FIRST LESSONS
IN
Natural Philosophy
FOR BEGINNERS.
BY
JOSEPH C. MARTINDALE, M.D.,
LATE PRINCIPAL OF MADISON GRAMMAR SCHOOL, PHILADELPHIA; AUTHOR OF A HISTORY OF THE UNITED STATES FOR SCHOOLS, ETC.
PHILADELPHIA:
ELDREDGE & BROTHER,
No. 17 North Seventh Street.
1881.
Entered, according to Act of Congress, in the year 1872, by
ELDREDGE & BROTHER,
in the Office of the Librarian of Congress at Washington.
J. FAGAN A SON,
ELECTROTYPERS, PHILAD’A.
KEYSTONE PRESS OF WM. J. DORNAN
Preface.
In preparing this little work, our desire has been to afford both pleasure and profit to those who may read or study its pages.
The most interesting, as well as the most useful things to us, are those with which we meet in every-day life; but we must have some knowledge of these things, ere we can contemplate them with much pleasure, or use them to our greatest profit.
Simple truths in natural science may be learned at an early age; for it requires no more than the ordinary intelligence possessed by boys and girls to understand them. Such facts are not only valuable in themselves, but they are also useful in disciplining the mind by teaching it to observe the many changes constantly going on around us, and to reason and reflect in regard to the same. Habits of observation and reflection thus formed are likely to go with us through life; and they may prove a blessing in a thousand ways; for they will not only add to our comfort and our enjoyment, but they will also produce many a pleasing thought.
We have endeavored to make a book that can be easily understood; but few scientific terms will be found in it, for its language is the language in familiar use; hence, it may serve to interest the fireside circle, and it may be referred to by intelligent boys and girls for answers to the many perplexing questions which are so often presented to their inquiring minds.
Many pupils have not the chance to attend even an Academy or a High School. It is to such that this book is especially adapted, for it is believed to contain as much matter as can be taught with satisfaction and profit in our public or our private schools; but to other pupils, it will prove a valuable introduction to larger and more comprehensive works.
Whenever a drawing would illustrate a fact, and make its application plainer, it has been made; but by far the best illustrations of hundreds of facts, explained in the text, will be found in the world around us, and the attention of both pupil and teacher should always be directed to this reliable and never-ending source.
In presenting these Familiar Lessons in Natural Philosophy to the youth of our country, it is with the earnest hope that it may make them better acquainted with the natural objects surrounding us; that it may lead them to observe the order, the harmony, and the beauty in nature everywhere; and that it may help to turn their thoughts more and more to Him, who is the Creator and Ruler of all things.
J. C. M.
CONTENTS
PAGE [CHAPTER I.—Astronomy.] 9 [The Earth and other Planets] 9 [The Moon] 14 [Eclipses] 18 [CHAPTER II.—Light.] 22 [Color] 22 [Refraction of Light] 32 [Reflection of Light] 34 [The Eye] 38 [CHAPTER III.—Heat.] 42 [Sources of Heat] 42 [Expansion by Heat] 46 [Conduction of Heat] 54 [Convection of Heat] 59 [Liquefaction] 61 [Latent Heat] 63 [Ebullition] 65 [Vaporization] 66 [Evaporation] 69 [Radiation] 71 [Reflection] 73 [Absorption] 74 [CHAPTER IV.—The Air.] 77 [Winds] 96 [CHAPTER V.—Water.] 99 [Springs and Streams] 104 [Fogs and Clouds] 106 [Dew] 109 [Frost] 112 [Rain, Snow, and Hail] 113 [Ice] 116 [CHAPTER VI.—Sound.] 118 [Conduction of Sound] 122 [Trumpets and Speaking-tubes] 125 [Velocity of Sound] 127 [Reflection of Sound] 128 [CHAPTER VII.—Matter.] 131 [CHAPTER VIII.—Attraction.] 137 [Cohesion and Adhesion] 137 [Capillary Attraction] 140 [Gravitation] 141 [Centre of Gravity] 145 [Specific Gravity] 150 [CHAPTER IX.—Motion.] 154 [CHAPTER X.—Mechanical Powers.] 164 [Lever] 164 [Wheel and Axle] 174 [Pulley] 176 [Inclined Plane] 177 [Wedge] 180 [Screw] 181 [CHAPTER XI.—Electricity.] 183 [Magnetism] 187 [Electro-Magnetism] 189
FIRST LESSONS
IN
Natural Philosophy.
