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.