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.