48. Inertia.—Matter has no power to put itself in motion. When it is moved it is moved by some force which is either outside of the matter or is communicated to it in some way. When your arm is moved it is not the matter in your arm that is the cause of its motion. It is caused by a force in you which I will not dwell upon here, because the subject belongs to Physiology. When air moves it is set in motion by some force acting upon it, as when you blow it from your lungs or move it with a fan. When the wind blows, the air is set in motion by heat and the attraction of the earth, as will be explained to you in another part of this book. I might multiply examples to any extent, showing that matter of itself can not move. This property of matter is termed inertia.
49. Inertia Shown in the Inability of Matter to Stop its Motion.—Matter, when once set in motion, has no power to stop itself. If it could stop itself it could not be said to be inert. And as it is inert, it would, when once set in motion, keep on moving forever unless stopped by some force. When a stone falls to the ground, it stops simply because the earth stops it. If the earth were not in the way, the stone would move straight on until it were stopped by something else. So, also, a stone that is thrown up in the air would keep on, and soon be out of sight, and never return to the earth, if it were not made to come down by forces acting upon it. One of these forces is the resistance of the air, which, from the moment the stone starts, is destroying its motion. Another force as constantly operating to retard the stone is the attraction, or drawing force, exerted by the earth upon it. This powerful though unseen force will be treated of fully in the next chapter.
50. Matter Equally Inclined to Rest and Motion.—It was formerly taught by philosophers that matter is more inclined to rest than to motion; and this is the popular notion now. This is because the chief causes that stop the motions that we see from day to day—viz., the air and the attraction of the earth—are not visible. For this reason, until we investigate the subject, it seems to us that motion has a natural tendency to stop, or is spent, as it is expressed. When friction has an agency in arresting motion we see it plainly; but the common idea is, that in this case the motion is in part spent and in part destroyed. But in no case is motion spent, but it is always destroyed by obstacles. The more thoroughly these are removed, the longer will the motion continue; and if they were wholly removed, the motion would never cease. Perpetual motion, then, is not in itself impossible; for matter has no more tendency to stop moving when once put in motion, than it has to begin motion when it is at rest. All motion would be perpetual if there were not forces opposing it. If there were only one body in the universe, and that were set in motion, it would move forever through empty space in a straight line; for there is no matter any where to resist its motion or to attract it away from its onward course.
51. Divisibility.—Though divisibility is a general quality of matter (§ 18), it is not an essential one. For if it be true that every portion of matter is composed of atoms that remain whole (§ 14), this property can not be said to belong to these atoms. It is only the bodies made up of these atoms that can be divided. When we come to the atoms themselves the division must stop.
52. Weight.—In speaking of the properties of matter I have said nothing about weight, although in the popular mind this is thought of as being one of the most prominent of the properties of some kinds of matter. This will be appropriately treated of when I come to speak of attraction, for it is a mere result of attraction. Suffice it to state here, that the weight of a body is the pressure occasioned by the attraction existing between it and another body. If when a stone is raised from the ground the attraction between it and the earth could be destroyed, the stone would remain there. It would not press down, and so would have no weight. It is plain, therefore, that weight, so far from being an essential property of matter, is not really a property at all. It is only an effect of a property—attraction. If there were only one body in the universe, it would have no weight, for it would press in no direction because there is nothing to attract it. But as it is, all matter has weight, for there is other matter to attract it.
[CHAPTER IV.]
ATTRACTION.
53. Nature of Attraction.—If you attempt to break a very tenacious solid substance, why do you not succeed? It is because the particles are so strongly fastened together. But how? By some kind of cement or glue, or by some mechanical contrivances as nails or hooks? No. They are fastened together by some unseen force. We know nothing of the nature of this force. We know only that it exists, and we call it attraction. The name is a proper one, for it simply expresses the fact that one particle attracts or draws another particle toward itself.
54. Newton's Idea of Attraction.—It was stated in § 20 that the particles of matter, even in the densest substances, are not in actual contact, but have spaces around them. Now it was supposed by Newton that there is some kind of ethereal substance pervading all these spaces, which causes this attraction between the particles. He supposed also that this ether was every where in space, causing attraction between masses of matter. But all this is mere supposition, and we know not whether there is this sort of ethereal glue keeping the universe together, or whether it is some property in the particles themselves that makes them thus attract each other. But the fact of the attraction we know, and we can observe the phenomena which it produces, and discover the laws or rules by which this force is regulated in its action.
55. Attraction in Solids.—Attraction is stronger in some solids than in others. The mason with his trowel easily divides a brick; but he can not do this with a piece of granite, for its particles have a greater attraction for each other than those of the brick. So a rap which would break a glass dish would not injure a copper one of the same thickness. A weight that would hang securely from an iron wire would break a lead wire of the same size; that is, it would tear the particles apart, because they are not strongly attracted to each other. Attraction has different modes of action in different solids. It therefore fastens their particles together in different ways, and thus produces all the various qualities, already noticed, which are so useful to us—tenacity, hardness, softness, ductility, flexibility, etc.