A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . ers, are used to impede the flow of heat into orout of bodies; that is, to keep hot bodies hot and coldbodies cold. 254. Davys Safety-lamp.—This is an instrument whichmakes use of the fact that the flame of a burning gas will,under certain conditions, not pass through a sheet ofwire gauze, the wire carrying off the heat so rapidly thatthe gas within the meshes will not burn. EXPERIMENT. Light a Bunsen burner and pressdown upon the flame a sheet offine wire gauze. Observe
A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . ers, are used to impede the flow of heat into orout of bodies; that is, to keep hot bodies hot and coldbodies cold. 254. Davys Safety-lamp.—This is an instrument whichmakes use of the fact that the flame of a burning gas will,under certain conditions, not pass through a sheet ofwire gauze, the wire carrying off the heat so rapidly thatthe gas within the meshes will not burn. EXPERIMENT. Light a Bunsen burner and pressdown upon the flame a sheet offine wire gauze. Observe that theflame does not, until the wire be-comes red-hot, pass through thegauze. To show that the gaspasses through unburned, hold alighted match above the gauze. The safety-lamp is used byminers in regions where theyare likely to meet with inflam-mable gases, which an ordinarylamp or candle would has (see Fig. 175) a cylinderand cover of wire gauze overthe flame. The inflammable fire-damp of the mines works its way into the lamp through the meshes of thegauze and causes the flame to grow larger and change,. I L I I FIG. 175. HEAT: TEMPERATURE, CONl/EYANCE OF HEAT. 305 color; and the miner, seeing this, leaves the dangerousregion before the flame can set fire to the gas outsidethe gauze. This lamp was the invention of Sir HumphryDavy, a famous English chemist who lived from 1778to 1829. 255. Convection.—This is the conveyance of heat byconveyance of the body containing the heat. Such amovement occurs naturally in a liquid or a gas which isunsymmetrically heated; for, as we shall see later, thehotter parts are, in general, less dense than the colderparts, and so the former tend to rise while the latter tendto fall. The movements thus produced are called con-vection-currents. It is evident that they should be espe-cially strong when heat is applied at the bottom. Itwould be a difficult matter, as the experiment of § 252has shown, to boil water at the bottom of a ves
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