CHAPTER I.
Astronomy.
The Earth and Other Planets.
Fig. 1.
What is the shape of the Earth on which we live?
It is round like a ball.
What other names are given to the Earth?
It is called the World or Globe.
What else is it sometimes called?
It is called a Planet.
Are there any other planets?
Yes; some of the stars are called planets.
Is our earth or world a star?
It is a star.
Why do the other stars seem to us different from the earth?
Because they are so far away from us.
What gives light to the planets?
The Sun gives light to the planets.
Of what is the sun the centre?
The sun is the centre of the solar system.
Do the planets remain in one place?
The planets do not remain in one place, but are always moving.
How do the planets move?
They move around the sun.
Are some planets nearer the sun than others?
Yes; some planets are much nearer the sun than others.
Which planet is nearest the sun?
The planet called Mercury.
Which is next to Mercury?
The planet called Venus.
And which is third in distance from the sun?
Our own planet, the Earth.
Are there any planets still farther from the sun?
Yes; Mars, Jupiter, Saturn, Uranus, and Neptune, and many smaller planets.
How does each planet move?
It moves in its own track or orbit around the sun, as is represented in [Fig. 2].
Fig. 2.
Does the earth always move in the same orbit around the sun?
Yes; the earth moves in the same orbit around the sun, year after year.
How long does it take the earth to move around the sun?
365 days, 5 hours and 49 minutes, or one year.
What makes a year to us?
A year to us is the time required by the earth to move around the sun.
If the earth is in a certain place at noon, on New Year’s day, when will it be in that place again?
5 hours and 49 minutes after noon on New Year’s day of the next year.
What, then, does the earth do every year?
The earth moves around the sun and comes back to the place whence it started, once every year.
Has the earth any other motion than the one around the sun?
It has; it turns over and over.
Does the earth stay in one place as it turns over and over?
It does not; it moves on in its track around the sun.
How many times does it turn over while going around the sun?
Three hundred and sixty-five times.
How many times, then, does it turn over in one year?
The earth turns over three hundred and sixty-five times in a year.
What is the time which it takes to turn over once called?
It is called a day.
How many days in a year?
There are three hundred and sixty-five days in a year.
What two motions, then, has the earth?
It turns over and over, and at the same time it moves around the sun.
What change is produced on the earth by its moving around the sun?
The change of Seasons.
Will you name the seasons?
Winter, Spring, Summer, and Autumn.
Do they follow each other in regular order?
They do; Spring follows Winter, Summer follows Spring, Autumn follows Summer, Winter follows Autumn, and Spring follows Winter again.
How often have we these seasons?
As often as the earth moves around the sun, which is once a year.
What change is produced on the earth by its turning over?
The change of day and night.
Where is it day?
It is day on that part of the earth which is turned towards the sun.
Where is it night?
It is night on that part of the earth which is turned away from the sun.
How often do we have the change from day to night?
As often as the earth turns over, which is once in twenty-four hours.
What is a day?
A day is the time from sunrise till sunrise again, from sunset till sunset again, from noon till noon again, or from midnight till midnight again.
What does a day include?
It includes the night-time as well as the day-time, of the twenty-four hours.
When it is day-time on one part of the earth, what is it on the opposite part?
It is night-time on the opposite part.
When it is morning to us, what is it to people living on the opposite side of the earth?
When it is morning to us, it is evening to them; when it is day-time to us, it is night-time to them; and when it is noon to us, it is midnight to them.
On how much of the earth is it always day?
It is day on one-half of the earth, while on the other half it is night.
The sun seems to move around the earth; does it really do so?
No; it does not move around the earth.
Does the earth move?
Yes; it turns over once every day.
Why does it not seem to us to be moving?
