Moreover, elementary actions are either attractive or repulsive, and neither of them can be conceived without intensity and direction. Now, no direction is possible unless there be two points distinctly ubicated in space. And therefore the action, no matter whether attractive or repulsive, cannot reach any material point which is not distant from the matter of the agent. But if so, the action is independent of a medium of communication; for the material medium, if it were needed, should lie between the agent and the patient in such a manner as to link them together, and fill by its material continuity the gap by which they are separated; and if this were the case, the medium could not be set in motion, as its contact with the agent would exclude distance, and consequently the possibility of any direction from the agent to the medium itself.
Some will say that this argument proves nothing, as the direction of the action can be sufficiently accounted for by the direction of the impulse. But this conclusion is evidently wrong. For what impulse can they imagine to proceed from the sun to the moon? Uncultivated minds are easily deluded by unlawful generalizations. They apply to all actions what they imagine to agree with some special phenomenon; and because they see that in the case of impact there is an impulse in a certain direction, they hastily conclude that the direction of every action depends on the direction of some impulse. We may remark that, even in the case of impact, it is not safe to conclude that the direction of the movement will follow the direction of the impulse, unless the impulse be central, and the body impinged upon homogeneous. But leaving aside the theory of impact, which has nothing to do with the present question, what impulse can explain the continuous resistance of a body to statical forces? What impulse can account for the expansive tendency of gases, and for their continuous pressure against the recipients in which they are contained? What impulse, above all, can account for universal attraction?
We have mentioned this objection, not because it needed any scientific or philosophical discussion, but simply because it is one of those notions to which the prejudices of our infancy give easy admittance into our minds when we allow ourselves to be guided, as is often the case, by our senses and imagination, in matters pertaining in great part to the intellectual order. Our mistakes in the appreciation of the character and conditions of natural facts most ordinarily originate in the unwarranted assumption that, since the facts are sensible, our knowledge of them must wholly depend on our senses; whilst the truth is that our senses perceive the movements, but not the actions which cause them, and therefore do not see the entirety of the natural facts, but that portion only which is most superficial. “A fundamental fact, like an elementary principle, never fails us,” says M. Faraday, speaking of natural philosophy; “its evidence is always true; but, on the other hand, we frequently have to ask, What is the [pg 733] fact? often fail in distinguishing it—often fail in the very statement of it—and mostly overpass or come short of its true recognition. If we are subject to mistake in the interpretation of our mere sense impressions, we are much more liable to error when we proceed to deduce from these impressions (as supplied to us by our ordinary experience) the relation of cause and effect; and the accuracy of our judgment, consequently, is more endangered.”[163]
And now, let no one imagine that we have any intention of denying the existence of a material medium between the celestial bodies. We only deny that there is a medium for transmitting actions. Again, we do not deny that when the earth, for instance, acts upon the moon, the elements of matter lying between the earth and the moon exert their activity on one another. But we maintain that their actions are their own, and proceed from their own intrinsic and permanent power, and not from any extrinsic agent, and that such actions are not travelling from element to element till they reach the moon. Neither do we deny that the elements located between the earth and the moon are also acted on, for it is clear that gravity must tend to alter their position in space; but we hold that the whole possible effect of gravity on all such elements is movement, and that movement is a mere change of place, and not a transmission of the action by which it is produced. How the movements themselves are communicated from element to element we shall explain presently.
Meanwhile, from the fact that the elementary actions are independent of all material medium of communication, we infer that bodies, in attracting and in repelling, act with equal promptitude, and without loss of time, whether the distance of the body acted on be great or small. Time, in fact, follows movement; for without movement there is no succession. Now, the action of a body does not reach the distant body through movement—that is, through successive transmission; on the contrary, each element is, of its own nature, determined to act directly and immediately on every other element existing in the indefinite sphere of its activity. Hence a body will indeed act with a greater intensity at a less distance, but will not act sooner than at a greater distance. There have been scientists who surmised that the solar attraction may perhaps need time for reaching the earth and the planets, and therefore that the attraction may reach Mercury in a shorter time than Jupiter or Neptune. From what precedes it is manifest that the surmise is wholly without foundation. Light needs time for its propagation, because it consists in a kind of movement; but attraction, as we have just remarked, is not movement, and therefore is not dependent on time.
Propagation of movements.
We have shown that there is no material medium for the transmission of forces, if the word “forces” is taken to mean “actions”; but if the word is intended to express “movements,” then the material medium is quite indispensable. We read very frequently in scientific books that actions are transmitted; but as this is not true of the actions themselves, we must suppose that the phrase is intended to express only the fact of a progressive development of the effects resulting from those actions. In the same way, when we read that [pg 734] actions are conveyed through a material medium, we interpret this expression as meaning that a material medium is strictly required for the progressive development of the series of effects due to such actions. We will explain the fact by an example.
If, a mass of air being at rest, a string is stretched in order to elicit sound, the vibrations of the string will be communicated to the neighboring molecules of air by the action (not by the movement) of the string itself; these first molecules, being thrust out of their position of equilibrium, will, by their action (that is, by the exertion of a power residing in each of their component elements, not of a power coming from the string, nor by their movement, nor by transmitted action), put in movement a following set of molecules, and so on indefinitely; so that, in the whole series of molecular vibrations, each preceding molecule causes the motion of the following one, and causes it by the exertion of its own powers, not of any power transmitted. It is evident that the string cannot give activity to the molecules of air. These molecules, whether the string vibrates or not, have already their own activity and their own mutual action; only their actions balance each other as long as the mass of air is at rest. But when the string begins to vibrate, the equilibrium being broken near it, those molecules of air which first cease to be in equilibrium begin to act on the following molecules with a different intensity, according to the change of the molecular distance. Thus the movement by which the distance is altered is not the cause, but the condition, of the phenomenon.
What we say of air and sound applies to any other medium, as ether with its vibrations, whether luminous or calorific. The molecules of ether have their own powers, and exert them continually, whether there exists a flame determining a series of vibrations or not; but with the flame the first molecules of ether which are displaced from their position of equilibrium will acquire a new local relation with regard to the following, and their actions will be of a new intensity, sufficient to cause the displacement of the next set of molecules, and so on. The flame, then, causes the displacement of the first set of molecules; the first set displaced causes the displacement of the second; the second displaced causes the displacement of the third, etc.; each set producing its own effect by its own inherent powers, not by the exertion of any power communicated to them by the flame, and their displacement being not a cause, but only a condition, on which the intensity of the exertion depends.
Hence it appears that in phenomena of this description it is not the action, and much less the power, that is transmitted, but only the movement, or the formal perturbation of the equilibrium; and even the movement is not properly transmitted, but only propagated; because the movement of each following molecule is not the identical movement of each preceding one, but is a movement really produced in the very impact of the one on the other, as our reader must have easily gathered from our preceding discussion. And therefore one movement succeeds another indefinitely, the one being a condition for the existence of the other; which constitutes propagation, not properly transmission.