568 HEAT of the doctrine of the convertibility of forces, will be found in the article CORRELATION OF FORCES. The experiments of Rumford and Davy were made about 80 years ago, but were not at the time regarded as conclusive; nor were the more refined demonstrations of Thom- as Young of the truth of Huygens's theory of light. It seems to have required the later investigations of Fresnel, Cauchy, Malus, Mel- loni, Tyndall, Sir William Hamilton, and others, to adapt the undulatory theory to the expla- nation of all the phenomena of radiation, to render the mechanical demonstrations accepta- ble. It is interesting to observe the clearness with which Rumford and Davy so long ago sta- ted their views upon the nature of heat. In a tract published in 1798, giving an account of his experiments at Munich, the former says : " It appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and com- municated in the manner that heat was excited and communicated in these experiments, ex- cept motion." In a tract contained in a vol- ume "published at Bristol in 1799, Davy says: " Heat, then, or that power which prevents the actual contact of the corpuscles of bodies, and which is the cause of our peculiar sensations of heat and cold, may be defined a peculiar mo- tion, probably a vibration of the corpuscles of bodies, tending to separate them." In his "Chemical Philosophy," published in 1812, he says: "The immediate cause of the phenome- non of heat, then, is motion, and the laws of its communication are precisely the same as the laws of the communication of motion." The dynamical theory of heat may therefore be stated in almost the words quoted above. It holds that heat consists in the vibratory motion of the particles of matter, and that it may be produced by mechanical force, such as friction, percussion, or compression, or by the electric current ; or that it may be communi- cated by the undulatory ether, the medium of radiation. Its communication from one body to another when they are in contact, or through a homogeneous body, from particle to particle, constitutes conduction. Sources of Heat. Ac- cording to the nebular hypothesis of Laplace, heat is a primal force which caused all matter at one time to exist in a gaseous condition, which by the action of gravitation and other forces has been aggregated into masses assu- ming solid and liquid conditions. But the opin- ion has bee^i advanced by J. R. Mayer and Waterson, and more recently elaborated by Helmholtz and Thomson, that the sun owes its heat to the force of gravitation acting upon the particles of matter, which at the beginning are assumed to have been at considerable dis- tances from each other, and causing by their clashing together the evolution of heat. Ac- cording to either theory, the sun is regarded as a vast storehouse of radiant heat from which the earth derives its supply, and has done for myriads of years, through most of the geolo- gic ages. Estimations have been made by Pou- illet which show that the sun emits a quantity of heat per hour equal to that which a layer of anthracite coal 10 ft. thick would yield in combustion. Chemical combination, including the combustion of fuel, is a secondary source of heat, originally derived from the sun, which furnished the energy necessary to the forma- tion of the fuel. The intensity of heat pro- duced by combustion, attains its maximum in the oxyhydrogen blowpipe, in which a heat approaching 4000 F. is reached. Mechanical action, either in the form of compression, per- cussion, or friction, develops heat in quantities equivalent to the force converted into it. The electric current is another source of heat, and an example of the equivalent conversion of one force into another. When the current is pro- duced by the rotation of magnets, there is a conversion of the mechanical force expended in effecting the rotation into electricity, and this under proper circumstances into heat. When the current is produced by the chemical action of a battery, the origin of whatever heat is obtained may be considered as arising from the combustion of the elements in the battery. The intensity of heat obtained by the electric current is considerably higher than that of the oxyhydrogen blowpipe, but the amount cannot be stated with any degree of accuracy. Gen- eral Effects of Heat. The most obvious effects of heat on matter are to cause it to expand and to assume different states, as the solid, liquid, and gaseous. Thus, under the ordinary pressure of the atmosphere, water at a temperature be- low 32 F. is a solid ; between 32 and 212 it is a liquid ; and above 212 it is in a gaseous con- dition. With a few exceptions, an increase of heat in bodies causes them to expand. Thus, a metallic bar which has a diameter just sufficient to enable it to be passed through an orifice, will by being heated become too large ; the heat vibrations have been intensified, and the bar has increased in bulk. In what this slight expan- sion consists it is impossible to say with cer- tainty ; whether the molecules require greater space in which to vibrate, or whether they ex- pand by a slight separation of the atoms of which they are composed, is not known ; it is possible that both causes unite to produce the effect. When, however, a body by the appli- cation of heat is converted into vapor, a repul- sive force is generated between the particles when a certain temperature is reached which produces a far different phenomenon from or- dinary expansion ; an active repulsion has been generated, which may exist between mole- cules, as in the vapor of water or alcohol, of between the atoms. Moreover, the atoms com- posing molecules may be completely separated, molecular disintegration, and consequently de- composition of the substance, taking place. Heat when it has such an effect is called the heat of dissociation, and is required in dif- ferent degrees by different bodies. (See DIS- SOCIATION.) Thus, the atoms composing tho
