CHAPTER III.

THE ESSENTIAL PROPERTIES OF MATTER.

44. =Extension.=--You can not conceive of any portion of matter, however small it may be, that has not shape or figure. It may be so small as to appear only as a point to the naked eye, but seen through the microscope its shape becomes obvious. Even an atom must have length, breadth, and thickness, though it be so small that we can not measure it, nor see its shape with the most powerful microscopes. _Extension_, which is the term commonly used to express this idea, is then an _essential_ property of matter; that is, it is a property of which no form or kind of matter can be destitute. The distinction, in this respect, between this property and those which I have before noticed may be made obvious to you by an example. Hardness is not an essential quality of matter, for some kinds of matter are destitute of it; but no portion of matter, hard or soft, can be destitute of extension or shape. The air is sometimes spoken of in common language as being shapeless. This is partly because it is invisible, and partly because no portion or body of air assumes any definite shape. But air is continually forced into definite shapes by confinement in rooms, boxes, etc.; and then its extension in different directions can be measured as accurately as the extension of a solid can be. And besides, the atoms of which air is composed are undoubtedly solid, and we can not conceive of their existence without attaching to them the idea of figure or extension.

45. =Impenetrability.=--In common language one substance is said to penetrate another. Thus a needle penetrates cloth, a nail penetrates wood, etc. But this is not strictly true. The needle does not go into the cloth, but goes between the fibres of it, pushing them to the one side and the other. So the nail goes between the fibres of the wood, and not into them. It does not occupy the same room that the fibres do at the same time. So, also, no atom of matter can penetrate or go into any other atom. It can only push it out of the way, and then occupy its place. Impenetrability is therefore said to be one of the essential qualities of matter. This means simply that no portion of matter can occupy the same space with another portion of matter at the same time.

46. =Illustrations.=--Many illustrations might be given of this property. I will give a few. If you press a tumbler into water with its open end downward, you can not fill it with water, for the air confined in the tumbler prevents it from rising. It can not occupy the same space with the air. It fills, indeed, a portion of the tumbler, but this is because the air is compressible. If you introduce a glass funnel, _a_, Fig. 6, into a jar of water, _b_, with your thumb on its mouth, _c_, the water will not rise to fill it. But if you take your thumb off, the water will rise to the level of the water outside of the funnel, pushing up the air before it. If you have not a funnel, a vial or bottle with its bottom broken off will answer for the experiment. The following is a very pretty experiment, illustrating the same point. Place a lighted taper, _a_, Fig. 7, on a large flat cork in a jar of water. Put over it an open jar or receiver, _b_, having a stop-cock, _c_. Closing the stop-cock, press the receiver down into the water, and you will see the taper sink with it, as represented in the figure, the air preventing the water from entering the receiver. If now you open the stop-cock the water rushes in, thrusting the air upward, and making the taper to appear as if rising out of the water. The diving-bell offers a good illustration. It consists of a vessel, _a a_, Fig. 8, shaped like a bell, made sufficiently heavy to sink in water. It is let down by a chain and cable, as seen in the figure. The water does not enter the bell any farther than the compressibility of the air permits it. In order that the men in it may remain under water for some time fresh air is supplied by the tube _b_, it being forced down by a forcing-pump. At the same time the vitiated air can be let off by a valve provided for that purpose. There are windows in the top of the bell to give the requisite light for work on the sea's bottom. Treasures are often recovered by this means which would otherwise be lost. You see a resemblance between the diving-bell and the arrangement in Fig. 7, the receiver representing the bell and the lighted taper the men in it.

47. =Other Illustrations.=--If you drop a bullet into a tumbler of water it pushes the particles to the one side and the other, and occupies the room thus made. If you drop in several there is a manifest rise of the water, and you may drop in enough to make it overflow. The same thing is true of the finest needle dropped in the water--it does not penetrate it, but like the bullet displaces some of its particles and occupies their room; and you can make the water overflow by dropping in many needles. We can truly say, then, that water can not be penetrated even by a needle. When any substance, as sugar, is dissolved in water, its particles do not penetrate the water, but go into the spaces between its particles. So, also, when particles from odorous substances are diffused in the air, they are not really in the air, but are between its particles.

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.