TRICITY (including MAGNETISM); V. ANIMAL FORCE; VI. VITAL FORCE, having, as some most irrationally suppose, an analogue in inorganic masses, which may be called crystalline force. (This idea is examined further on.) Of these, I., II., and some forms of III., are mare immediately connected with matter than the others—that is to say, that the remainder almost necessitate the hypothesis of the existence of some medium unlike ordinary matter, or, in popular language, an imponderable. The almost universal opinion or physicists, however, seems to be, that even 'the former must be accounted for in sonic such way. Newton, in his second letter to Bentley, says, with respect to gravi tation (and it is obvious that similar language is applicable to molecular forces gener ally): " You sometimes speak of gravity as essential and inherent to mutter. Pray, do not ascribe that notion to me, for the cause of gravity is what I do not pretend to know." And again in the third letter: "It is inconceivable that inanimate brute matter should, without the mediation of something else, which is not material, operate on, and affect: other matter without mutual contact, as it must do, if gravitation, in the sense of Epicurus, be essential and inherent in it; and that is one reason why I desired yotL would not ascribe innate' gravity to me. That gravity should be innate, inherent., and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity, that. I. believe no man who has in philosophical- matters a competent faculty of thinking, can ever fall into it. Gravity must be caused by an agent acting constantly according to certain. laws; but whether this agent be material or immaterial, I have left to the ,con sideration of my readers." Of what that medium may consist, we cannot, of course, hazard even a conjecture; but if it be composed of separate atoms—i.e., not continuous —it is evidentthat a second medium will be required to help the particles of the first to act on each other (for without this, the first medium would be merely obstructive), and so on. This must stop somewhere; why not, then, at the first? But in the present state
of our knowledge of mechanics, a continuous medium is barely conceivable, and its motions, etc., present considerable difficulties to even plausible mathematical treatment. If we take the view opposed to Newton's, as Mosotti and others have done (their ideas are considered further on), we can, in a very artificial manner, however, account for gravitation and molecular action; but, as before said, the foundations of this attempt at explanation are hardly tenable.
Just as sound depends on the elasticity of the air and vibrations thereby maintained and propagated, light and radiant heat, which arc certainly identical, most probably consist in the vibrations of some very elastic fluid. This has been provisionally named ether (q.v.). If it be continuous, it may help us to account for the first two categories of force also, as we have already seen; if not so, as is more likely, fresh difficulties arise. Light and heat, however, undoubtedly depend on motion, and correspond, there fore, to so much vis-viva or actual energy. Even heat in a liquid or solid body must cor respond to some vis-viva in the material particles, since a hot body can give out both light and heat, and a body may be heated by luminous or calorific rays which are vibratory, as we have seen.
Class IV. contains perhaps the most puzzling of all these forces. That there is something in common in all the forms of electricity, and that magnetism is nearly related to them, is certain; it is probable, .also, that frictional electricity, when stat ical, consists in something analogous to a coiled spring, or is a form of potential ene•gy—the others being forms of kinetic energy. Some have supposed magnetism to be also a form of potential energy, but Ampere's discoveries have materially lessened the probability of the truth of this hypothesis. We shall consider this again.