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.
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:
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: