CHAPTER XVIII.

ATTRACTION IN SOLIDS AND FLUIDS.

You saw by what I told you in the latter part of the last chapter that the great difference between a solid and a fluid is that the particles of a solid are fastened tightly together, while those of a fluid are not. If you should tie some people tightly together so that they could not move away from each other at all, they would be like the particles of a solid. If you moved them, you would move them all together as you do a stick of wood, a lump of ice, or any thing else that is solid. You can not move them, one one way, and another another way, as you can the particles of water; but if they are all pretty close together, and yet can move about among each other, as you often see in a crowded company, they are like the particles of a fluid. You can make your way among them just as you do among the particles of water when you wade.

[Sidenote: Attraction of the particles of solids illustrated.]

But you will ask, Are the particles of a solid really tied together in any way? No; but there is something that does the same thing to them as tying together would. It makes them stick together very tight. We know not what it is, but we call it attraction. We say that the particles of a solid attract each other very much. This is really just what a child would mean by saying that they stick together very close or very tight. Why they thus attract each other, or how they do it, no one has ever yet found out.

[Sidenote: Experiment with India-rubber and with bullets.]

It seems to be necessary that the particles should be very near together to attract each other as hard as they do in a solid. If a solid is divided in any way, you know that you can not make the two parts stick close together again. The reason is that you can not bring the particles near enough to each other to hold together. This is commonly so, but not always. If you divide a piece of India-rubber, making a smooth cut with a very sharp knife, you can press the two parts together so as to make them adhere. Boys often try the following experiment: A piece is cut off from two bullets, and each cut place is scraped as smooth as it can be. The two bullets are then pressed together at these smooth surfaces, and they adhere so well that it takes considerable pulling to get them apart. Here enough of the particles on the surfaces are brought near enough together to hold on to each other, or to attract each other, as it is commonly expressed.

[Sidenote: Drops of water.]

The particles of solids, then, attract each other very much, and it is this attraction that makes them solid. But how is it with the particles of liquids? Do they not attract each other? See that drop of water on a window. Why is it in the shape of a drop? If the particles of water did not attract each other they would be spread out on the glass. They would not be in the shape of a drop. They do not attract each other very much, but enough to keep them together in that shape.

But you can spoil that drop very easily. Put your finger on it, and it is gone. It is all spread out now, partly on your finger and partly on the glass. Why is this? It is because the particles attract each other so little that they are easily separated.

[Sidenote: Drops of water and shot compared.]

Put your finger on a shot, and it remains shot still. Why is it not gone like the round drop of water? Because its particles attract each other so much that they are not easily separated. A mere touch will separate the particles of the drop of water, and make them roll about any way; but you can not do this to the shot without heating it very hot. You can melt it, and then it will be, like the water, a liquid. Its particles now attract each other but little, just as the particles of water do. And then, again, you can freeze the water, and its particles attract each other like the particles of the solid shot.

[Sidenote: Quicksilver.]

In some fluids the particles attract each other more strongly than they do in others. And the more they attract each other, the better they keep their drop shape. Pour a very little quicksilver on a flat surface. See the round drops of it roll about! How well they keep their shape! If you touch them you do not spoil them, as you do a drop of water when you touch it. If you break one as you touch it, its parts make only so many little drops or balls. Why is this? It is because the particles of the quicksilver attract each other so much more than the particles of water do. They are so attractive to each other that they are disposed to keep together in little companies.

[Sidenote: Drops on leaves.]

You sometimes see drops of water on the leaves of plants more round and separate than you see them on window-panes. They roll about like the little balls of quicksilver. See the reason of this. The particles of the drop like each other, as we may say, better than they do the leaf. They are more ready to stick together than they are to stick to the leaf, and so they roll about on it like little balls. As you see the drops on the glass, they are not round, because the particles on one side stick to the glass--that is, they are attracted by it; but the leaf does not attract the particles so much as the glass does, for it lets them keep together in a round form. There is a difference between different leaves about this. On some, the drops of water act as they do on the window-pane, and on others they do as I have just told you; and then, on the same leaves, the drops act differently at different times.

[Sidenote: Oil on water.]

If you pour a little oil on water, you see the oil floating in drops. This is for the same reason that water stands in round drops on some leaves. The water has no attraction for the oil, and so the particles of the oil hold together in little companies on the surface of the water. It is different when oil is spilled upon cloth or wood. It has so much attraction for them that it mingles up with their fibres, instead of forming into round companies as it does on the water.

[Sidenote: How shot are made.]

[Sidenote: How shot are round.]

Whenever there is a little of any liquid by itself, it tends to take a round shape, as seen in the quicksilver, and in the drops of water on windows and leaves. We see a pretty example of this in the manufacture of shot. Perhaps you have seen a shot tower. It is very high. All the shot that are made drop from the top to the bottom. At the top they have the melted lead. They pour it into a sort of cullender--that is, a vessel with holes in it. These holes are quite small. From each one of these holes come out, one after another, drops of the melted lead. Each drop is round. It cools as it goes down all this long distance in the air, and by the time that it gets to the bottom of the tower, it is cold and solid. The shot all fall into a tub of water, so that they may keep their round shape.

Now why is it that the shot are round? Simply because when they begin to fall they are melted lead--that is, a fluid. Their particles are disposed, therefore, to hold together in a round form, like the particles of quicksilver, or of a drop of water.

[Sidenote: Bullets.]

Bullets are made by pouring the melted lead into moulds. Think, now, why they can not be made in the same way that shot are. The reason is that there are more particles in a bullet than can hold together in a round shape while the lead is fluid. You can not have very large drops of any fluid. The particles will hold together only in small companies.

[Sidenote: Making soap-bubbles roll.]

There is one thing that you can do with soap-bubbles which perhaps you have never thought of. You can make them roll on a table or on the floor by blowing them along. The reason is that the particles of soap and water mixed together hold on to each other, or attract each other, better than the particles of water alone.

_Questions._--What is the great difference between a solid and a fluid? Give the comparison about a crowd. Do we know what it is that fastens the particles of a solid together? What is it called? What is said about the particles being near together? Tell about the experiment with the India-rubber and the lead. How do liquids differ from solids in attraction? Why is water on a pane of glass often in drops? Why is it that you can spoil a drop by a touch? Tell how a shot differs from a drop of water. Is the attraction between the particles alike in all fluids? Tell about the quicksilver. Tell about the drops of water on leaves. Tell about oil dropped upon water. How is it with oil spilled upon cloth or wood? Describe shot-making. Why are the shot round? How are bullets made? Why can not they be made in the same way that shot are? What is said about soap-bubbles?