nize another reason for believing that Nature will never attain absolute equilibrium, from the variety of forces ever abiding together within her sphere.
Fourthly, our present inability to obtain the movement of masses from, the motion of molecules or small masses—that is, the derivation of work from uniformly heated matter—does not decide that such conversion is impossible in Nature, or even to science in the future. While it is perfectly right to reason from what we know, it is of yet higher importance to constantly bear in mind how little we know. It would require infinite knowledge to say that the motion of uniform heat may not be transformable into phases of energy quite as diverse as high and low temperatures.
Such a change would not contravene the truth of the uncreatability or indestructibility of force, but would simply be an enlargement of the known, which every one feels to be indefinitely small as compared with the knowable. The possibility here suggested may be conceivable as depending on the definiteness in size of atoms; or, on the variety of motions to which the differences between atoms, as chemical elements, may give rise—differences in size and form; or, on the variety of motions implied by the checks offered to steady accessions of temperature, already explained in this paper.
One of the first principles to which the mind clings as fundamental is, that every truth has its converse. Although this may seem an axiom, yet its demonstration may be often very difficult, and, at times, even impossible. A knife-blade held over a gas-flame for a moment shows that hydrogen and oxygen combine to form watery vapor; yet the proof of the converse—the decomposition of water into its elements—demands extensive and powerful apparatus. Oersted, in a happy hour, noticed that an electric wire moved a magnetic needle; but years of experiment had to elapse before the electro-magnet and the magneto-electric machine established the complementary principles in a practical form. The analysis of compounds, chemically, is vastly easier than the building them up from their elements. We know the exact percentages of carbon, hydrogen, and oxygen, that go to make up sugar, and can express to a nicety the dynamical relations of the compound to its elements; but how to bring about the changes desired, with economy, is what puzzles us.
When we see high and low temperatures coming to an equality, it is certainly permissible to entertain faith in the possibility of the converse; in a change equivalent to a mass becoming, in its several parts, hotter and colder. To have recourse to such a supposition is less straining to the mind than the alternatives usually proposed by the theory under consideration.
If that theory be true, the question suggests itself, "Why has not the universe come to death by this time, for limits cannot be imagined to past duration?"