英文科學(xué)讀本 第五冊(cè)·Lesson 10 General Properties of Matter

Lesson 10 General Properties of Matter

You all can tell me what we mean by the properties of a body said Mr. Wilson. "Our lessons, from the first, have been concerned in investigating the properties of all sorts of bodies. We have found one body to be tough, another brittle; one transparent, another opaque; one soluble, another insoluble; one rigid, another flexible. The toughness or the brittleness, the transparency or the opacity, the solubility or the insolubility, the rigidness or the flexibility is in each case a property of some particular body, but not of all bodies.

We are now going to take this word property in a much wider sense, because it has been found, after very careful investigation, that there are some properties which do not belong to particular bodies only, but to every kind of matter. Such properties as those we have just named, we might call special properties, because they belong only to special bodies. Those which we have next to consider, we shall call general properties, because they are common to matter of every kind. There is no kind of matter to which they do not belong, in a greater or less degree.

These general properties of matter are extension, divisibility, weight, porosity, compressibility, elasticity. Let us take first the property of extension.

See: I will place my closed hand on the table—so. Now one of you shall come to the front, and, without moving my hand, shall put his own in exactly the same position—not on top of it, not at the side, but exactly in the same place. You cannot do it; of course not. Here are two large stones about the same size. Could you place one on the table, and, without taking that away, put the other exactly where it stood? No, you cannot do it.

The hand and the stone must occupy a certain amount of space or room, and what is true of them is universally true. Every kind of matter—solid, liquid, or gas—must occupy a certain amount of space or room. This little word room will convey to you the simplest idea, then, of what we mean by extension.

Sometimes we consider this property of extension in one direction only, as when we measure the length, the breadth, or the thickness of a body. Sometimes we have to consider the length and breadth of a body combined. This too is extension—extension of surface. Sometimes, in addition to the length and breadth, we have to consider the thickness. This again is extension—extension of space or volume. I fill this basin with water to the brim. Watch what happens when I thrust my fist into it. The water overflows. As soon as I remove my hand, the water falls in the basin, and we can see how much was forced out. Let us fill the basin again, and instead of my hand, I will drop the stone in it. The result is the same; some of the water overflows as before.

Why was the water driven out each time? No doubt you will say that it was to make room for the hand in the one case, and the stone in the other. So it was. The amount of water driven out would have occupied exactly the room, or space, which the hand and the stone required respectively. The water and the stone cannot occupy the same space, at the same time. When I drive a nail into a piece of wood, the nail gets in only by thrusting aside the particles of the wood. It would not be possible for the wood and the iron to occupy the same space at the same time.

What is true of these things is universally true of every kind of matter. No two bodies can occupy the same space at the same time. Let us pass on now to the property of divisibility.

Our lessons have shown us that matter of all kinds can be separated, or divided, into minute particles by various means—sometimes by mechanical grinding, sometimes by dissolving, sometimes by boiling and evaporating. I have here a test-tube filled with water, and I will drop into it a small piece of some powerful dye. Even a piece of common stone-blue will answer the purpose as well as anything. I stir it well, and soon the whole of the water is colored. More than this, the least drop I could remove from the tube would retain the color. Imagine the test-tube to contain ten thousand such drops, and it is evident that the piece of coloring matter must have been divided up into at least ten thousand particles. This is what we mean by divisibility. It enables man to reduce all bodies to any required size. On the other hand, it is this property that causes bodies to wear away; even the rocks wear away by reason of it, and become particles of loose dust.

Weight is another property which every kind of matter possesses. I think you can tell me what it is that gives bodies weight.

The force of gravity attracts all bodies towards the center of the earth, sir, in proportion to the amount of matter which they contain, said Fred. "A drop of water poured from a glass falls down, and not up, for the same reason. We know that bodies have weight, for we can feel the muscular exertion of holding them in the hand, while gravity is attracting them downwards."

Quite right, Fred, replied Mr. Wilson. "But gases too have weight. Can you tell me how we learn this?"

Yes, sir. You have only to take an air-tight, stoppered bottle, exhaust all the air from it by means of the air-pump, and weigh it. Then, if you remove the stopper, allow the air to enter the bottle, and weigh it again, the difference in weight will be the weight of the air. If your bottle held exactly 100 cubic inches, the air in it would weigh 31 grains.

Right again, my lad. But if I had filled the bottle with hydrogen instead of air, the increase of weight would have been only 2 grains. That is, 100 cubic inches of hydrogen weigh just 2 grains. But this shows us that even hydrogen, the lightest of all bodies, has some weight.