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). [Illustration: FIG. 136.--Convection in a liquid.] =156. Draft of a Chimney.=--When a fire is started in a stove or a furnace the air above the fire becomes heated, expands, and therefore is less dense than it was before. This warm air and the heated gases which are the products of the combustion of the fuel weigh less than an equal volume of the colder air outside. Therefore they are pushed upward by a force equal to the difference between their weight and the weight of an equal volume of the colder air. The chimney soon becomes filled with these heated gases. (See Fig. 138.) These are pushed upward by the pressure of the colder, denser air, because this colder air is pulled downward more strongly by the force of gravity than are the heated gases in the chimney. Other things being equal, the taller the chimney, the greater the draft, because there is a greater difference between the weight of the gases inside and the weight of an equal volume of outside air. [Illustration: FIG. 137.--Convection currents in a room.] [Illustration: FIG. 138.--Fire place showing draft of a chimney.]