LATENT HEAT
52. Latent Heat of Fusion. — The fusion, or melting, of a solid body, whether it is ice, steel, or any other solid capable of being melted, is due to the fact that, when a certain temperature is reached, the rapid vibration of the molecules overcomes that force of attraction of the molecules for one another by which the body was enabled to retain its solid state. If a piece of ice is placed in a suitable vessel and heat is applied, the ice will gradually melt; but the temperature of the water surrounding it will not rise above 32° F. until the ice is fully melted. The ice has received heat constantly, but the heat has been utilized in changing the body from a solid to a liquid state. The heat that is added to a body to change its state, without changing its temperature, is called latent heat. If the state is changed from a solid to a liquid, the heat required to accomplish it is called the latent heat of fusion. It is customary to use 1 pound of a substance as the basis for comparing latent heats of fusion. A pound of ice requires 144 British thermal units to convert it into water at 32° F. Hence, the latent heat of ice is said to be 144. Every substance capable of being liquefied has its own latent heat of fusion, which is the number of heat units required to convert 1 pound of it from the solid to the liquid state without change of temperature.
53. Latent Heat of Vaporization. — If a quantity of water is boiled in the open air and its temperature is noted, it will be found that the temperature remains at 212° F. until the water entirely disappears. The water has absorbed a large quantity of heat while being converted into vapor without change of temperature. The amount of heat thus expended in converting a pound of water at the boiling point into steam at the same temperature is called the latent heat of vaporization of steam. The latent heat of steam at 212° F. is 965.8; that is, it requires 965.8 British thermal units to convert 1 pound of water at 212° F. to steam at the same temperature, under atmospheric pressure. The latent heat
