1. Add a little water to some baking powder and cause the gas that forms to pass through lime-water. What causes the biscuits to "rise"? 2. Mix flour and water in a jar, add a bit of yeast cake and a little sugar, and let stand in a warm place. Test the gas that forms, for carbon dioxide. What causes bread to rise? 3. Uncork a bottle of ginger ale, shake the bottle, and lead the gas that comes off through lime-water. 4. Most portable fire extinguishers depend on the generation of carbon dioxide. 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 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?