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.
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.
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.
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.