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.