SOLIDS, LIQUIDS, AND GASES
Arrange a collection of objects of various shapes, sizes, colours, and weights, as cork, glass, lead, iron, copper, stone, coal, chalk. Show that these are alike in one respect, namely, that they have a shape not easily changed, that is, they are solids. Compare these solids with such substances as water, alcohol, oil, molasses, mercury, milk, tar, honey, glycerine, gasolene. These latter will pour, and depend for their shape on the containing vessel. They are liquids. Compare air with solids and liquids. Such a material as air is called a gas. Other examples of illuminating gas, and dentists' "gas"; others will be studied in future lessons. Pupils may think all gases are invisible. To show that some are not, put a few pieces of copper in a test-tube or tumbler and add a little nitric acid. Watch the brown gas fall through the air; note how it spreads in all directions. Some gases fall because they are heavier than air; others rise because lighter. All gases spread out as soon as liberated and try to fill all the available space. Spill a little ammonia and note how soon the odour of the gas is smelled in all parts of the room.
CHANGE OF STATE
Heat some lead or solder in a spoon till liquid. Let it cool. Do the same with wax.
Heat some water in a flask till it becomes steam. Steam is a gas. Cool the steam and form water again. (See distillation.) Refer to lava (melted rock), moulding iron, melting ice and snow, softening of butter.
All solids may be changed to liquids and even to gases if sufficiently heated. Likewise all gases may be changed into liquids and then to solids.
EXPANSION OF SOLIDS
In winter pupils may find that the ink is frozen. The teacher directs attention to this and inquires why it has occurred. It may be that in a lesson on rocks the teacher will ask the pupils to account for all the little stones. The following experiments will aim at solving the foregoing problems:
1. A brass ball and ring are shown. Pupils handle these and note that both are cold and that the ball just passes through the ring. They are asked to compare the size of the ball with that of the ring.
2. The spirit-lamp is lighted and examined. Pupils hold their hands over the flame to note the heat.
3. The ball is heated in the flame for a short time by one of the pupils, and felt cautiously. An attempt is made to pass it through the ring. How has the ball changed in feeling? In size? How does one know it is larger? What has caused these changes?
4. Cool the ball. Feel it. Try to pass it through the ring now. How has it changed in feeling? In size? What caused these changes? How does heat affect the ball? How does cold affect it?
The teacher may now give the words expand and contract, writing them on the black-board and explaining their use. Pupils may then state their conclusions: A brass ball expands when heated and contracts when cooled.
A blacksmith can make the following very serviceable apparatus: A scrap of iron about eleven inches long, one inch wide, and one-eighth inch thick, has one inch bent up at each end. A rod one-eighth inch in diameter is made just long enough to pass between the upturned ends of the first piece when both are cold. The rod is heated and the experiment conducted as in the case of the ball. Two additional facts are learned: (1) Iron expands as well as brass; (2) solids expand in length as well as in volume. The pupils may now be told that other solids have been tried and expansion has invariably followed heating. The conclusion may then be made general.
PRACTICAL APPLICATIONS
1. When your ink-bottle was placed on the stove, which end became warmer? Which expanded the more. Why then did it crack?
2. What other examples like this have you noticed? (Lamp chimneys, fruit jars, stove plates)
3. The earth was once very hot and is now cooling. How is the size of the earth changing? Does it ever crack? What causes earthquakes?
4. Find out by observation how a blacksmith sets tires.
5. Invent a way to loosen a glass stopper stuck in the neck of a bottle.
6. What does your mother do if the metal rim refuses to come off the fruit jar?
7. Next time you cross a railway, notice whether the ends of the rails touch. Explain.
8. What allowance is made for contraction in a wire fence? A railway bridge? Why?
9. Why do the stove-pipes crack when the fire is first started?
10. Why does the house go "thump" on a very cold night?
11. Draw the ball, ring, and spirit-lamp in position.
12. Describe in writing the experiments we have made.
QUESTIONS FOR FURTHER INVESTIGATION
You have seen that iron and brass both expand. Do they expand equally? Let pupils have a few days to invent a way of answering the question. The experiment may then be tried with the compound bar. See The Ontario High School Physics, pages 217-218, also First Course in Physics, Milliken and Gale, page 144.
