THE CONSERVATION OF ENERGY.
The total amount of Energy, Potential and Kinetic, existing in the universe is always a fixed quantity. It is not, however, like Force, rigidly bound up with the individual particles in which it is from time to time manifested. As we have already seen, it can be transferred from one particle or set of particles to another. For this reason it has been deemed desirable to embody the principle in different language from that which we employed in the somewhat analogous case of Force. While Forces persist, Energies are conserved. The concrete and practical results of this difference are enormous.
It does not come within the scope of the present work to give a full account of the quantitative relations subsisting between the various species of Energy; it will be sufficient to trace their equivalence in its broader qualitative aspect. For this purpose we may consider the phenomena of Conservation under three heads: the passage of Energy from the Potential Mode to the Kinetic, the passage of Energy from the Kinetic Mode to the Potential, and the passage of Energy from one species of the Kinetic Mode to another.
Potential Energy or relative statical separation has a tendency constantly to pass into the Kinetic Mode, under the influence of Force. Every free body or particle, unless restrained by an antagonistic Force, or kept in separation by a continuous Kinetic Energy, is aggregated at once with other bodies or particles which attract it. A mass poised on a ledge or suspended by a rope is prevented from aggregating with the earth by the Force of cohesion; but when some external Energy has pushed it off the ledge or severed the rope, its Potential Energy passes at once into the Kinetic Mode, under the influence of gravitation. Two molecules of water vapour are prevented from aggregating under the relatively feeble attraction of cohesion at a distance by their inertia — that is, by the relatively strong cohesion of surrounding or intervening matters (just as a mass on the table, though attracted by the earth, is prevented from aggregating by the intervention of the cohering boards) — but when some external Energy brings them within such a distance of one another that the resistances are overcome by their mutual attractions, their Potential Energy becomes Kinetic, and they aggregate with one another. Two atoms (having affinities for one another) are similarly prevented from aggregating by inertia; but when brought within the sphere of their mutual attraction, their Potential Energy becomes at once Kinetic, and they combine with one another. So also, two electrical units are prevented from aggregating in the Leyden jar by the electrical neutrality of the glass partition; but when a conducting medium is made to connect them, their Potential Energy passes into the Kinetic Mode and they rush together at once.
Kinetic Energy or motion often passes into the Potential Mode. The Kinetic Energy of actual separation always exhibits this interchange. A cannon ball fired in the air, the piston of a steam-engine forced up by the expansive Energy of the steam, a weight hauled by a pulley to a height, a man who has climbed a mountain, are all of them instances where Molar Kinetic Energy has become Potential. The liquid condition of water melted from ice, the diffused state of vapour raised from water, are instances where Molecular Kinetic Energy has become Potential. The free hydrogen and oxygen of an electrolytic bottle, the iron and oxygen driven from their combination by heat, are instances where Atomic Kinetic Energy has become Potential. The negative and positive electricities of a Leyden jar, of a thunder-cloud and the earth, of the knobs of an electrical machine, are instances where Electrical Kinetic Energy has become Potential.
Finally, Kinetic Energy often passes from one of its species to another. Molar motion passes into Molecular motion whenever one mass interferes with the motion of another. This is true whether the motion is aggregative, or separative, or continuous. If a cannon ball be allowed to fall to the earth from a position of Potential Energy, all the Kinetic Energy which the mass acquires in its fall passes to the molecular species when it touches the ground. If it be fired into the air, and immediately checked by an iron target, the same result occurs. And if a top be stopped in spinning or the moon checked in her course, exactly like effects are or would be produced. Molecular motion passes into molar motion whenever the free separation of the moving molecules is interfered with by the cohesion of enclosing masses. Thus the steam in a cylinder pushes up the piston by its expansion; the freed nitrogen in a discharge of gunpowder in like manner pushes out the ball; and the energetic movement of a heated gas bursts the vessel within which it is confined. Molecular motion also passes into atomic motion in decomposition by heat, and into electrical motion in the friction machine. Atomic motion passes into molecular motion when heat is generated by chemical combination. It also passes (apparently) into electrical motion in the galvanic current. Electrical motion passes into molecular motion when an interrupted current produces heat. Light, which is a phenomenon connected with the ethereal medium, must be neglected for the present.
This relation is quantitative — that is to say, a definite amount of Potential Energy passes always into a definite amount of Kinetic, and vice versa, while a definite quantity of each species is equivalent to a definite quantity of each other species, in either Mode. The law of conservation may therefore be subsumed under the following formula, where A stands for Potential and B for Kinetic Energy; 1, 2, 3, and 4 for the Molar, Molecular, Atomic, and Electrical species, and 5 for the Kinetic Energy of the ether (of which more hereafter):
A1 + A2 + A3 + A4 + B1 + B2 + B3 + B4 + B5 = a constant quantity.
But while the total of Energy, like the total of Force, is thus constant, the total of each mode and species varies from moment to moment. Whereas the total of each Species of Force is as constant as the sum of their totals.
Again, while each unit of Force is rigidly bound up with each atom of matter (with which it is perhaps identical), each unit of Energy may pass from one mass, molecule, atom, or electrical unit to another. It may also pass from matter to the ethereal medium, and vice versa. This can only happen, however, to Energy in the Kinetic Mode.
A mass in motion parts always with portions of its motion to all other bodies with which it comes in contact. It does so either by imparting to them a portion of its motion in the molar form (as when one billiard ball strikes another), or in the molecular form (as when heat is generated by friction). Hence every moving mass tends to part with all its Kinetic Energy more or less quickly, according as it is more or less impeded in its motion by more or less cohesion and gravitation. Thus a cannon ball parts with all its Molar Kinetic Energy at once when it strikes an iron target, and very quickly when it is fired in the air; a billiard ball parts with it more slowly, as it hits the other balls and the cushions; a quoit on ice more slowly still, as it meets the resistance of the air and the gentle friction of the ice; while a pendulum under an air pump hardly parts with it perceptibly by friction on its knife-edge, and a planet only by infinitesimal decrements to the ethereal medium. A molecule in motion parts similarly with a portion of its motion to every other molecule with which it comes in contact. When the two molecules, however, possess equal motions, or, as we oftener say, are at the same temperature, the amounts of gain and loss neutralise one another. But when the motions of the Molecules differ, the more energetic parts with a disproportionate amount of its motion to the less energetic, until the Energies of both are equal. Hence it happens that whenever the molecules of any mass have a higher Kinetic Energy than that of surrounding bodies, the motion of its molecules is imparted to the surrounding bodies till a state of equality is reached. As to Atomic and Electrical motions, we know too little of their nature to speak with any confidence, but we see at least that they also tend to pass away from the bodies with which they were associated, and to assume the forms of light and heat. In short, without fully anticipating the chapter on the Dissipation of Energy, we may say that whenever masses, molecules, atoms, or electrical units are free to act in accordance with their aggregative tendencies, without interference of antagonistic Forces or restraining power of continuous Kinetic Energies, they immediately unite, and impart their former Potential Energy in the Kinetic Mode directly to surrounding bodies, and ultimately to the ethereal medium.
We may thus summarise the contents of the present chapter: the sum total of all Energies in the Universe is a constant quantity; and whenever one mode or species of Energy disappears it is replaced by an equivalent quantity of another mode or species.