HEAT

is that particular form of energy which consists in the kinetic and potential energy of the molecules of In matter (see Enerey). The ultimate particles of any ma are, at all temperatures above the lowest, in the state of agitation; each atom may be moving as 2 whole, and it may have internal motions; the molecules (consisting of an a emblage of two or more atoms chemically alike or different) may have similar motions, and with them there may be a ociated potential energy due to the separation of their constituent atoms. All energy dependent on such motion or position is cla ed as heat, using the term in its strictest sense. The popular acceptation of the term is different, and is a ociated with the relative hotne or coldne of a body as perceived by the senses—1.e., it is related to the temperature of the body, not to the heat-energy contained in the body. The term latent heat is applied to heat which when supplied to a body produces no change in temp. so as to be appreciated by a thermomenter—e.g., when a solid becomes liquid, the heat required for the change is called the latent heat of fusion of the solid; from liquid to vapour we have the latent heat of vaporization. Correctly speaking, the heat has been transformed into potential energy due to the separation of the molecules of the ice during the pa age of the substance into the liquid form. Radiant heat is not heat in the sense used above, but is energy of wave motion in the ether, and only differs from light in the length of its waves. In ordinary circumstances—+.e., when no change of physical state takes place— any change in the heat-energy contained in a body produces a change in the state of hotne or coldne of the body, and to this state the name temperature is given. In order to give numerical expre ion to temp. a scale is chosen. The common attribute of all temp. scales is that they have two definite points, fixed by reference to two definite conditions of a standard substance as regards its hotne or coldne , and this interval of temp. is divided into a _ certain number of degrees. Any instrument which will indicate by means of such a scale the temp. of a body is termed a thermometer. Being one form of the objective reality which we term energy, heat must be capable of measurement. ‘The unit of heat generally employed in scientific work is the calorie, which is the amount of heat required to raise the temp. of 1 gram of water from 15° to 16° GC. For engineering purposes the Brit. thermal unit is employed. It is the amount of heat required to raise the temp. of 1 Ib. of water by 1° F., and is equal to 252 calories. Another unit in practical use is the amount of heat required to evaporate 1 lb. of water at the boiling point under standard atmospheric pre ure, and is equal to 243,583 calories. The determination of a given quantity of heat in terms of these units is dealt with below. Heat must also have quantitative relations with other forms of energy, and it is found that 1 calorie is equivalent to 42 million ergs, and that the British thermal unit is equivalent to 776 foot-pounds. Heat may be transferred from one body to another or from one part to another of the same body in three ways— viz., conduction, convection, radiation. In most cases all three modes of heat transference operate together. In conduction, heat is pa ed from one particle to an adjoining particle at a lower temperature (or on a larger scale from one body to another in contact with it) until the temperatures are uniform. In some substances (good conductors—e.g. silver, copper, and metals generally) the transference takes place rapidly. In others (bad conductors—e.g., woollen cloth, wood, etc.), it takes place slowly. Heat has also very important effects when supplied to or withdrawn from a body. Indeed, there are few physical properties of matter which, otherwise constant, are not thus affected. The first effect is, in general, a change in temp. The relation between the amount of heat added to a body and the consequent rise in temp. is expre ed by the specific heat of the substance, and this is defined as the number of units of heat required to raise the temp. of unit ma of the substance by 1° C. The method of determining specific heats is detailed below. The next important effect of adding heat to a body is to change the volume, and in general all bodies increase in vol« ume when heated. The increase is expee numerically by the co-efficient of inear expansion, which is the increase in length of unit length of a substance when heated by 1° C. The co-efficient of cubicle | (or volume) expansion is correspondingly defined, and it is approximately thrice the linear coefficient. In gases, the coefficient of volume expansion is approximately *00366. The expansion of bodies when heated and their contraction on cooling finds many useful applications in industry and in the arts. Obviously, there is need for a substance that will not be so affected, and this has been found in an alloy of steel and nickel, containing 36 per cent. of nickel, known as Invar, which has the remarkably low coefficient of expansion of *00000087, about thirteen times smaller than the coefficient of either of its constituents. I¢ is now used for pendulum rods in clocks, and in various measuring instruments. The third important effect of heat on matter is change of physical state. In general, the continuous addition of heat to a body causes it to pa from the solid to the liquid, and then to the gaseous (or _ vaporous) state. It is probable that if the means were available, all bodies could be made to a ume any of these three states, provided that such a umption involves no. chemical change. Changes of temp. also cause changes in the viscosity of fluids, electric resistance magnetic properties of matter, ete. Lastly, there must be noted the important fact that ‘the velocity with which a chemical system strives to reach its state of equilibrium increases enormously with the temp.’ (Nernst). Usually, the tate at which a chemical reaction takes place is doubled or trebled by a rise in temp. amounting to 10° C. Calorimetry is concerned with the measurement of quantities of heat. The apparatus used is termed a calorimeter. In the metric system the unit quantity of heat is the calorie, and great calorie= 1,000 calories. Substances other than water, except liquid hydrogen, do not require so much heat for 1° rise—e.g., ice, mercury,copper, air, steam. Mercury absorbs about.003, copper about -091 calories. ‘These numbers are the specific heats (S.H.), and increase with increase of temp. of measurement. Carbon, boron, and silicon show this increase remarkably, so that at high temperatures they tend to conform to Dulong and Petit’s law, specific heat—atomic weight =6'4 nearly. A simple calorimeter may made from a small sheet-copper ve el supported on corks and isolated from air-currents by being placed inside a larger ve el. Such a _ ve el has a definite water-equivalent, which may be found by placing a given ma of heated water (A) at temp. ¢ inside it and stirring it, when a rapid fall takes place due to heat taken by the calorimeter and the stirrer. If tc is the original temp. of the calorimeter and tf the final temp. of the water and calorimeter, the water equivOnce this is found, the specific heat of a body may be found where it is practicable to place such a body at a given temp. in a given ma of liquid in the calorimeter. If M be the ma of the substance heated to temp. ts m the ma of the water (any liquid of known specific heat would do), to the original temp. of the water, we the water equivalent of the calorimeter, and éf the final temp. of the mixture, then the specific heat (S.H.) of the substance is given by some other liquid of specific heat sh be to). _ For any body the specific heat — ma gives the water equivalent or thermal capacity. A substance with a low specific heat like mercury comes rapidly to the temp. of its environment and is suited for thermometers. Water, again, stores much heat, is heated and cooled slowly, and is adapted for heating purposes. This property of water also causes certain meteorological effects. For gases and vapours there exist a specific heat when the volume is kept constant, and a specific heat when the pre ure is constant. The latter is greater than the former, owing to the work done in expansion against the pre ure. The specific heats of steam in steam-engine theory and of air in gas and oil-engine theory are of high importance. Again, the latent heat of fusion of a solid, e.g. ice, is determined by finding the number of-calories of heat required to convert 1 gram of ice at 0° C. into water at the same temp. Similarly the latent heat of steam (more correctly of vaporization of water) is determined.