But let us suppose the air removed or annihilated, and the projectile force (in the direction of a tangent), to be sufficient to carry the stone as far from the earth as gravity would cause it to descend each instant, and the stone would describe a circle round the earth, and if there were nothing to stop or obstruct it, it would thus continue to revolve without end. We have indeed this principle exemplified in the motion of the moon, which revolves in a void about the earth; we see moreover the same renewed, perpetual motion in the planets which pass in like manner through spaces destitute of all material resistance. We are hence lead to believe that matter is incapable of effecting any change in itself, either with respect to motion or rest, and once put into either of these states, it would continue in this state so long as it should remain undisturbed by any cause foreign to itself. This indifference to motion and rest, this want of all power of self-direction, has obtained the name of inertia. There is one class of bodies, however, that seem to form an exception to this law of matter. It comprehends those which we call animated, which put themselves in motion or stop themselves by an act of the will; but even in these the material elements which constitute their parts or members, and these members themselves, are perfectly inert. It is their union or combination that possesses the quality of life. Separated, they have no longer this power, but return to the condition of ordinary matter. We are entirely in the dark with regard to the cause of this remarkable difference in the bodies that surround us. As to what constitutes a state of life, we can pretend to no knowledge whatever. But seeing matter under all other circumstances destitute of the power of self-direction, and knowing also that in living beings it loses this faculty by death and by sleep, we are led to regard it as foreign to the essence of matter, and to consider the volition of animated beings, as the act of an immaterial principle which resides within them. We are unable to say in what part this principle is seated, or in what it consists, and still less how, being immaterial, it is capable of acting upon matter; but with the little attention that we have paid to ourselves and to the objects about us, these obscurities, unfortunately too common, in which our imperfect knowledge has left us, ought not to be made the grounds of an objection against the essence of things with which we must be contented to remain unacquainted. So that we here proceed philosophically, according to the rule adopted in other cases, by bringing together things that are analogous, and making the motion of animated beings to depend upon a cause foreign to matter; matter being found inert under all other circumstances in which we have been able to examine it. Another rea son is given in the schools of philosophy for attributing spontaneous motion to an immaterial principle; namely, that the will, by the very nature of its acts, can proceed only from a simple being, and that consequently it cannot belong to a substance essentially compounded, or at least divisible and decomposable, like matter; but this metaphysical argument would carry us too far from our subject. We content ourselves with merely suggesting it; for all experimental purposes, it will be sufficient to consider the immateriality of the principle of volition as a distinction founded upon analogy, and the inertia of matter as a general property in the actual state of the world. 9. We are moreover made acquainted by experiment with several other properties of matter which are also accidental, that is, which seem not to be absolutely necessary in order that material bodies may manifest themselves to our senses, but the co-existence of which with the primitive conditions of materiality is important to be known, since it supplies the want of other evidence, in a great number of cases in which the essential properties do not admit of being recognised. Such, for example, is gravity. Among natural bodies which we can see and touch, none is to be found which is not heavy, that is, which does not tend to fall toward the centre of the earth when left to itself; and since these properties are always found to accompany each other, the presence of the one is with respect to us always a sufficient ground to infer the existence of the other. Thus, although we can neither see nor touch the air, as we can see and touch other bodies, still we believe it to be a material substance, because it is heavy, capable of being confined in vessels and of exhibiting other phenomena, all similar to those which belong to a heavy fluid. A careful examination of these properties teaches us at length that there are airs of very different kinds, which are all so many substances differing essentially from each other in the action which they are capable of exerting on other bodies, and which is exerted in turn upon them by these bodies. 10. Moreover attraction is one of those contingent properties which supplies what is wanting in the evidence furnished by the immediate testimony of the senses. I have said that the particles of all known bodies exert upon one another attractive and repulsive forces. On the other hand when we can demonstrate the existence of these forces in an unknown principle, we infer that this principle is material. Thus, light is not tangible, it is not, so far as we can perceive, extended; it has no weight, or at least none capable of being appreciated by our balances. It is so subtle as to elude all the ordinary methods by which matter manifests itself to the senses. But by causing it to pass through transparent bodies, as glass, water, &c., it deviates from a direct course in its passage, and is bent precisely as if it were repelled by a force proceeding from the surface, and attracted on the other hand within by the particles which compose the transparent body. We know also that it employs a certain time, very short indeed, but yet capable of being estimated, in passing from luminous bodies to us. In fine, by subjecting rays of light to certain tests, we find that transparent bodies attract and repel them differently on certain sides from what