Show the similarity between our bodies and the candle. The candle needs oxygen; it produces heat, and yields water and carbon dioxide. Much of our food is somewhat similar in composition to the wax of a candle; we breathe oxygen, our bodies are warmed by a real burning within, and we exhale water and carbon dioxide.

After exercise why do we feel more hungry? Why do we breathe faster? Why do we feel warmer? Why does the fire burn better when the damper is opened?

IMPURITIES OF AIR

All air contains carbon dioxide. If the amount exceeds 6 parts in 10,000, it becomes an impurity, not so much on its own account as because it indicates a poisoned state of the air in a room, since organic poisons always accompany it when it is emitted from the lungs.

Other impurities of the air, dependent on the locality and the season, are smoke, dust, disease germs, sewer gas, coal-gas, pollen dust.

SOLUTIONS OF SOLIDS

(Consult the _Science of Common Life_, Chap. VII.)

Have the pupils weigh out equal quant.i.ties of sugar, salt, soda, alum, blue-vitriol. Shake up with equal quant.i.ties of water to compare solubilities. Repeat, using hot water. Is it possible to recover the substance dissolved? Set out solutions on the table to evaporate, or evaporate them rapidly over a stove or spirit-lamp. Try to dissolve sand, sulphur, charcoal, in water. Obtain crystals of iodine and show how much better, in some cases, alcohol is as a solvent than is water.

APPLICATIONS:

1. Most of our "essences", "tinctures", and "spirits" are alcoholic solutions.

2. Digestion is the effort of the body to dissolve food.

3. The food in the soil enters the plant only after solution.

4. The solvent power of water makes it so valuable for washing.

5. Maple sap is water containing sugar in solution.

6. In the salt region along Lake Huron, holes are drilled to the salt beds, water is poured in, then pumped out and evaporated. Explain.

7. Meat broth is a solution of certain materials in the meat.

8. How could you manufacture salt from sea water?

SOLUTION OF LIQUIDS

Try to mix oil and water, benzine and water, oil and benzine. Only in the third case do we find a permanent mixture, or solution. Try to dissolve vinegar, glycerine, alcohol, mercury, with water.

APPLICATIONS:

1. Paint is mixed with oil so that the rain will not wash it off so easily.

2. Water will not wash grease stains. Benzine is necessary.

3. Why is it necessary to "shake" the bottle before taking medicine?

SOLUTION OF GASES

Study air dissolved in water, by gently heating water in a test-tube and observing the bubbles of air that gather on the inner surface of the test-tube. Aquatic animals, such as fish, clams, crayfish, crabs, subsist on this dissolved air.

LIMESTONE

Pieces of this rock may be found in all localities. Teach pupils to recognize it by its gray colour, its effervescence with acid, and the fossils and strata that show in most cases. If exposed limestone rocks are near, visit them with the pupils and note the layers, fossils, and evidences of sea action. Compare lime with limestone as to touch, colour, and action on water and litmus. Try to make lime by putting a lump of limestone in the coals for some time; add water to this. Other forms of limestone are marble, chalk, egg-sh.e.l.ls, clam-sh.e.l.ls, scales in tea-kettles.

Geographically, the study of limestone is of great importance. Grind some limestone very fine, add a very little of this to water, and bubble carbon dioxide through for some time; note the disappearance of the limestone. This explains how limestone rocks are being slowly worn away and why the water of rivers, springs, and wells is so often "hard".

Catch some rain-water in the open and test it for hardness. It will be found "soft". Place a few limestone pebbles in a tumbler with this soft water and after a day or two test again. The water will be "hard".

Compare, as to hardness, the water from a concrete cistern with that from a wooden one.

CARBON

Procure specimens of hard and soft coal, c.o.ke, charcoal, graphite, peat, and petroleum. Note the distinctive characteristics of each. Discuss the uses. Try to set each on fire. Note which burns with a flame when laid on the coals or placed over the spirit-lamp. Put a bit of soft coal into a small test-tube; heat and light the gas that is produced. This gas, when purified, is one kind of illuminating gas. Note the _c.o.ke_ left in the test-tube.

Fill the bowl of a clay pipe with soft coal and seal it up with plaster of paris. After this has hardened, place the bowl in hot coals or in the flame of a spirit-lamp and light the coal-gas at the end of the stem.

After all the gas has been driven off, look for the c.o.ke inside.

Heat a bit of wood in a small test-tube and light the gas that is evolved. Note the charcoal left.

Cover a piece of wood with sand or earth; heat, and note that charcoal is formed. This ill.u.s.trates the old method of charcoal-burning. This subject is closely related to industrial geography.

HYDROGEN

A convenient way to prepare hydrogen is to use zinc and hydrochloric acid with a test-tube for a generator. (Consult any Chemistry text-book.) Make the gas and burn it at the end of a tube, holding a dry, cold tumbler inverted over the flame. Note that water is formed.

Conclude what water consists of, namely, oxygen and hydrogen. Water may be decomposed into oxygen and hydrogen, hence a use of hydrogen may be shown by attaching a clay pipe to the generator and filling soap bubbles with the gas. When freed these rise quickly.

MAGNETS

If bar magnets cannot be obtained, use a child's horse-shoe magnet.

Procure small pieces of cork, wood, iron, bra.s.s, gla.s.s, lead, etc., and let pupils discover which the magnet attracts.

Have pupils interpose paper, wood, slate, gla.s.s, iron, lead, etc., in sheets between the magnet and the iron and note the effect on the force exerted.

Note that when one end of a magnet touches or comes near the end of a nail, the nail becomes a magnet, but not a permanent one.

Magnetize a needle by drawing one of the poles of the magnet from end to end of the needle, always in the same direction, about twenty times.

Suspend the needle horizontally with a piece of silk thread and note its position when at rest.

Get a small compa.s.s and show how it is related to the foregoing experiments. Emphasize its use to mariners. If possible, get a piece of lodestone and show its magnetic properties.

ELECTRICITY

Half fill a tumbler with water and add about a teaspoonful of sulphuric acid. Set in this a piece of copper and a piece of zinc, but do not let them touch. Make a coil by winding insulated wire around a block of wood about ten times. Remove the wood and place a compa.s.s in the centre of the coil. Join the ends of the wire to the two metals in the tumbler.

The sudden movement of the needle will be taken as the indication of a current.

Let pupils try experiments with many pairs of solids, such as lead and silver, carbon and gla.s.s, wood and iron, tin and zinc, and liquids such as vinegar and brine.

Show pupils how to make a simple battery. See home-made apparatus, page 50, and consult _Laboratory Exercises_ by Newman. Two or three dry cells will be found sufficient for any experiments, but the home-made battery is to be preferred.