If the equipment of the school is limited, it may be necessary to dispense with the ball and ring and generalize from one experiment.
Another easily made apparatus consists of two iron rings with handles. One ring will just pass through the other when both are cold. The stove may take the place of the spirit-lamp.
A still simpler plan consists in driving two nails into a block at such a distance apart that an iron rod (six-inch nail, poker, bolt, etc.) will just pass between. On heating the rod the increase in length becomes evident.
EXPANSION OF LIQUIDS
Fill a common bottle with coloured water; insert a rubber stopper through which passes a glass tube about sixteen inches long. Set the bottle in a pan of water and gradually warm the water. The rise of the liquid in the tube will indicate expansion. On setting the bottle in cold water the fall of the column of coloured water shows contraction. See The Ontario High School Physics, page 218, also Science of Common Life, page 48. Macmillan Co., 60 cents.
Set the flask or bottle in a mixture of ice and salt and note that the extreme cold causes contraction for a while, then expansion. Note that when expansion begins, the water has not begun to freeze, but that it does so soon after.
The night before this experiment the children should set out in the cold air, tightly corked bottles of water. In the morning they will be found burst by the expansion.
APPLICATIONS
1. Why did some of the ink-bottles burst in the cold room?
2. Find large stones split up into two or more fragments. Explain.
3. Why is fall-ploughed land so mellow in spring?
4. Why does ice float? Think what would happen if it did not.
5. Explain the heaving of oats, clover, wheat.
6. Do all liquids expand on freezing? Try melted paraffin.
THE THERMOMETER
Besides the ordinary thermometer the school should possess a chemical thermometer graduated from 0° Fahrenheit to 212°.
1. Our sensations vary so much under different circumstances and in different individuals that they cannot be depended on. Find examples of this and show the need of a measuring instrument.
2. The pupils can learn, by examination of the common wall instrument, the parts of the thermometer—tube, bulb, liquid (alcohol or mercury), and scale.
3. Repeat the experiment for expansion of liquids, showing wherein the apparatus resembles the thermometer, warm the thermometer bulb and watch the column rise; cool it and note the fall.
4. Set the bulb of the chemical thermometer in boiling water. The mercury comes to rest near 212°. Bury the bulb in melting snow and notice that the column falls to 32°. Give names for these points. Explain that a degree is one of the 180 equal parts which lie between boiling point and freezing-point. Show that 32° below freezing must be 0°, or zero.
5. The uses of thermometers for indoors and outdoors; for dairy, sick room, incubator, and soils; maximum and minimum. Dairy thermometers registering 212° Fahrenheit may be obtained; they are cheaper than chemical thermometers.
EXPANSION OF AIR
Half fill a flask with water and invert it uncorked over water in a plate. Apply a cloth soaked in boiling water to the part that contains air. Why does the water leave the flask? Apply cold water. Why does the water return? Any ordinary bottle may be used in place of the flask, but it is more liable to crack.
Make an air thermometer. See The Ontario High School Physics, page 223, also Science of Common Life, page 41. Try to graduate it from the mercurial thermometer. Have the boys make a stand for it.
Inferences.—Heated gases rise because they expand. Hot-air balloons, winds, and heating with hot-air furnaces, all depend on this principle.
SOURCES OF HEAT AND LIGHT
NOTES FOR A SERIES OF LESSONS
1. The Sun.—Our dependence on it. Valuable results of its heat. Simple notions as to its size, distance, and nature. Our earth catches a very small fraction of the sun's heat; our sun is but one of millions—the fixed stars. Show the burning effect of a lens.
