SOLIDS, LIQUIDS, AND GASES
Arrange a collection of objects of various shapes, sizes, colours, and weights, as cork, gla.s.s, 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, mola.s.ses, 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 s.p.a.ce. 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 b.u.t.ter.
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 bra.s.s ball and ring are shown. Pupils handle these and note that both are cold and that the ball just pa.s.ses 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 pa.s.s 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 pa.s.s 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 bra.s.s ball expands when heated and contracts when cooled._
A blacksmith can make the following very serviceable apparatus: A sc.r.a.p 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 pa.s.s 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 bra.s.s; (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 gla.s.s 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 bra.s.s 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 pa.s.s 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 pa.s.s 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 pa.s.ses a gla.s.s 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 circ.u.mstances 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.