Because we move along with it. If we are on a railroad car, the trees and houses along the road appear to move, and the car to stand still; but the car really moves, while the trees and houses stand still.
In what direction does the earth turn on its axis?
The earth turns from west to east.
Where do the sun, moon, and stars rise?
The sun, moon, and stars rise in the east.
The teacher may, by means of a ball or small globe, readily make these things so plain that they can be understood even by small children.
When is it sunrise to us?
When the part of the earth on which we are, first comes into the sun’s rays.
When is it sunset to us?
When the part of the earth on which we are, moves out of the sun’s rays.
The Moon.
We have been told that the earth moves around the sun; does any planet move around the earth?
Yes; one planet moves around the earth.
What is that planet called?
It is called the Moon.
Fig. 3.
How often does the moon move around the earth?
Once in 27 d. 7 h. 43 min.
How long is the time from new moon to new moon again?
It is 29 d. 12 h. 44 min.
How often do we have new moon?
Once every 29 d. 12 h. 44 min.
What is the period from one new moon to another called?
It is called a Lunar month.
Are the lunar months all of the same length?
Yes; they are all of the same length.
Is there any other kind of month?
Yes; the Calendar month, as it stands in almanacs.
How many calendar months in a year?
There are twelve calendar months in a year.
Are the calendar months all of the same length?
No; some are thirty-one days long, some thirty days long, and one only twenty-eight days long.
When can we see the moon and stars best?
We can see them best when it is night.
From what do the moon and the other planets get their light?
They get their light from the sun.
What do they do with this light?
They reflect it.
How do we see them?
We see them by the light which they reflect.
Why cannot we see the moon and stars in the day-time?
Because the bright light of the sun hides their light.
Are there stars in every direction around the earth?
There are stars in every direction around the earth.
How do we know this?
Because stars may be seen from every part of the earth, when it is night.
Stars are in every direction around the earth, just as we may imagine the apples to be in every direction around a boy who has climbed into a tree full of that fruit.
What planet moves around the earth?
The moon moves around the earth.
Upon what part of the moon does the sun shine?
It shines upon the part towards the sun.
What does the moon do with the light which it receives from the sun?
The moon reflects the light received from the sun.
What is this reflected light called?
It is called moonlight when reflected from the moon.
Is that part of the moon on which the sun shines always towards the earth?
It is not always towards the earth.
When all of the moon on which the sun shines is towards the earth, how does the moon appear?
The moon appears to be round, and the whole of it seems to shine.
What is the moon then called?
It is called the Full Moon.
When no part of the moon on which the sun shines is towards the earth, how does the moon appear?
We do not see the moon at all, and we say there is no moon.
When a small part of the moon on which the sun shines is first turned towards the earth, what is it called?
It is called the New Moon.
Does the lighted part of the moon grow?
It seems to grow larger and larger, until the whole of the moon is lighted.
What is it then called?
It is then called the Full Moon.
What then takes place?
It seems to grow smaller and smaller, until the whole of the lighted part is gone.
Just before the last part is gone, what is it called?
It is called the Old Moon.
How long is it from one full moon to another full moon?
It is a lunar month.
How many full moons are there in a year?
There are thirteen full moons in a year.
Is the sun much larger than the moon?
The sun is many thousand times larger than the moon.
Why does the moon appear nearly as large as the sun?
Because it is so much nearer to us.
How far is the moon from the earth?
The moon is nearly 240,000 miles from the earth.
How far is the sun from the earth?
The sun is nearly 92,000,000 miles from the earth.
How long does it take the light of the sun to reach the earth?
It takes about eight minutes; therefore, the light of the sun must travel about twelve millions of miles in a minute, or two hundred thousand miles in a second of time. At this rate, the light would come from the moon to the earth in a little more than one second of time. It would take no longer for it to come than it does for the pulse to beat once, or the clock to make one tick.
Eclipses.
How do we know that the earth is round, like a ball?
We know that the earth is round like a ball, because its shadow is similar to the shadow of a ball.
How else do we know that the earth is round?
We also know that the earth is round, because we can circumnavigate or sail around it.
