5. Is iron better than brick or porcelain as a material for stoves? Explain.
6. Which is better, a good or a poor conductor for keeping a body warm? for keeping a body cool?
7. Should the bottom of a teakettle be polished? Explain.
8. How are safes made fireproof?
9. Explain the principle of the Thermos bottle.
10. Explain why the coiled wire handles of some objects as stove-lid lifters, oven doors, etc., do not get hot.
(5) Transmission of Heat in Fluids. Heating and Ventilation
155. Convection.—While fluids are poor conductors, they may transmit heat more effectively than solids by the mode called convection. To illustrate: if heat is applied at the top of a test-tube of water, the hot water being lighter is found at the top, while at the bottom the water remains cold. On the other hand, if heat is applied at the bottom of the vessel, as soon as the water at the bottom is warmed (above 4°C.) it expands, becomes lighter and is pushed up to the top by the colder, denser water about it. This circulation of water continues as long as heat is applied below, until all of the water is brought to the boiling temperature. (See Fig. 136.)
When a liquid or a gas is heated in the manner just described, the heat is said to be transferred by convection. Thus the air in the lower part of a room may receive heat by conduction from a stove or radiator. As it expands on being warmed, it is pushed up by the colder denser air about it, which takes its place, thus creating a circulation of the air in the room. (See Fig. 137.) The heated currents of air give up their heat to the objects in the room as the circulation continues. These air currents may be observed readily by using the smoke from burning "touch paper" (unglazed paper that has been dipped into a solution of potassium nitrate ["saltpeter">[ and dried).
