"But we don't know whether there are any people there," continued Mary, "and if there are, they might not mind the heat at all. You can get used to the heat, just as Uncle Robert did when he went to India. Don't you remember how he felt the change when he came home, and how he shivered? He missed the heat just as we would suffer from it if we went to India for the first time." [Illustration: COMPARATIVE SIZE OF SUN AS SEEN FROM THE PLANETS.] "Then Uncle Robert would not mind going to Mercury," said Harry, laughing, "if he is getting to like the heat in India. But I do not want him to go yet, as he might never come back again; and what would we do without him?" "What would we?" said Nellie mournfully, her eyes filling with tears at the very thought. "Is a planet made of earth and stones and trees and flowers, just like planet Earth?" asked Harry. [Illustration: COMPARATIVE SIZE OF THE PLANETS.] "Yes, dear," replied his sister; "only some planets, like Jupiter and Saturn, are still wrapped up in a blanket of clouds and steam, and we cannot see them yet. They are very hot indeed, and all the water that will make the oceans and seas and bays is now steam and clouds hiding the true planet from view. Water could no more rest on the surface of the planets Jupiter and Saturn than it could rest on red-hot iron. Don't you remember, the other day, when nurse upset a cup of water on the hot stove, how the water sizzled and turned into steam in a moment? "Now planet earth, a long time ago, when it was a very young world, was very hot like Jupiter. All the lakes and seas and oceans were turned into steam and blankets of cloud. It would have been a very uncomfortable world to live on then. But it became cooler and cooler, and the clouds changed into the oceans and seas and lakes that make our earth so beautiful. "Some day this little world will grow old, and the oceans will get smaller and smaller, and the earth colder and colder. Then there will be scarcely any air to breathe, and we would gasp, and die just like that poor fish that Uncle Robert caught last week and threw in the bottom of the boat. Don't you remember, Nellie, how the poor little thing gasped and jumped around? It could not live out of the water, so it died. Now, we cannot live without air, and if this earth had not any air we would die. But this will not happen for a very long time." "Are you quite sure?" asked Harry, with an anxious look on his face; "because I don't want to die yet, sister." "Quite sure, my little brother," she said, kissing him tenderly; "for hundreds and hundreds of years must pass away before anyone will have any idea that the earth is growing old." "And what will become of the poor little fishes when the oceans dry up?" asked Nellie sadly, as she clasped her dollie closely in her arms, as though to protect it from the coming trouble. "I expect they will all die," said Harry wisely; "because you know, Nellie, they can't live out of water. Can they?"