296. =Convection.=--This mode of diffusion of heat is in operation in those substances whose particles are movable among each other--viz., liquids and aeriform substances. I have already alluded to examples of this mode in speaking of the movements which heat causes in these substances. The heat goes along with the particles which are moved, or is _conveyed_ along with them, and hence the term convection. In this movement the heated particles always ascend, for the reason given in § 275. Of the multitude of examples of convection I will present but a few. In the upward current about a stove-pipe you have an example of convection, the heat generated being carried upward by the particles of this current. This being so, the heat of a stove has no effect upon the air _below_ it by convection, though it does have by radiation, as you will soon see. Any hot fluid becomes cool chiefly by convection. The air coming in contact with it taking some of its heat rises, and other air comes in its turn to be also heated, and so on till the fluid becomes of the same temperature with the air, and then the currents of air cease. The liquid cools more rapidly by stirring it, because the air is brought into contact with a greater extent of surface, and so the heat is conveyed away more rapidly. The result is the same whether we disturb the surface by stirring it or by blowing upon it. In the latter case, however, the effect is increased by making the air to come more rapidly upon the disturbed surface. So in fanning, it is the bringing of the air faster upon the surface of the body that causes the more rapid, convection of heat from it. Every one must have observed the fact that a buckwheat cake cools much more quickly than a flour or rice cake. It is because it has so many pores and little projections, and so presents a much larger amount of surface to the heat-conveying air than the smoother and more solid cakes. Viscid fluids, as molasses, oil, etc., when heated do not cool as readily as water, because their particles are not as movable, and therefore heat is not conveyed as rapidly upward to be given off to the air. [Illustration: Fig. 211.] 297. =Conduction.=--In this mode of diffusion the heat goes through or among the particles of substances. For example, if one end of a bar of iron be held in the fire, it travels through or among the particles to the other end. The gradual progress of the heat may be seen by the following simple experiment: Take a rod of iron and attach to it, as seen in Fig. 211, some little balls of wood by means of wax. By heating one end with a lamp the balls will drop one after another as the heat passing along melts the wax which holds them. [Illustration: Fig. 212.]