"What is it that moves up and down in the thermometer?" asked Susie.

"It is mercury," answered Uncle Robert, "which is sometimes called
quicksilver."

"It looks like silver," said Susie, examining it closely.

"Perhaps you can see this better," said Uncle Robert, taking a small
bottle of mercury from his pocket and pouring a little into Donald's
hand.

"How heavy it is!" exclaimed he, letting it roll about. "It feels just
like lead."

"It is almost twice as heavy as lead," replied Uncle Robert.

"Put it in my hand, Donald," said Susie. "There, you've spilled it on
the floor! Just see it run around!"

"Is it always soft like this?" asked Frank.

"No, it becomes hard when it is very, very cold."

"How cold, uncle?" asked Donald, looking at the thermometer.

"Thirty-nine or forty degrees below zero," was the reply. "In the
coldest of countries alcohol thermometers are used. It must be much
colder than that to freeze alcohol."