Fig. 58. Convection currents carrying the heat of the stove about the room.
Since heat is so often carried to us by convection,—by warm winds, warm air from the stove, warm ocean currents, etc.,—it seems as if air must be a good conductor of heat. But if you shut the air up into many tiny compartments, as a bird's feathers do, or as the hair on an animal's back does, so that it cannot circulate, the passage of heat is almost completely stopped. When you use a towel or napkin to lift something hot, it is not so much the fibers of cotton which keep the heat from your hand; it is principally the very small pockets of air between the threads and even between the fibers of the threads.
Fig. 59. Diagram of a hot-water heater. What makes the water circulate?
Cold the absence of heat. Cold is merely the absence of heat; so if you keep the heat from escaping from anything warm, it cannot become cold; while if you keep the heat from reaching a cold thing it cannot become warm. A blanket is just as good for keeping ice from melting, by shutting the heat out, as it is for keeping you warm, by holding heat in.
Application 32. Explain why ice is packed in straw or sawdust; why a sweater keeps you warm.
Select from the following list the good conductors of heat from the poor conductors (insulators): glass, silver, iron, wood, straw, excelsior, copper, asbestos, steel, nickel, cloth, leather.
Inference Exercise
Explain the following:
171. If the axle of a wheel is not greased, it swells until it sticks fast in the hub; this is a hot box.
172. When you have put liquid shoe polish on your shoes, your feet become cold as it dries.
173. The part of an ice-cream freezer which holds the cream is usually made of metal, while that which goes outside and contains the ice and salt is usually made of wood.
174. The steam in a steam radiator rises from a boiler in the basement to the upper floors.
175. When you throw a ball, it keeps going for a while after it leaves your hand.
176. Clothes keep you warm, especially woolen clothes.
177. The Leaning Tower of Pisa does not fall over.
178. It is almost impossible to climb a greased pole.
179. Heat goes up a poker that is held in a fire.
180. A child can make a bicycle go rapidly without making his feet go any faster than if he were walking.