=179. Heat Capacity of Water.=--The large capacity for heat shown by water is useful in regulating the temperature of the air near lakes and the ocean. In hot weather the water rises slowly in temperature absorbing heat from the warm winds blowing over it. In winter the large amount of heat stored in the water is slowly given out to the air above. Thus the climate near the ocean is made more moderate both in winter and summer by the large capacity of water for heat. This large heat capacity of water may seem to be a disadvantage when one is warming it for domestic purposes since it requires so much heat to warm water to boiling. However, it is this capacity that makes hot-water bottles and hot-water heating effective. If one takes a pound of ice at 0°C. in one dish and a pound of water at 0°C. in another, and warms the dish of ice by a Bunsen flame until the ice is just melted, and then warms the water in the other dish for the same time, the water will be found to be _hot_ and at a temperature 80°C., or 176°F. =180. The Heat of Fusion of Ice.=--This experiment indicates the large amount of heat required to change the ice to water without changing its temperature. As indicated by the experiment, it requires 80 calories to melt 1 g. of ice without changing its temperature or, in other words, if one placed 1 g. of ice at 0°C. in 1 g. of water at 80°C., the ice would be melted and the water would be cooled to 0°C. =181. Heat Given out by Freezing water.=--Just as 80 calories of heat are required to melt 1 g. of ice, so in freezing 1 g. of water, 80 calories of heat are given out. The fact that heat is set free or given out when a liquid solidifies may be strikingly shown by making a strong solution of sodium acetate. On allowing this to cool quietly it will come to the room temperature and remain liquid. If now a small crystal of sodium acetate is dropped into the liquid the latter quickly becomes a solid mass of crystals, at the same time rising markedly in temperature. The amount of heat now liberated must enter the sodium acetate when the mass of crystals is melted again. The large amount of heat that must be liberated before water freezes accounts for the slowness of the formation of ice. It is also the reason why the temperature never falls so low in the vicinity of large lakes as it does far inland, the heat given out by the freezing water warming the surrounding air. The heat that disappears on melting and reappears on solidifying is called the _heat of fusion_. It is sometimes called _latent heat_ since the heat seems to become hidden or latent. It is now believed that the heat energy that disappears when a body melts has been transformed into the _potential energy_ of partially separated molecules. The heat of fusion therefore represents the work done in changing a solid to a liquid without a change of temperature.