. The Ontario high school physics. Fig. 302.—Water is a poorconductor of heat. 312. Conduction in Liquids. If we except mercury and molten metals, li(|uids are poor conductors of heat. Take water for example. We may boil the upj)er layers of water held in a test-tube over a lamp (Fig. 302) without perceptibly heating the water at the bottom of the tube. The poor conductivity of water is also strikingly shown in the following experiment. Pass the stem of a Galileo air-thermometer (§ 255) through a per-forated cork inserted into a funnel asshown in Fig. 803. Then cover thebulb of the thermometer
. The Ontario high school physics. Fig. 302.—Water is a poorconductor of heat. 312. Conduction in Liquids. If we except mercury and molten metals, li(|uids are poor conductors of heat. Take water for example. We may boil the upj)er layers of water held in a test-tube over a lamp (Fig. 302) without perceptibly heating the water at the bottom of the tube. The poor conductivity of water is also strikingly shown in the following experiment. Pass the stem of a Galileo air-thermometer (§ 255) through a per-forated cork inserted into a funnel asshown in Fig. 803. Then cover thebulb of the thermometer to a depthof about J cm. with water. Now poura spoonful of ether on the surfaceof the water and set fire to it. Theindex of the thermometer shows thatlittle, if any, heat is transmitted bythe water to the bulb from the flame. Fio. 303.—Illustration of thenon-conductivity of water. at the surface. 276 TRANSFERENCE OF HEAT 313. Conduction in Gases. Gases are extremely poorconductors of heat. The conductivity of air is estimatedto be only about ,049 of that of copper. Manysubstances, such as wool, fur, down, etc., owe their poorconductivity to the fact that they are porous and con-tain in their interstices air in a finely divided state. Ifthese substances are compressed they become better con-ductors. Light, freshly fallen snow encloses within it largequantities of air, and consequently forms a warm blanketfor the earth, protecting the roots of plants from intensefrost. Heat is conducted with the greatest difficulty through avacuum. For holding liquid air Dewar introduced glass flaskswith hollow walls from which the air has been inner surfaces of the walls are silvered to preventradiation ( § 570). The familiar Thermos bottle is con-structed in this way. When contained in such a vessel aho
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