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 ill.u.s.tration. 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 b.u.t.ton 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 sh.e.l.ls, 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 b.u.t.ter, 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 gla.s.s, 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 bra.s.s 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 bra.s.s 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, (a.n.a.line blue, blueing, or pota.s.sium 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 subst.i.tute 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 gla.s.s, 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