they do on others. From these properties, taken together, we are led to conclude that light is a material substance, composed of particles extremely small, the form of which is symmetrical on certain faces, which are susceptible of particular attractions and repulsions, and which move in free space, and through transparent bodies, with a given and determinable velocity. Such 11. There are still other principles which act upon material bodies without being either visible or tangible, or susceptible of being weighed by our balances, which even present much fewer indications of materiality than light, and which notwithstanding are believed to be material substances. is the unknown principle of electricity. Nothing absolutely material has yet been detected in the cause of electrical phenomena, nothing indeed which does not admit of being explained without the supposition of matter. Still in its distribution over bodies, in its passage from one to the other through the obstacles which separate them, this principle acts in a manner so exactly conformable to the laws of equilibrium and motion which belong to fluid substances, that we can on this hypothesis calculate with the utmost precision, and in all their details, the phenomena that It seems extremeare to take place under given circumstances. ly probable, therefore, that the principle in question is a fluid, and that it is accordingly material. The same reasoning is applicable to the principle of magnetism, which manifests itself in several metals. 12. We have still less evidence of any thing material in the principle of heat. Not only does it want, like the preceding, the sensible properties by which matter is characterized, but the laws of its motion and equilibrium, not being completely known, we cannot arrive at the same probable conclusion in this case as in the former. By following it however in our experiments, we find that it diffuses itself in bodies, passes from one to another, modifies the disposition, the distances, and attractive properties of their particles. But all this does not prove incontestibly, that the principle in question is itself a body. The strongest argument in favor of its materiality is derived perhaps from certain analogies, lately discovered, between the radiant properties of heat and those of light, which lead us to believe that one of these principles may change itself gradually into the other, that is, they may acquire and lose successively the modifications by which they are respectively distinguished. The developement of these analogies furnishes a most important subject of investigation. 13. It will be perceived from what has been said, that all bodies of a sensible magnitude, the materiality of which can be immediately determined, consist in the grouping together of a multitude of material particles of extreme minuteness, in which a difference in the mode of aggregation is the only circumstance that constitutes a body solid, liquid, or gaseous. There are moreover strong reasons for believing, as we have seen, that these particles are inert masses, incapable from any inherent power of their own, of modifying themselves, and susceptible only of of obeying causes from without; whether this want of choice and self-direction is, in fact, as observation seems to prove, a general and essential characteristic of matter, or whether we so regard it intellectually for the purpose merely of considering by themselves those properties which remain to matter, after it is deprived of this. Now material particles being considered as in this inert state, there will hence arise, in the phenomena which their aggregation presents, certain necessary conditions, which are applicable to all bodies, independently of the chemical nature of their constituent parts, being the simple consequences of Mech. 2 their materiality. Such are the general laws of equilibrium and motion, which are deduced indeed mathematically from the sin gle property of inertia. 14. We have already used the words rest, motion, and force, as making a part of ordinary language. It now becomes necessary to fix their meaning with precision. We begin with defining the place in which the phenomena under consideration are supposed to occur. In order to this let us conceive of space without bounds, immaterial, immoveable, and of which all the parts, similar among themselves, are capable of being penetrated by matter without opposing the smallest resistance. Whether space in this sense exist in nature or not, is of little consequence; the definition presents to us merely an abstract extension. Now imagine in this space the particles of which we have been speaking, the material elements of bodies, and let us first consider with respect to them the mere circumstance of their existence. This simple fact will be capable of two distinct modifications; it may be that the same particle shall remain without change in its actual place, or that by the influence of external causes it shall leave its place to pass to some other part of space. The first of these states constitutes absolute rest, and the second motion. 15. But we can conceive further, that two or several particles are displaced at the same time, and impressed with a common motion, preserving with regard to one another their respective positions. Then if we consider them with reference to immoveable space, they will actually be in absolute motion; but if we consider them simply in their mutual relations to one another, these will continue the same as if the whole group had remained at rest; and if there were upon one of these particles an intelligent being who should observe all the others, it would be impossible for him to decide from this observation alone, whether the whole system were in motion or not. The permanence of these relations in the midst of a common motion, is what we understand by relative rest. This will be the condition of a number of bodies placed in a boat and abandoned to the course of a smooth stream. This is indeed the condition of all the bodies about us so long as they remain fixed to the same point of the terrestrial surface. They are at rest among themselves; but the earth which turns daily on its axis, impresses upon them a common |