In spite of this, it was left for Joule and Colding, who worked almost simultaneously and by well-devised experimental methods from about the year 1840, independently to discover, and by degrees to enuntiate, by means of arguments founded on the only admissible basis—experiment, the grand law of the Conservation of Energy. In its most general form, the statement of the conservation of energy is merely a completed version of the passage we have already quoted from Newton; and the experimental discoveries of Rumford and Davy, extended and completed by Joule and Colding, allow us now to put Newton’s second or alternative interpretation of his Third Law of Motion into the modern statement of the Conservation of Energy.
In any system of bodies whatever, to which no energy is communicated by external bodies, and which parts with no energy to external bodies, the sum of the various potential and kinetic energies remains for ever unaltered.
In other words, while the one form of energy becomes changed into the other,—potential into kinetic and kinetic into potential, or one species of either into another;—yet each change represents at once a creation of one kind of energy and a simultaneous and equal annihilation of another, the total energy present, as we have already said, remaining for ever unaltered.
103. Taking as our ‘system of bodies’ the whole physical universe, we now see that, according to the test we have already laid down, energy has as much claim to be regarded as an objective reality as matter itself. But the forms of statement are most markedly different for the two. We before spoke of the quantity of matter without qualification, but we now speak of the sum of the two kinds of energy. Let us think for a moment of this, and we see that whereas (to our present knowledge, at least) matter is always the same, though it may be masked in various combinations, energy is constantly changing the form in which it presents itself. The one is like the eternal, unchangeable Fate or Necessitas of the antients; the other is Proteus himself in the variety and rapidity of its transformations.
Φύσις, διαδόχαις σχημάτων τρισμυρίοις
ἀλλάσσεται τύπωμα, Πρωτέως δίκην,
πάντων ὅσ’ ἔστι ποικιλώτατον τέρας·
τῆς δ’ αὖτ’ Ἀνάγκης ἐστ’ ἀκίνητον σθένος,
μόνη δ’ ἁπάντων ταὐτὸ διαμένουσ’ ἀεὶ
βροτῶν τε καὶ θεῶν πάντ’ ἀποτρύει γένη.[37]