"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."
"Why is mercury used, uncle?" asked Frank.
"Because it takes a very great heat to make it boil." said Uncle Robert. "Then you have seen how quickly it shows a change of temperature. When it is warm we call it a high temperature, and when it is cold it is called a low temperature."
"That is because the mercury goes up when it is hot, and down when it is cold, isn't it?" said Donald. "I wonder how it would feel if it was forty degrees below zero. See, it is away down to there!"
"Do you remember that day last winter when Peter froze his ears driving to town?" asked Frank. "Well, it was twenty below that day at the post office. I saw it. But father is calling me; I must go."