Where can we see the shadow of the earth?
We can see the shadow on the moon, when the moon is eclipsed.
What is the shadow on the moon like?
It is round, like the shadow of a ball.
When the Earth comes between the sun and the moon, where does the shadow of the earth fall?
The shadow of the earth falls on the moon, as is represented in [Fig. 4].
Fig. 4.
What is this shadow on the moon called?
It is called an Eclipse of the moon.
How do we know that the moon is round?
We know that the moon is round, because its shadow is similar to the shadow of the earth.
When is the moon eclipsed?
When it comes into the earth’s shadow.
When the moon comes fully into the earth’s shadow, what is it called?
It is called a total eclipse of the moon.
When it comes partly into the earths shadow, what is it called?
It is called a partial eclipse of the moon.
When the moon comes between the sun and the earth, where does the shadow of the moon fall?
The shadow of the moon falls on the earth, as is represented in [Fig. 5].
Fig. 5.
What does this produce?
It produces an eclipse of the sun.
When the moon is between the sun and the earth, can we see the sun?
No; we cannot see the sun, because the moon prevents the rays of the sun from coming to us.
When the moon prevents the sun’s rays from falling on us, what is it called?
It is called an eclipse of the sun.
When the moon prevents all the sun’s rays from falling on us, what is it called?
It is called a total eclipse of the sun.
When the moon prevents only a part of the sun’s rays from falling on us, what is it called?
It is called a partial eclipse of the sun.
When will the sun be eclipsed?
Whenever the moon comes between the sun and the earth.
When will the moon be eclipsed?
Whenever the earth comes between the sun and the moon.
Who made the sun, the moon, and the stars?
God, the creator of all things, made the sun, the moon, and the stars. He placed them in the heavens, where they remain in obedience to His will. He made the bright rays from the sun to light up the day, and give beauty to the world; but the feebler rays of the moon and the stars are seen best at night, when the sun is hid.
CHAPTER II.
Light.
Color.
In what way is the sun useful to us?
It gives heat, which makes us warm, and it gives light, so that we can see.
Is it useful in any other way?
It gives light and warmth to all animals as well as to all plants.
What make the flowers and leaves so beautiful?
The light and heat from the sun make them beautiful.
What do the light and heat from the sun ripen?
They ripen the apples, cherries, and other fruits; also, the wheat, corn, and other grains, and make them fit for food.
When it is night-time or dark, can we distinguish objects?
No; because all things are then without color, and they cannot be seen.
As it grows light in the morning, what takes place?
At first, we see the objects around us faintly; but as it grows lighter, we see them more distinctly.
Do they all show the same color in the light?
They do not; they show different colors.
What enables us to see the color of different things?
Light enables us to see the color of everything.
What kinds of light have we?
We have the light of the sun, moon, and stars; also, the light from the burning of any substance.
Does the light show many colors?
It does show many colors or shades of color.
How may some colors be made?
They may be made by mixing other colors together.
How can we make a purple color?
By mixing a red and a blue color together.
How can a green color be made?
By mixing a blue and a yellow color together.
How can an orange color be made?
By mixing a red and a yellow color together.
How does the light come from the sun?
It comes in rays or straight lines.
What may be done with a ray of light?
It may be separated into different colors.
Into how many colors may a ray of light be separated?
Into seven colors, like the colors of the rainbow.
What names are given to these colors?
Red, orange, yellow, green, blue, indigo, violet.
How may these colors be separated?
By a prism, or three-sided piece of glass.
How may this be done?
If a sunbeam, shining through a hole in the window-shutter of a dark room, should fall upon a prism, it will be broken up as it passes through the prism, and be shown on the opposite wall, in the seven rainbow colors.
Fig. 6.
In [Fig. 6], D is a sunbeam passing through a shutter. When it passes through the prism, at E, it is separated into the seven rainbow colors, as is shown on the opposite wall.
In what order will these colors appear?
They will always appear one above the other, in the order named.
How may this order be remembered?
By the word Roy-g-biv, made from their initial letters, beginning at the bottom.
