"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?"