"Peter and I were driving to town," began Frank, "and the tire of one of the wagon wheels slipped right off. We managed to get to the blacksmith's shop, and he put the tire in the fire until it was hot. Then he put it on the wheel, but it was still loose. We couldn't have gone a step without its coming off again. He brought cold water and poured over it, and soon it was as tight as could be. I thought the water made the wood of the wheel swell up--you know water does that to the pails and tubs when they leak; but now I believe the fire made the tire larger, and then the cold water made it small again. That is just what happened." [Illustration: The blacksmith shop.] "But air can't grow bigger, can it?" asked Donald. "If you can find an empty bottle, Donald," said Uncle Robert, "perhaps we can soon find out about it." Uncle Robert took a piece of thin rubber out of his pocket and tied it tightly over the mouth of the bottle." "By the way," he said, "is there anything in this bottle? "No," said Susie, looking through the glass. "Oh, yes," said Donald, "there is air in it." "Well," replied Uncle Robert, "please get a pan of hot water, Frank." Frank brought the water, and as Uncle Robert began to put the bottle into it they all exclaimed: "Be careful; you'll break the bottle!" "What will make it break?" asked Uncle Robert, pausing. "Why, the hot water," said Susie. "It always breaks glass if you put it in too quickly," said Donald. [Illustration] "Well, we'll warm it a little first," holding the bottle close to the water. "I think I can try it now." As he spoke he lowered the bottle into the water, and the rubber tied over the neck began to bulge out. "See!" cried Susie. "What makes it do that?" "Try the cold now," said Uncle Robert. "Here, Donald, hold the bottle in this pail of cold water." "The rubber is going down," said Donald in a moment. "It is going right into the bottle." "Does the air in the bottle pull the rubber in with it?" asked Susie. "But, Uncle Robert," said Donald, "what if wagon tires, apples, and air do swell up when they are hot? I don't see what all that has to do with the thermometer." "I think I see," said Frank. "Why wouldn't this gray stuff in the thermometer get bigger when it's hot, if everything else does?" "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.