Convection

(-věk's hun.) [From L. con, with, and Conversion, veho, to carry.] (Phy.) When a liquid is heated from above, the temperature of the ma rises with extreme slowne , because liquids po e but little conducting power for heat; thus water may be boiled on ice, although separated from it by a very thin stratum of water. But if the liquid be heated from below, we notice at once that currents of liquid ascend from the bottom to the top of the ve el, and the liquid acquires a uniform temperature. This transport of heat by ma es of matter is known as C. The layers ofa liquid or gas which are nearest to the source of heat are expanded, and thus become specifically lighter than surrounding portions, consequently they rise; while colder, and consequently heavier, portions descend, are heated in their turn, and then ascend to make way for other colder portions. Thus, however badly a liquid or gas conduct heat, it can rapidly acquire a uniform temperature by the C. of heat; and C. takes place in gases far more readily than in fluids, because for equal increments of heat they expand to a greater extent than liquids. [s. 224]