LATENT HEAT. The heat absorbed when unit mass of any substance changes its state without change of temperature; e.g., when I gm. of ice at o° C melts to form water at o° C, 79.6 calories of heat are absorbed. Latent heat of fusion and latent heat of vaporization are the most familiar examples of the phe nomenon, but a similar absorption (or liberation) of heat occurs in all changes of state, e.g., sublimation, solution, dilution, expan sion. The term was introduced by Joseph Black, c. 176o. For details see the articles on HEATS THERMODYNAMICS STEAM.
Under the Kinetic theory of heat, latent heat is considered to be the potential energy of separation of attracting masses. The Kinetic theory is based on the assumption that heat is a "mode of motion." The exact character of this motion is unknown, but is sufficiently understood to present a plausible picture of what goes on within matter. The molecules of a substance, the smallest divisions that can be broken up without losing their identities, in turn are structures composed of atoms and the atoms are struc tures made up of still smaller units called electrons. Electrons may arrange themselves in many different ways, each constituting a different kind of atom which is the unit of a definite substance known as a chemical element. The atoms of different elements unite in a variety of combinations to form the molecules. In studying the forces which hold the atoms in place to form a defi nite combination, the molecule is considered to be a miniature solar system. An entirely molecular (e.g., solar) system, while its component parts are involved in mutual relative motions, may be participating as a unit in motion relative to surrounding molecular systems. Heat is Kinetic energy represented by this inter-molecu lar motion. The quality of heat is determined by the aggregate of the mass of molecules involved together with the intensity of the energy. Temperature is indicative only of the intensity of the
Kinetic energy. The proximity or distance of the molecules of a substance determine its state as a solid, liquid or vapor. In a solid state the molecules are relatively near each other and the force of attraction is great. When the solid is heated the molecular velocities increase until the centrifugal force is sufficient to break up the molecular systems. As heating continues no further increase of velocity takes place, and therefore no rise in temperature occurs until uniformity of condition throughout the mass has been at tained. The solid changes to a liquid and the latent heat of the liquid is accounted for in the breaking up of the molecular systems.
With further application of heat, velocity increases until a con dition is reached when the force of separation is sufficient to cause some of the molecules to be thrown out of the liquid entirely and vaporization begins. The wide displacement of particles means a great increase in potential energy of separation and a consequent large supply of heat. The separation is so great that the volume of the entire mass is enormously increased, necessitating a con siderable amount of energy to enable the substance to find room for itself against the environing pressure. This is the external work part of the latent heat of vaporization. Whether heat is the energy of motion of molecules alone or whether atomic and electronic motions within the molecule participate in the effect, does not alter the general conception since it is the aggregate of the kinetic effect that is perceived as heat.