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_.” Questions and Exercises. 1. How is rust formed? 2. Explain why things which are made of iron should be painted. 3. A grate full of coals burns away and only a small quantity of ash is left in the grate. What has become of the coals? VIII.—THE FIRE-BALLOON. One day Uncle George made the children a fire-balloon. He took twelve strips of tissue-paper shaped as you see in the picture. These he pasted neatly together at the edges so as to form a kind of bag with a round opening at the bottom. A ring of wire was then fixed at the bottom to keep it firm, and across the ring was stretched another piece of wire. This was to hold a dry sponge by and by. Uncle George swung the balloon till it was filled with air. He told Frank to hold it by the ring while he heated the air inside the balloon. This was done by holding the mouth of the balloon over a piece of rag which had been dipped in spirits and set on fire. [Illustration: Strip for Balloon.] Soon Frank felt the balloon rising. He lifted it up away from the flame, while Uncle George moved the little sponge along the wire to the middle of the ring. Then he soaked the sponge with spirits and set fire to it. “Let go!” said Uncle George; and away went the balloon, soaring up towards the sky. Higher and higher it rose, moving with the wind. The children watched it until at last it seemed a mere dot in the sky, and then it went quite out of sight. “What makes the balloon rise up?” Dolly asked, as they returned to the house. For answer Uncle George took a cork and held it under a trough of water. When he let it go, the cork at once rose to the top of the water. “What makes the cork rise up?” he asked. “Because it is lighter than the water,” said Tom.