From what are the colors of the rainbow made?
From the rays of light coming from the sun.
When is a rainbow seen?
A rainbow is seen when the sun shines through the drops of water as they fall through the air.
How do the drops of water help to make a rainbow?
The drops of water act like little prisms, and separate the rays of light passing through them into the colors of the rainbow.
What two things, then, are needed to make a rainbow?
A shower of water, and a bright sunshine.
In what part of the sky does the rainbow appear?
In the part opposite to that in which the sun is, at the time of the shower.
How does the rainbow appear?
It appears like a great arch, spanning the sky, and its splendid bright colors are very beautiful.
What may we remember in reference to the rainbow?
We may remember that God set his “bow” in the cloud, as a token that the world should no more be destroyed by a flood of waters.
Where else may we see a variety of colors?
In the soap-bubble.
Why are these colors constantly changing?
Because the walls of the bubble are constantly growing thinner, and different colors are, therefore, reflected from them.
Why do the walls of the soap-bubble grow thinner?
Because the water runs to the bottom of the bubble until its top becomes so thin as to burst.
Can we tell the color of any object by touching it?
No; we must see it in order to tell its color.
How do we know this?
If there are two coats, one blue and the other black, we cannot tell which is the blue one or which is the black one, by feeling them.
What is necessary in order to see the colors?
Light is necessary; because all colors come from the rays of light.
Can we see the different colors in the ray itself?
We cannot; because God has so blended them together that they cannot be seen by us.
When most of the rays of light pass through a substance, what is said of it?
The substance is said to be transparent.
Name some transparent substances.
Glass, ice, diamonds, air, and clear water.
When only a few rays of light pass through a substance, what is said of it?
The substance is said to be translucent.
Name some translucent substances.
Flint, isinglass, scraped-horn, and china-ware.
When no rays of light pass through a substance, what is said of it?
The substance is said to be opaque.
Name some opaque substances.
Wood, iron, coal, and granite.
Can we see through a glass window?
Yes; because the glass is transparent, and the light passes through it.
Can we see through a looking-glass?
No; because the back of the looking-glass is covered with quicksilver, which prevents the rays of light from passing through it.
What becomes of the light falling on the looking-glass?
It is thrown back from the glass.
What is this called?
It is called reflection.
What objects are good reflectors of light?
Those having smooth and polished surfaces; such as tin, silver, gold, and quicksilver.
What objects are poor reflectors of light?
Those having dull, uneven surfaces; such as iron, wood, cloth, leather, and calico.
Of what is every ray of sunlight composed?
It is composed of the seven colors of the rainbow.
What does a looking-glass do with these colors?
A looking-glass throws them all back from its surface.
What does a glass window do with these colors?
A glass window permits all these colors to pass through it.
What do opaque substances do with these colors?
They throw back or reflect some of them.
Do they all reflect the same color?
No; some reflect one color, and some another color.
What color does the grass reflect?
It reflects the green color; therefore, the grass is green.
What does the grass do with the other colors?
It absorbs or hides them in itself.
How do we know the color of anything?
We know it by the color which it reflects.
Why is the rose red?
Because it reflects the red color, and absorbs or hides the other colors in itself.
When anything absorbs all the colors of a ray of light, what color is it?
It is black.
When it reflects all the colors of light, what color is it?
It is white.
Why are the letters on this page black?
Because they absorb all the colors of light, and reflect none.
Why is the paper white?
Because it reflects all the colors of light, and absorbs none.
Why are all things black in the dark?
Because there are no rays of light falling on them; hence, there is no color for them to reflect.
Does an object reflect its color in every direction?
It does reflect its color in every direction.
How do we know this?
Because thousands of persons, in different directions from the same object, can see it at the same time.
How are the rays of color reflected from an object?
They are reflected in straight lines.
Where must a person be so as to see an object?
He must be in a position so that there is nothing to prevent the rays of light passing in a straight line from the object to his eyes.
What color does a red ribbon reflect?
It reflects a red color.
Why are some red ribbons of a brighter red than others?
When they reflect a great many red rays, they are of a bright red color; but when they reflect only a few red rays, they are of a dull red color.
