SPECIFIC HEATS OF This difference in specific heat in the same gas is due to two causes When the gas ex pands, not only do the molecules acquire greater kinetic energy, but in pushing each other far ther apart against the attractive force of co hesion, they require a further amount of energy of the potential sort, and in pushing back the restraining pressure of the atmos phere still another large supply of energy is needed. It appears from several independent considerations that in gases far removed from their liquefying points the cohesion effect is exceedingly small, and so we conclude that the excess of specific heat of an expanding gas is almost entirely due to work done on the ex ternal pressure applied to the gas.
In the last column of the table the ratio of the two specific heats of the gases is given. This ratio is found to vary, decreasing from simple gases like mercury vapor, the molecules of which have single atoms, to complex gases like ether vapor, the molecules of which have 15 atoms. With complex molecules a large part of the energy is internal, much being stored up in the rotating motion of the individual mole cules, and in the relative motion of, their atoms, leaving the energy of translation of the mole cules and the energy due to the pushing back of the external pressure about the same as for mercury vapor. It follows then that the energy associated with the external pressure is a smaller fraction of the whole energy, and that therefore, as observed, the ratio between the heat energy imparted to an expanding gas and the heat energy imparted to a non-expanding gas must be smaller for such complex mole cules. The value of this ratio is the principal means of judging of the number of atoms in a molecule of an element in the gaseous state.
Before leaving this subject it should be re marked that the specific heat of water varies slightly with the temperature, and so it is con venient to take as the value of the calorie one hundredth of the heat energy required to raise the temperature of a gram of water from 0° C. to 100° C.
Latent If heat energy be imparted to a mass of ice at the point of melting, the ice will proceed to melt, but will not grow any warmer as it does so. The heat energy thus added without increasing temperature is called latent heat. Latent heat is devoted only to shaking the molecules of ice asunder, not to increasing their speed. Temperature depends upon the energy of motion, (kinetic energy) of the molecules; latent heat only stores up energy of position (potential energy) of the mole cules, and so does not produce an increase of temperature (this simple statement must be modified in cases of change of polymerization). Again, when water is being boiled, a large amount of heat energy becomes latent. The latent heat of vaporization and of melting for a variety of substances is given below.
It should be remarked that the latent heat devoted to converting a liquid into vapor, be sides increasing the internal potential energy of the molecules, also does work in pushing back the atmosphere, but with water this external work bears a very small ratio to the internal work against cohesion, namely, a little more than one-twelfth.
Heretofore we have supposed the energy for melting or for vaporization to be derived from some external source of heat. It is, however, possible to secure a change of state through the consumption of the heat energy of the body itself. If water be left in an open vessel it will presently have evaporated entirely away. Dur ing the progress of this vaporization a ther mometer placed either in the water or in the moist air above the water will show a tem perature lower than that of the surrounding air. The reason of this is as follows: At the surface of the liquid, with all the irregularities of position and velocity possessed by the mole cules, some of them find opportunity to fly off from the liquid surface. On the average it will be the faster going molecules that spring away first, thus leaving the more slowly going ones behind, which is the same as saying that the remaining liquid is cooler. Also in going away, the molecules fly against the back pull of co hesion, and so their velocity is checked. Indeed many are entirely stopped and drawn back into the liquid, though others escape quite beyond the range of cohesion of the liquid and diffuse among the molecules of the air. The reduced motion of these escaping molecules causes the low temperature, referred to above, of the vapor. Common illustrations of cold by evap oration are frequently met with. The function of perspiration is a means of regulating the temperature of the human body. In the healthy state when we are overheated the skin. becomes very moist, and the evaporation of this moist ure, assisted by a breeze or by fanning, cools the surface. In disease the proper action of the skin may be interfered with, and becoming dry, may fail through lack of evaporation to provide the normal cooling effect. An exalted temper ature of the body ensues; in other words, a fever. Certain drugs tend to promote perspi ration and thus reduce the temperature of the patient. Another large factor in the tempera ture regulation of the body is in the water evaporated from the lungs in the process of breathing. The evaporation of ammonia that has been liquefied by pressure furnishes the cold employed in some ice machines. In the case of liquefaction the necessary latent heat may be derived from the body itself. This occurs when a salt is dissolved in water, a process that is generally accompanied by a 411 of temperature, though occasionally a rise in temperature is noted. The factors governing the result in such cases are rather complicated. We have to take account of the work done by the solvent in tearing molecules away from the solid lump and in some cases the tearing of these molecules apart into electrically charged parts called ions. On the other hand a certain amount of kinetic energy is furnished by the attraction of the molecules of the dia. solving substance by the molecules of the sol vent. According as the back pulls or the forward pulls predominate, will the temperature of the solution tend to be lowered or raised. If much chemical combination takes place between the substance and the solvent, the solution is almost always warmed.