2. Fuels.—Wood, oil, coal, alcohol, gas, peat, straw: where obtained; special uses of each under varying conditions; need of economy. (This is closely related to geography.)
3. Electricity.—In urban schools use the electric light or some heating device for illustration. In rural schools a battery of two or three cells (see "Apparatus") will melt a fine strand drawn from a picture wire.
Applications: ironing, toasting, cooking; advantages or disadvantages compared with gas or wood.
4. Friction.—Pupils rub hands together; rub a button on a cloth; saw a string across the edge of a board or across the hand; bore a hole through a hardwood plank, then feel the auger-bit.
Applications: restoring circulation; "hot-boxes" in machinery; lubricants and their uses; lighting matches.
5. Pounding.—Hammer a nail flat on an anvil or stone; feel it. Bullets fired against an iron or stone surface may be picked up very hot. Note sparks that can be struck from a stone; percussion caps, flint-lock muskets.
6. Pressure.—After using a bicycle pump for some time, feel the bottom, also the top. If possible, examine an air-compressor and find out the means used for cooling the air.
7. Sources of Light.—Sun, moon, oil, tallow, gas, electricity, wax, acetylene; advantages of each; relative cost.
Primitive Methods of Obtaining Fire: Most savages obtain fire by friction; rubbing two pieces of wood together till hot enough to set fire to some dry, light material. The natives of Australia placed a flat piece of wood on the ground and pressed against this the end of a round piece, which they twirled rapidly with their hands till fire was produced. The North American Indians did the twirling with their bow strings; the Eskimo's plan is somewhat similar. It is impossible to say when flint and steel were first used, but we know they continued to be the chief means of producing fire till about 1834, when matches were invented. Let pupils try to produce fire by these means.
The earliest lamps consisted of shells, skulls of animals, and cup-shaped stones filled with fat or fish oils which burned on a wick of cloth or the pith of rushes. The Tibetans burn butter, the Eskimos whale- or seal-oil, the Arabians palm- or olive-oil. For outdoor lighting, torches carried in the hand were used till gas came into general use about 1792.
CONDUCTION
Give to four boys strips of copper, aluminium, wood, and glass, respectively. They hold these by one end and heat the other end till one or more are forced to drop the piece on account of the heat. The boys with the metals will soon find them hot throughout, but the other two will be able to hold on indefinitely. The teacher gives the terms "good conductor" and "poor conductor".
PROBLEMS
1. Are metals generally good conductors? Try with strips of zinc, lead, iron, a silver spoon.
2. Are all good conductors equally good? Devise a means of ascertaining. See Science of Common Life, Chapter VI; also The Ontario High School Physics, page 274.
3. Is water a good conductor?
Lists of good and poor conductors may then be made, the teacher adding to the list. Good: metals; poor: wood, horn, bone, cloth, leather, air, water, hair, asbestos, ashes, rock, earth.
PROBLEMS
1. If the interior of the earth is very hot, why do we not feel it?
2. How can the cold snow keep the earth warm?
3. Why does your hand freeze to metals but not to wood?
4. Let the children try to find other instances: wools or furs for clothing, fur coats on northern animals, feathers on birds, down quilts, tea cosies, sawdust for packing ice, double windows, wooden handles for hot irons, asbestos coating for steam pipes.
The Miners' Safety-Lamp: This is a most important application of conduction. Get from the tinsmith a piece of brass gauze six inches square. Raise the wick of the spirit-lamp causing it to give a high flame and bring the gauze down upon the flame till it touches the wick. Note that the flame does not rise above the gauze. Hold a piece of paper above the gauze near the flame and note that it does not take fire. Note also that the gauze soon becomes hot. The brass wires conduct the heat of the flame rapidly away so that there is not heat enough above the gauze to cause combustion. Now roll the gauze into a hollow cylinder, pin the edges together, insert a cork at each end, and have a short candle fastened to the lower one. Try to light the candle with the lamp through the gauze. It is not easily done.
