"You mean thermometer, don't you?" said Fred. "What can we do with that?"
"Don't you see? There is a great deal of quicksilver in this glass ball, and we can play with it. I'll show you how." And away they went downstairs to find their grandfather.
"Grandpa, can we have this?"
Mr. Lenox looked up from his newspaper.
"Let me see it a moment. What do you wish to do with it?"
"We will break it and take out the quicksilver, and then I will show you. Let me ask Ellen for a dish to catch the drops."
"Not quite so fast; wait a moment, Harry," replied Mr. Lenox. "I wish you to notice something about it first. The top of the tube is slightly broken, which makes it of no exact use, for to measure heat or cold the quicksilver must be entirely protected from the air. If you had noticed it when you first came in, you would see that the warmth of the room has caused it to rise in the tube. This is shown by the marks on the plate to which it is fastened. Now, if you hold it close to the stove, the quicksilver will rise still higher. Let it stand outside the window a moment, and it will sink."
By this time the boys were much interested.
"But what makes it do so, grandpa?" they asked.
"Quicksilver is very sensitive to heat and cold. If the weather is warm, or if the room it is in is warm, it expands—swells out—and so rises in the glass tube, as you have seen. The least coolness in the air will cause it to contract, or draw itself into a smaller space; then, of course, it sinks in the tube.