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 quantities of sugar, salt, soda, alum,
blue-vitriol. Shake up with equal quantities 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,