The theory is a plausible one, and the reputation of its authors must command for it respectful consideration; but it is as yet a long way from being an established theory which can be accepted as a true representation of facts. In the first place it is based solely on mathematical theory, and not, as in the case of atoms and light-waves, upon actual facts of weight and measurement tested by experiment, and to which mathematical reasoning affords only an aid and supplement. No one has proved the existence of such a medium or of such vortex rings, much less weighed or measured them.
Moreover the theory is open to some very obvious objections. How can aggregations of imponderable matter acquire weight, and become subject to the law of gravity, which, as we have seen, is one of the essential and permanent qualities of atoms? If a cubic millionth of a millimetre of ether formed into a big vortex ring of, say, an atom of mercury, has a weight equal to 200 times that of an atom of hydrogen, which itself has a definite weight, why has it no weight in its original form? And if it had weight, however small, how could the enormous mass of ether filling all space produce no perceptible effect on bodies, even of attenuated cometic vapour, revolving through it with immense velocities? Again, how could these innumerable vortex rings be formed out of the ether without disturbing the uniformity and continuity of the medium, which are essential for the propagation of the light-waves through it? And how could the motions requisite to form the vortex rings be impressed on them de novo consistently with the principle of the conservation of energy? Energy can no more be created out of nothing than matter, by any process known in nature or conceivable by the human intellect; and to assume it is simply a more refined manner of falling back on the supernatural, which is itself only a more refined manner of saying that we know nothing.
For the present, therefore, we must be content with atoms and ether as the ultimate terms of our knowledge of the material or quasi-material components of the universe.
CHAPTER IV.
ENERGY.
Energy of motion and of position—Energy can be transformed, not created or destroyed—Not created by free will—Conservation of mechanical power—Convertibility of heat and work—Nature of heat—The steam-engine—Different forms of energy—Gravity—Molecular energy—Chemical energy—Dynamite—Chemical affinities—Electricity—Produced by friction—By the voltaic battery—Electric currents—Arc light—Induction—Magnetism—The magnetic needle—The electric telegraph—The telephone—Dynamo-electric engine—Accumulator.
Those ultimate elements, however, atoms and ether, only give us what may be called the dead half of the universe, which could not exist without the constant presence of the animating principle of force or energy. Energy is the term generally adopted in the language of science, for force is apt to be associated with human effort and with actual motion produced, while energy is a comprehensive term, embracing whatever produces or is capable of producing motion. Thus, if we bend a cross-bow, the force with which it is bent may either reappear at once in the flight of the arrow, if we let go the string; or it may remain stored up, if we fix the string in the notch, ready to reappear when we pull the trigger. In the former case it is called energy of motion, in the latter energy of position. It is important to realise this distinction clearly, for many of the ordered and harmonious arrangements of the universe depend on the polarity, or conflict with alternate victories and defeats, between those two forms of energy.
Thus if A B is a pendulum suspended at the point A, if we move it from its position of rest A C to A B and hold it there, its whole energy is that of position. If we let it go it swings backwards and forwards between the positions A B and A D, and but for the resistance of the air and the friction at the point of suspension, it would so swing for ever. But in thus swinging what happens? From A B to A C energy of motion keeps gaining on energy of position, until when the pendulum reaches C, it has annihilated it. Energy of position has entirely disappeared, and the whole original force expended in raising the pendulum to A B exactly reappears in the force or momentum of the pendulum at its lowest point. But is this victory final? By no means; energy of position having touched bottom, gathers, like Antæus, fresh vigour for the contest, and from the position A C upwards it gains ground on its adversary until when the pendulum reaches A D it is in its turn completely victorious.