Upon what does the brightness or dullness of a color depend?
It depends upon the number of rays of that color reflected.
What variety does this produce?
It produces a great variety in the shades and tints of color.
Where may many of these shades and tints be seen?
In the pretty flowers that grow in the woods, fields, and gardens; in the beautiful tints of the leaves in autumn; and in the plumage of the birds that visit us every spring, and cheer us with their songs.
What may these things teach us?
They may teach us the goodness of God, in giving us a world so full of beauty; and we should feel ever thankful to Him for the great pleasure that we have in seeing them.
Why cannot we see the light of a lamp or candle distinctly, in the day-time?
Because the strong rays from the sun hide the more feeble rays from the lamp.
Why cannot we see the stars in the day-time?
Because the bright light from the sun hides their more feeble light.
When can we see the glow-worms or fire-flies best?
At night, when it is dark; because, at other times, the light of the sun hides their feeble light.
Where may we see them at night?
Flying about in all directions, looking like bits of fire moving in the air.
Does a looking-glass reflect all the colors of light falling upon it?
It does; hence, the ray of light is not changed.
When the object is of a blue color, why is blue only reflected?
Because it is the only color falling upon the glass, and there is no other color to reflect.
Is the color of any object changed by falling upon a looking-glass?
No; the color of every object is reflected from a looking-glass without change.
Do other objects generally reflect the colors falling upon them?
They do not; they absorb or hide the color of all other objects, and reflect only their own color.
If every object was a good reflector of color, like a looking-glass, what would result?
Then every object would reflect the color of every other object around it, and there would be an endless confusion of colors.
What objects reflect the most light?
Those of a white or light color.
What objects reflect the least light?
Those of a darker color; and the nearer the color approaches to black, the less light it reflects.
Why is it darker when the sky is covered by dark-colored, than by light-colored, clouds?
Because the dark-colored clouds reflect less light, than the light-colored clouds do.
What advantage do we derive from the green color of the grass and the leaves?
They reflect a soft, pleasant light, which does not dazzle or hurt our eyes.
How would it be if those objects were white?
They would dazzle us by the amount of light reflected.
If we wish to make a room light, of what color should the walls be?
They should be white; and when papered, it should be with light-colored paper.
Why does dark-colored paper darken a room?
Because it absorbs some of the rays of light which come in at the windows; hence, there is less light in the room. Dark-colored carpets, or dark furniture, darken the room for the same reason.
Does the light from a burning body differ from sun-light?
Yes; it differs in intensity, in color, and in many other ways.
What effect does the lamp-light have upon the color of some substances?
It changes their color.
Where may we find an example of this?
In trying to match the colors of thread or silk at night.
Why is the color different by lamp-light from what it is by daylight?
Because the yellow color of the lamp’s rays acts upon the color of the silk, forming a third color. Blue silk becomes green by lamp-light, and red silk becomes orange.
Are all persons able to distinguish one color from another?
No; some are color-blind. To such persons all colors appear to be alike.
What curious instances of color-blindness may be mentioned?
Dr. Mitchell tells of an officer who bought a blue coat and a red waistcoat, thinking them to be of the same color. He also tells of a tailor who patched a black waistcoat with a piece of crimson; and of another, who put a red collar on a blue coat. Dr. Dalton lost a piece of red sealing-wax in the grass, and he could not find it because it and the grass seemed of the same color to him.
Refraction of Light.
How do the rays of light from the sun come to us?
They come to us in straight lines.
When the rays of light pass from one substance to another more dense, what takes place?
They are bent or turned from a straight course.
What is said of these bent rays?
They are said to be refracted.
What is meant by refraction?
Bending a ray of light, as it passes from one substance to another of different density; as from air to water, or from water to air.
Where may we see an example of refraction?
In the rays of light from a rod which has one end in the water.
Fig. 7.
Why does the rod appear to be bent?
Because light coming from the part of the rod under the water, is refracted or bent at the surface of the water, while the light from the part above the surface, comes in straight lines to the eye; hence, the rod appears to be bent at the surface, as is represented in [Fig. 7].
