evaporation?
It is the gradual change of a liquid into a vapor.
When water is left in a shallow vessel, what becomes of it?
The water evaporates or passes off into the air.
Do liquids generally evaporate?
Yes; when exposed to the air, nearly all liquids evaporate.
Does water change into steam when it evaporates?
Water does not change into steam when it evaporates.
How, then, does water evaporate?
It passes off into the air in particles or drops so very small as not to be visible.
Is evaporation always going on around us?
Yes; wherever there is water exposed to the air, evaporation is taking place.
Where does the most evaporation take place?
From the surface of the rivers, the lakes, and the ocean.
What becomes of the water when evaporated?
It is formed into fogs and clouds.
In what kind of air does evaporation go on most rapidly?
In air that is warm and dry.
Does evaporation take place in cool air?
It does; because clothing will dry during the coldest weather of winter.
Why does water evaporate more in warm air than in cold air?
Because the warm air can hold more of the fine particles of water, and, therefore, can absorb more.
Why does water evaporate more slowly in damp weather than in dry weather?
Because the damp air contains so much moisture that but little more can pass off into it.
What example have we of this?
Wet clothes dry much more slowly when the air is damp than when it is dry.
Why do clothes dry more rapidly in windy weather?
Because the damp air in contact with them is blown away, and the dry air is constantly taking its place.
Is heat necessary in evaporation?
Heat is necessary to produce evaporation.
Why do our hands feel cold when wet?
Because the water on our hands evaporates, and thus heat is taken from them.
Why is the air cooler after a rain?
Because evaporation then takes place from every wet object, and this absorbs heat from the air.
Why does watering streets and roads cool the air?
Because the water evaporates and absorbs heat from the air, making it cooler.
Why do we feel cold when our clothes are wet?
Because the moisture in them evaporates and absorbs heat from our bodies.
Why is wet land always cold?
Because so much heat is absorbed from the land in the evaporation which is constantly taking place.
What becomes of the dew that collects on plants and other objects at night?
It is evaporated.
How does the sun hasten the evaporation of the dew?
By supplying the heat needed to change the dew into a vapor, the sun hastens its evaporation.
Why do bread and biscuit become hard when kept a few days?
Because the moisture in them passes off into the air, and thus they become dry and hard.
How can they be kept moist and soft for a longer time?
By keeping them in a covered vessel, so that evaporation will take place more slowly, they may be kept moist for a longer time.
Radiation.
When we stand near the fire, why do we feel warm?
Because the fire gives off heat which makes us feel warm.
How does the fire give off heat?
It gives off heat in straight lines, and in every direction.
What is this giving off of heat called?
It is called _Radiation_ of heat.
Does the sun radiate heat?
The sun does radiate or give off both heat and light.
How do heat and light from the sun come to us?
They come together in the sunbeam.
Does heat travel as rapidly as light?
Yes; heat and light travel together at the rate of 200,000 miles in a second of time.
Do all substances radiate heat equally well?
No; some substances are much better radiators than others.
What kind of objects are the best radiators?
Those with rough and dark surfaces.
What kind of objects are the poorest radiators?
Those with smooth and bright surfaces.
Why does water keep hot a long time in a bright tin vessel?
Because the tin is a poor radiator, and prevents the heat of the water from passing off into the air.
Why does water soon cool in a dull earthen vessel?
Because the earthen vessel is a good radiator, and gives off the heat of the water rapidly.
If the bright tin vessel be painted, will the water keep hot so long?
No; the tin becomes a good radiator when painted, and the water is soon cooled.
Why does a stove cool so quickly?
Because the iron is a good radiator, and gives off its heat rapidly.
Do the rays from the sun give off much heat while passing through the air?
The rays from the sun give off about one-fourth of their heat while passing through the air.
What becomes of the remaining heat from the sun?
It is absorbed or reflected at the earth’s surface.
What does the earth do with the heat?
The earth radiates the heat, and thus warms the air.
Reflection.
When a piece of bright tin is held near the fire, does the tin become hot?
No; the tin is scarcely warmed.
What does the tin do with the rays of heat from the fire?
The tin throws them back from its surface.
What is this called?
It is called _Reflection_ of heat.
Do all substances reflect heat equally well?
No; some reflect much more heat than others.
What kind of objects reflect heat the best?
Those with bright and smooth surfaces.
What kind of objects reflect heat the least?
Those with rough and dark surfaces.
Does bright tin reflect heat better than glass?
Yes; bright tin reflects about eight times as much heat as glass does.
Why does it take so long to heat water in a new tin cup?
Because the bright tin is a good reflector, and throws off the greater part of the rays of heat that come to it from the fire.
Why does a piece of tin or other bright metal, placed under a stove, protect the carpet from burning?
Because the tin being a good reflector, remains cool, and cannot burn the carpet under it.
Why will not a piece of iron protect the carpet from burning?
Because the iron, being a poor reflector, soon becomes heated, so as to burn the carpet under it.
Absorption.
When a piece of iron is held near a fire, does the iron become hot?
Yes; the iron soon becomes hot.
How is the iron heated?
The iron absorbs or takes in the heat from the fire.
What is this power to take in the heat called?
It is called _Absorption_ of heat.
What is this process like?
It is somewhat like a sponge, that absorbs or sucks up moisture.
Do all substances absorb heat equally well?
No; some substances absorb more heat than others.
What kind of objects absorb heat the best?
Those with rough surfaces absorb heat the best.
What kind of objects absorb heat the least?
Those with bright and smooth surfaces.
What colored objects absorb heat the best?
Objects of a dark or black color.
Do objects that absorb heat well, radiate well?
Yes; good absorbers are good radiators.
Why is this?
Because every substance must absorb heat before it can have heat to give off.
Do objects that absorb heat well, reflect it well?
No; objects that absorb heat well, cannot reflect it well.
Why is this?
Because a ray of heat cannot be reflected from an object, and be absorbed by that object, both at the same time.
Why is water rapidly heated in a tea-kettle?
Because the rough, dark iron of the tea-kettle absorbs the heat from the fire, and gives it off rapidly to the water contained in the kettle. The iron is also a good conductor of heat; therefore, the heat from the fire passes rapidly through it to the water.
Why does black cloth, when spread on the snow, cause the snow to melt more rapidly?
Because the cloth absorbs heat from the rays of the sun, and, therefore, melts the snow beneath it.
Why does not white muslin, when spread on the snow, hasten its melting?
Because the white muslin reflects the heat from the rays of the sun, so that but little of it enters the snow.
Does the snow receive much heat from the sun?
Snow is a good reflector, and throws back most of the heat from the sun.
What causes the snow to melt?
The heat it receives from touching the earth, causes the most of it to melt.
Why does snow melt first on dark earth?
Because dark earth is a good radiator, and gives off much heat to the snow.
Why does snow remain longest on light-colored earth?
Because the light-colored earth is a poor radiator, and gives off but little heat to the snow.
Why does the air feel cold in winter time?
Because it is so much cooler than our bodies that it absorbs heat from them rapidly, and thus makes them feel cold.
Why does the air feel warm in summer time?
Because it is then nearly as warm as our bodies, and, therefore, absorbs heat from them slowly.
Why are greasy shoes warmer than those polished with blacking?
Because the greasy shoes absorb heat from the sun, but the polished shoes throw off the heat of the sun by reflection.
What may we remember about heat?
And God said, Let there be light; and there was light. But along with the light, came its companion—heat; for when the sunbeam first came forth at His command, the twin sisters, light and heat, together touched the earth, causing it to bloom with life and beauty. And the earth brought forth grass, the herb yielded its seed, and the tree its fruit: And God saw that it was all good.