英文科學(xué)讀本 第五冊(cè)·Lesson 05 Solids, Liquids, and Gases

Lesson 05 Solids, Liquids, and Gases

Suppose we begin our new lesson, boys, said Mr. Wilson, "with one or two experiments.

I have here a piece of wash-leather. I will pour a small drop of mercury into it, and squeeze it between my fingers and thumb. The squeezing, you see, breaks up the one drop of mercury into thousands of very tiny drops, and forces them through the pores of the leather. I have purposely let some of them fall on my coat-sleeve; you can see the little silvery balls shining out from the dark cloth. Now I will pick up two or three of these tiny balls with my knife, and spread them over this piece of black cloth, just as one spreads butter on bread. Come and find them now, Fred.

I cannot see them now, sir.

No, Fred, you cannot see them with your naked eye, but look for them through this magnifying glass, and tell me what you see.

There seem to be thousands of tiny drops; I could not see one of them before. But did they all come from those two or three little balls, sir?

Yes, Fred, they did. But you have seen something of the same kind before.

Oh, I remember, sir. You boiled a few drops of mercury in a test-tube. We saw it boiling, and you told us that it was passing off as vapor, but we could not see it till we called in the help of the magnifying glass. We then found, all round the cool upper part of the tube, an immense number of silvery balls, which our eyes were not sharp enough to see, and you told us that each of these little balls was made up of untold numbers of still smaller globules, so small as to be invisible even to the most powerful microscope.

That's right, Fred, said Mr. Wilson. "And now what have we learned? We have learned that mercury can be split up into extremely small particles, so small as to be absolutely invisible. It was only after condensation had united millions of them into the form of little tiny balls that the magnifying glass was able to detect them. This means, of course, to say that the mercury is made up of these tiny particles, and what is true of mercury is true of every sort of matter. Matter of all kinds is made up of extremely small particles, which are called molecules.

This being the case, how do we account for the fact that matter is not all alike? Why should there be solid, liquid, and gaseous bodies?

This is due to the force of cohesion, sir, said Will. "In all solid bodies the molecules are held together by this force, so that each individual molecule has its own particular position, from which it cannot be moved unless this force of cohesion be first overcome."

Quite right, Will, said Mr. Wilson. "What this force of cohesion really is, we don't know; we only give it this name because the word cohesion means the act of holding together. This we do know, that if there were no such force, the molecules of all matter would fall asunder. Instead of solid bodies, there would be nothing but impalpable powder, and in place of liquids, nothing but vapor.

This force is not equally strong in all solids. We can break a piece of lead more easily than a piece of steel, a brick more easily than a piece of marble. Cohesion is stronger in the steel and marble than it is in the lead and brick respectively. In liquids the force of cohesion is slight as compared with that in solids. The molecules have no fixed position, but are free to move about, and roll and tumble one over another. In a solid the molecules are bound so firmly together that we are unable to move a single part of it without moving the whole body, but in a liquid we can set in motion some of the particles, and leave the remainder at rest. This freedom of movement among the molecules of a liquid explains why it flows when unsupported.

But let us notice next the manner in which different liquids flow. I will pour this water from one tumbler into the other, and then I will do the same with some oil, molasses, and tar. Why should the molasses, tar, and oil flow slowly as compared with the water? The force of cohesion is stronger in them than in water; the particles take longer to separate. It is easier, for the same reason, to thrust one's hand into a basin of water, than into a basin of molasses. In gases there is no force of cohesion, but rather a repelling force. The molecules of a gas are constantly trying to separate farther and farther from each other. This explains why gases can always be compressed into a smaller space than they once occupied."