How does a pole appear when one end is placed in the water?
It appears to be bent where it meets the surface of the water.
How does the part of the rod under water appear?
The part under the water appears to be raised up, as may be seen in [Fig. 7].
How does the bottom of a pond or stream of water appear?
It appears to be raised up, and the water does not seem so deep as it really is.
Why is this the case?
Because the light from the bottom of the pond is refracted or bent at the surface of the water; hence, the bottom appears to be raised up.
How much does the bottom of a pond or stream of water seem to be raised up?
About one-third of the depth of the water.
If the water is really six feet deep, how deep does it appear?
It appears to be only four feet deep.
What has frequently happened from not knowing this fact?
People have been drowned from getting into water deeper than it appeared to be.
Do the fish and other things floating in the water appear to be raised up?
Yes; like the bottom of the stream, they also appear to be only two-thirds as far from the surface as they really are.
If we wish to spear the fish, what must we do?
We must aim below the place where they seem to be, or we must strike perpendicularly at them.
Reflection of Light.
When a ball is thrown against a wall, what is the path through which it goes called?
The line of incidence.
When the ball bounds back, what is its path called?
The line of reflection.
What is the angle at which a ball strikes the wall called?
The angle of incidence.
What is the angle at which the ball rebounds called?
The angle of reflection.
When a ray of light falls upon a looking-glass, what is the path through which it goes called?
The line of incidence.
When the ray is reflected, what is its path called?
The line of reflection.
The angle of incidence is the angle between a perpendicular and the line of incidence; and the angle of reflection is the angle between the perpendicular and the line of reflection. The two angles are always equal to each other, as is represented in [Fig. 8].
Fig. 8.
When a ray of light strikes a looking-glass obliquely or slantingly, what is done with it?
The ray is reflected as obliquely from the glass as it strikes or falls upon it.
Why is the reflection of the sun in a pond of water seen near the edge at noon?
Because the angle of reflection is equal to the angle of incidence; and the observer must be nearly over the place where the rays strike, so that, when reflected, they may meet his eye.
Where is the reflection seen in the morning or in the evening?
It is seen nearer the middle of the pond; because the sun’s rays then fell more obliquely on the pond, and are reflected more obliquely to the eye.
When an opaque object is placed between a candle and a wall, why will there be a shadow?
Because the opaque object prevents the rays of light from falling on the wall.
Why will the shadow be larger, the nearer the object is to the candle?
Because the rays of light diverge, in every direction from the candle, in straight lines, and the nearer the object, the more rays will it intercept, as is represented in [Fig. 9].
Fig. 9.
In [Fig. 9], let L represent the flame of a candle. When the object is placed at A, the shadow on the wall will extend from C to D; but when the object is moved nearer, as at B, the shadow will extend from E to F.
Why does an object seem to be smaller, the farther it is from us?
Because the angle at which the light from it strikes the eye, is less when at a distance, than when near to the eye.
Fig. 10.
In [Fig. 10], the diameter of the near circle is from A to B; but the diameter of the more distant circle, although just as large, seems only to be from C to D.
Why does an object grow more dim the farther it is from us?
Because the rays of light from it spread out in every direction; and fewer rays enter the eye when it is at a distance than when it is near by.
What is the use of the telescope?
It is used to view distant objects.
Why can we see a distant object more distinctly with a telescope, than without one?
Because the telescope enlarges the image of the object, and it also collects more light from that object than is collected by the unaided eye; thus, making the image distinctly visible.
What class of objects do we view with a telescope?
We view the heavenly bodies; the sun, moon, and stars.
What is a spy-glass?
It is a glass mostly used to view distant objects on the earth.
Is the spy-glass like the telescope?
The spy-glass is much like the telescope, only it is smaller.
When looking through a spy-glass, how do objects appear?
They seem to be brought nearer to us, and they appear larger.
By whom are spy-glasses much used?
Spy-glasses are much used by persons on board of vessels, while at sea.
What instruments have we for magnifying small objects?
We have magnifying glasses and microscopes.
What are magnifying glasses?
