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 shells, 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 butter, 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 glass, 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.