The miner carries a lamp made like this, so that if he should be in the presence of the explosive gas, "fire damp", it would not explode because of the wire gauze shield.
CONVECTION
Water is not a conductor, how then is it heated?
Drop a few pieces of solid colouring matter, (analine blue, blueing, or potassium permanganate) into a beaker of cold water. Place the beaker over a heater and observe the coloured portion rise.
Wet sawdust will make a good substitute for the colouring matter. A sealing jar or even a tin cup will do instead of the beaker. The stove or a dish of hot water will take the place of the lamp.
PROBLEMS
1. Using a thermometer, see whether the water at the bottom is warmer than that at the top while the beaker is being heated.
2. Heat some oil and pour it over the surface of some cold water. Lower a thermometer into this. Does the water at the bottom soon become warm?
3. If your kitchen is provided with a hot-water tank, find out what part of the tank first becomes warm after the fire is lighted.
4. In bathing, where do you find the coldest water of a pond or still river? See Science of Common Life, Chapter VI; also The Ontario High School Physics, page 280.
CONVECTION IN GASES
A good apparatus may be made by cutting two holes one inch in diameter in one side of a chalk box, replace the lid with a piece of glass, place a lamp chimney over each hole and a lighted candle under one of the chimneys. Hold a piece of smoking touch-paper at each chimney in turn and note direction of air current.
APPLICATIONS
1. Winds are caused by the rising of air over heated areas, allowing cooler currents to take its place. (Geography)
2. Rooms are ventilated by heating some of the air more than the rest, thus producing a current. (Hygiene) Winds are nature's means of ventilating the earth.
RADIATION OF HEAT
This should be taken up as an introduction to dew, frost, winds, climate, etc.
1. Make an iron ball hot (the end of a poker will answer). Hold the hand a few inches below the iron. Does the heat reach the hand by convection? By conduction? By means of suitable questions, lead the pupil to see that it is not by convection, for the hand is below the hot object while heated air rises; it is not by conduction, for air is one of the very poorest conductors; moreover, the heat is felt instantly from the poker, but it takes an appreciable time for it to come by conduction and convection. We say this heat is radiated from the iron. The velocity of radiated heat is about 186,000 miles a second.
2. The above experiment may be varied by bringing the hot iron gradually toward the bulb of the air thermometer and noting the greatest distance at which it will affect the thermometer.
It is by radiation that the sun's heat and light reach us. We get much of the heat of stoves, fire-places, and radiators by the same means.
Why does the earth cool off at night? Why does dew form? Why can no dew form on a cloudy night? Why is a mountain top or a desert so cold, especially at night?
3. Take two tin cans (baking powder boxes will answer) and make holes in the lids large enough to admit a thermometer. Blacken one box in the flame of an oil lamp. Fill both with boiling water and put in a cool place. Test with a thermometer from time to time. Which cools most rapidly?
4. Fill the tin cans with cold water, find the temperature, and then place them near a hot stove. Which warms faster? Usually dark or rough surfaces radiate heat and absorb heat faster than bright or smooth ones. An excellent way of testing this is to lay a black cloth and a white one side by side on the snow where the sun is shining brightly. The snow will melt more rapidly under the black cloth. Painted shingles may be substituted for the cloths. Try different colours. The day chosen should not be extremely cold.
PROBLEMS
1. Why should we have the outside of a tea-kettle, teapot, or hot-air shaft of a bright colour? Why should we have stoves and stove-pipes dull black?
2. Why does a coat of snow keep the earth warm?
3. Which is the warmest colour to wear in winter? Does this account for the colour of Arctic animals?
4. Which is the coolest colour to wear in the hot sun?
5. Gardeners sometimes strew the ground with coal-dust to help ripen their melons. Show the value of this.
6. Suggest a method of protecting a wall from the heat of a stove.