To go back, then: if we looked at a piece of iron under a microscope that would show us the molecules,--and remember, no such powerful microscope could exist,--we should see these quivering particles, and nothing more. Then if some one heated the iron while we watched the molecules, or if the sun shone on it, we should see the molecules move faster and faster and separate farther and farther. That is why heat expands things. When the molecules in an object move farther apart, naturally the object expands. _Heat is the motion of the molecules._ When the molecules move faster (that is, when the iron grows hotter), they separate farther and the iron swells. [Illustration: FIG. 42. A thermometer made of a flask of water. It does not show the exact degree of heat of the water, but it does show whether the water is hot or cold.] HOW WE CAN TELL THE TEMPERATURE BY READING A THERMOMETER. The mercury (quicksilver) in the bulb of the thermometer like everything else expands (swells) when it becomes warm. It is shut in tightly on all sides by the glass, except for the little opening into the tube above. When it expands it must have more room, and the only space into which it can move is up in the tube. So it rises in the tube. [Illustration: FIG. 43. Will the hot ball go through the ring?] Water will do the same thing. You can make a sort of thermometer, using water instead of mercury, and watch the water expand when you heat it. Here are the directions for doing this: [Illustration: FIG. 44. When the wire is cold, it is fairly tight.] EXPERIMENT 28. Fill a flask to the top with water. Put a piece of glass tubing through a stopper, letting the tube stick 8 or 10 inches above the top of the stopper. Put the stopper into the flask, keeping out all air; the water may rise 2 or 3 inches in the glass tube. Dry the flask on the outside and put it on a screen on the stove or ring stand, and heat it. Watch the water in the tube. What effect does heat have on the water? Here are two interesting experiments that show how solid things expand when they are heated: EXPERIMENT 29. The brass ball and brass ring shown in Figure 43 are called the expansion ball and ring. Try pushing the ball through the ring. Now heat the ball over the flame for a minute or two--it should not be red hot--and try again to pass it through the ring. Heat both ball and ring for a short time. Does heating expand the ring? EXPERIMENT 30. Go to the electric apparatus (described on page 379) and turn on the switch that lets the electricity flow through the long resistance wire. Watch the wire as it becomes hot.