They are single glasses or lenses, used for magnifying small objects.
By whom are magnifying glasses used?
They are used by jewellers, watchmakers, engravers, and others who examine small objects.
What are microscopes?
They are instruments used for examining very small objects.
How does a drop of rain-water or vinegar appear under the microscope?
It appears to be full of strange-looking creatures, which are always in motion.
How do the smallest insects appear?
They appear to be as perfectly formed as the larger ones which are seen with the naked eye.
What may be seen with a microscope?
Thousands of things too small to be seen by us unless they are magnified.
What kind of spectacles do near-sighted people use?
Near-sighted people use glasses, thickest at the edge and thinnest at the centre.
What kind of spectacles do we use as we grow old?
As we grow old we use glasses, thinnest at the edge and thickest at the centre.
Of what use are spectacles?
When the vision is defective, spectacles enable us to see objects more clearly.
How should spectacles be made?
They should be so made as to render objects distinct, but neither to magnify nor diminish them.
As people grow old, how do their eyes change?
Their eyes change so that they see objects at a distance better than those which are nearer, hence, in reading they hold a book farther from the eye.
The Eye.
What is that part of the eye called, which is blue, gray, or brown, in different persons?
It is called the iris.
What is the pupil of the eye?
The pupil is the black spot or hole in the centre of the iris.
Fig. 11.
What is the use of the pupil?
The pupil is the window of the eye, and the rays of light which enter the eye must pass through it.
What power has the iris over the pupil?
The iris has the power of making the pupil larger or smaller, according to the amount of light which it receives.
How does a bright light affect the iris?
It causes the iris to contract, so that the pupil becomes smaller.
How does a faint light affect the iris?
It causes the iris to dilate, so that the pupil becomes larger.
Of what advantage is the iris to the eye?
The iris acts like a sentinel to protect the eye from any sudden light.
How does a sudden light affect the eye?
A sudden light causes pain to the eye.
Of what shape is the pupil of the eye?
In man, the pupil of the eye is circular.
What is the size of the pupil of the eye?
The pupil varies from the one-twentieth to the one-third of an inch in diameter, depending upon the brightness of the light entering the eye.
How else are our eyes protected from any sudden light?
By means of our eyelids, which may be closed, and thus shut out all the light.
When we pass from a well-lighted room into the open air, at night, why does it seem darker at first than it does afterwards?
Because the pupil is contracted at first; but it soon dilates, and allows more rays of light to pass into the eye, so that the night seems less dark.
Why does the pupil become larger in the twilight?
So that more rays of light may enter the eye, whereby objects may be more distinctly seen.
Why can an owl or a bat see at night?
Because the pupil of the eye in them is quite large, and admits much light.
Why do they stay in dark places through the day?
Because the bright light of day hurts their eyes.
May the image of an object be retained in the eye after the eyelids are closed?
It may for a very short time, as any one can prove by first looking at some bright object, and then closing the eyelids.
Why does a burning coal moved rapidly around, seem like a circle of fire?
Because the light from it is retained a short time by the eye, thus seeming to form a complete circle.
Why cannot we count the posts in a fence, when riding rapidly in a car?
Because the light from each post falls in such quick succession upon the eye, that it cannot distinguish one post from another.
Can objects be seen distinctly when placed near the eye?
No; when objects are within six inches of the eye they cannot be seen distinctly.
Can objects be seen when at a great distance from the eye?
Yes; objects may be seen even when millions of miles away from us; thus, the sun is seen although ninety-five millions of miles from the earth, and some of the stars which we behold in the heavens, are still farther away than the sun.
Of what use is the eye to us?
The eye gives us ideas of the size, the shape, the color, the place, the distance, and the movements of things around us, so that we can use them the better to promote our comfort and our happiness.
What may we remember about the light?
And God said, “Let there be light;” and there was light. Thus the day was separated from the night, and the sun was made to send forth his silvery rays upon hill and valley, field and forest, causing the rarest buds and most beautiful blossoms to come forth from the lifeless earth, although no eye, save the All-Seeing One, was there to behold them!
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.