Then Uncle George called the boys and asked them what had taken place in the bell-jar.
“The tacks have rusted, and some of the air in the jar has been used up,” said Frank.
“How much air has been used up?” Uncle George asked.
“Just exactly the same as was used up when we burned the candle,” said Tom, pointing to the top mark.
“Let us see, then,” said Uncle George, “what part of the air has gone.”
He poured water into the large dish until it was level with the water inside the bell-jar. Then he put a lighted taper into the bell-jar as before. It went out at once.
“It is the same part of the air as the burning candle used up,” said Frank.
“Then we have found out,” said Uncle George, “that when a thing burns it uses up a certain part of the air; and that when iron rusts, exactly the same part of the air is used up.
“In the first case, the burning of the candle changed part of the air into a gas which dissolved in the water. In this case, that same part of the air has joined up to part of the iron tacks to form that red powder which we call rust.”