The specific heat of any substance is “the number of units of heat required to raise one pound of such substance one degree.” We can explain this farther. When heat is communicated to a body it has two or three functions to perform. Some of it has to overcome the resistance of the air in expanding the body, more of it expands, and the remainder increases the temperature of the body. So some heat disappears as heat, and is turned into energy,—“molecular potential energy,”—as it is called, and the rest remains. Of course in objects the molecules vary very much in weight and in their mutual attraction, and the heat requisite to raise equal weights of different substances through the same number of degrees of temperature will vary. This is called capacity for heat, or specific heat. The capacity of different metals for heat can easily be shown. The specific heat of water is very high, because its capacity for heat is great. We can cool a hot iron in very little water, and it takes thirty times as much heat to raise a given weight of water a certain number of degrees, as it would to raise the same weight of mercury to the same temperature. Water has greater specific heat, generally speaking, than other bodies, and it is owing to this circumstance that the climate is so affected by ocean currents. Nearly all substances can be melted by heat, if we go far enough, or frozen, if we could take the heat away. Solid can be made liquid, and these liquids can be made gases and fly off in vapour. Similarly, if we could only get heat away sufficiently from the atoms of a substance we could freeze it. We cannot freeze alcohol, nor make ice from air, nor can we liquify it, for we are unable to take away its heat sufficiently. But we can turn water into steam, and into ice; or ice into water, and then into steam. But there is one body we cannot melt by heat, that is carbon. In the hottest fire coal will not melt, it becomes soft. We call this melting _fusion_, and every body has its melting point, or fusing point, which is the same at all times if the air pressure be the same. It is a curious fact that when a body is melting it rises to a certain temperature (its fusing point), and then gets no hotter, no matter whether or not the fire be increased;—all the extra heat goes to melt the remainder of the substance. The heat only produces _changes of state_. So this heat above fusing point disappears apparently, and is called _Latent Heat_. This can easily be proved by melting ice. Ice melts at 32° Fahr., or 0° Cent., and at that temperature it will remain so long as any ice is left; but the water at 32°, into which the ice has melted, contains a great deal of _latent_ heat, for it has melted the ice quickly, and yet the thermometer does not show it. It is just the same with boiling water.