When a body is heated, it absorbs one part of the heat; the other part raises its temperature. The part absorbed increases the bulk or volume of the body, the expansion being the exact measure, or mechanical equivalent of the heat absorbed. In fact the coefficient of expansion is the fractional part of the expansion in length, surface, or volume of the body when its temperature is raised one degree. When the body is cooled, its volume is diminished, and then the contraction is an exact measure, or mechanical equivalent of the heat given out, and thus expansion and contraction are correlatives with and represent heat and cold.
Specific heat is the quantity of heat required to raise a given bulk or a given weight of a body a given number of degrees. In the one case it is distinguished as the specific heat for a constant volume, in the other for a constant weight.
Although the specific heat of a substance remains the same, its sensible and absorbed heat may vary reciprocally to a great extent.
As there can be no direct measurement of heat independent of matter, its mutations and action on matter are the sole means we have of forming our judgment concerning its agency in the material world.
Mr. Joule has proved that the quantity of heat requisite to raise the temperature of a pound of water one degree of the centigrade thermometer is equivalent to the mechanical work or force that would raise the same mass of water to the height of 1,389 feet. This is the unit, or mechanical equivalent of heat.
In fact, for every unit of force expended in percussion, friction, or raising a weight, a definite quantity of heat is generated; and conversely, when work is performed by the consumption of heat, for each unit of force gained, a unit of heat disappears. For since heat is a dynamical force of mechanical effect, there must be an equivalence between mechanical work and heat as between cause and effect. That equivalence is a law of nature. The mechanical force exerted by the steam engine is exactly in proportion to the consumption of heat, neither more nor less; for if we could produce a greater quantity than its equivalent we should have perpetual motion, which is impossible. When steam is employed to perform any work, the temperature of the steam is lowered; the heat that disappears is transformed into the force that performs the work, and is exactly proportional to the work done, and vice versâ.
The heat which is the motive force in the steam engine is due to the chemical combination of the carbon of the fuel with the oxygen of the atmosphere. A pound weight of coal when consumed in one of our best steam engines produces an effect equal to raising a weight of a million of pounds a foot high, yet marvellous as that is, the investigations of recent years have demonstrated the fact, that the mechanical energy resident in a pound of coal and liberated by its combustion is capable of raising to the same height ten times that weight.[[3]] The quantity of coal existing in the whole globe is believed to be inexhaustible, hence the energy in abeyance is incalculable. The chemical energy continually and actually exerted in the great laboratory of nature is greater than that which maintains the planets in their orbits.
The act of the combination of the atoms of carbon and oxygen in combustion is ‘now regarded exactly as we regard the clashing of a falling weight against the earth, and the heat produced in both cases is referable to the same cause;’[[4]] so chemical combination in combustion is only a particular case of falling bodies. Drummond’s light, the most brilliant of artificial illuminations, is produced by a simultaneous shower of the atoms of oxygen and hydrogen gas upon lime; and platinum, the least fusible of metals, is vaporized by a similar shower from the oxy-hydrogen blowpipe, and thus impetus generates both light and heat, for although the atoms are too small to admit of an estimation of their individual vis viva, there can be no doubt that like causes produce like effects.
In what manner or under what form magnetism and electricity exist when quiescent in matter we know not, but the compass needles show that numerous lines of magnetic force, subject to periodic and secular variations, perpetually traverse the earth and the ocean; and that waves of magnetic force occasionally sweep rapidly over great tracts of the globe. These phenomena would seem to stand in some periodic connection with the solar spots. Professor Lamont of Munich has discovered that a permanent and regular current of electricity is propagated parallel to the equator all over the earth, and another similar to it in the atmosphere. Besides these, there are currents of electricity in the surface of the earth, sometimes in one direction and sometimes in another, which decrease with the depth; and M. Lamont conceives that this electric system is the cause of terrestrial magnetism. Electricity of intense power and inappreciable quantity certainly exists in abeyance in the atmosphere and in all terrestrial matter till the equilibrium between the antagonist forces be disturbed, and then it bursts forth with terrific violence in the lightning flash and stunning crash of thunder. Since it requires electricity equivalent to that in activity during a thunderstorm to form one drop of water, what must that power have been which the Omnipotent wielded when he created that deep over the face of which ‘darkness brooded.’
Electricity, though the most formidable power in nature, is made available to man by the voltaic battery, and by the electro-magnetic induction apparatus, in the battery of which it is generated by the chemical action of dilute sulphuric acid on zinc. The positive and negative electricities thus produced pass in opposite directions through the two conducting wires of the machine by a continuous transmission of force or vibration from atom to atom, a circulation that is accompanied by a continual development of heat in overcoming the resistance it meets with in the wires. The electricity decreases as the heat increases, and vice versâ; the action is reciprocal. Thus electricity is merely a transmission of force. Mr. Joule has proved that the quantity of heat produced in a unit of time is proportional to the strength of the current, whatever may be its direction, and that its power to overcome resistance is as the square of the force of the current. The force is exactly in proportion to the chemical action which produces it, and that is measured by the quantity of zinc consumed in the battery. Thus chemical action produces electricity, and conversely electricity is a powerful agent in the chemical composition and decomposition of matter.