  “How is it, Uncle George, that there is always a clear space between the spout of the kettle and the puff of steam?” It was Frank that asked the question. He had been watching the kettle boiling for a long time. “It is the same with the railway engine,” said Tom. “There is always a big space between the funnel and the puff.” “That is so,” said Uncle George. “But if you watch carefully, you will notice that the space between the funnel of the engine and Engine puffing on a hot day. “No,” said Frank, “I have noticed that the white puff is farther away from the funnel on a hot day than on a cold day.” “That is true,” said his uncle. “Perhaps you have also noticed that, as the engine rushes along, it leaves a long white cloud trailing in the air behind it. Sooner or later this long white cloud melts away from sight. It melts away sooner on a hot day than on a cold day. Where does it go?” “It goes into the air,” said Frank; “just as the cloud from this kettle goes into the air of the room.” “Very good,” said Uncle George. “Tom, will you please fetch me a tumbler full of cold water, and see that the outside of the glass is quite dry?” When Tom came back with the glass of cold water, Uncle George wiped it outside with a clean dry cloth. When he was sure that the outside of it was dry, he placed the tumbler of water on the table in the middle of the warm room. “Now,” he said, “let us try to answer Frank’s question about the clear space between the spout of the kettle and the puff of steam. “The fact is,” Uncle George went on, “the white puff which we call steam is not steam at all. We might just as well call it ‘water-dust.’ For it is made up of tiny droplets of water—so tiny that they float in air. Steam is water in the form of gas. Like the air we breathe, it cannot be seen. In fact, this water-gas forms part of the air around us. The clear space between the spout of the kettle and the puff is made “If you hold any cold object, such as a knife, in the puff, these water particles run together and form large drops upon it. The cloud of water-dust melts away in the room, as Frank told us. What takes place is this. The tiny droplets, when spread out into the warm air, become real steam, or ‘water-gas,’ again. “The outside of this tumbler of cold water was quite dry when I placed it on the table. Run your finger along it and tell me what you find.” Frank did so, and said, “Why, it is quite wet now.” “Yes,” said Uncle George, “it is covered all over with very small drops of water. Where did this water come from. It could not come through the glass.” “It is like dew,” said Tom. “It is dew—real dew,” said Uncle George. “The water in the glass is much colder than the air around it. The film of air next the “The earth is heated during the day by the sun, and the layer of air next to it becomes filled with water-gas. At night the earth gets cold. The water-gas, if the night is calm, comes out of the film of air next to the earth. It settles in the form of tiny drops on everything around. “When the earth gets very cold, the water freezes as it changes from gas to water, and instead of dew we have frost.” “Oh, that is why we have frost on the inside of the window panes in winter,” said Frank. “That is so,” said his uncle. “The frost on your window pane is the water-gas of the warm room changed into particles of ice. But let us come back to our steam puff. We spoke about the long white streak of water-dust which the engine leaves behind it. Do you know of anything else like that outside?” “Oh yes,” said Tom, “the clouds far up in the sky are very like it.” “They are,” said Uncle George. “In fact, the clouds in the sky and the cloud behind the engine are just the same kind of thing. They are both made up of tiny particles of water. “We have learnt that the streak of cloud left by the steam-engine melts away quickly on a hot day, also that the puff is farther from the funnel on a hot day. This shows us that the warmer the air is, the more water can it take up and hold. We have also learnt that warm air is light and rises up. “What happens when warm air, which holds much water-gas, rises up to the higher and colder parts of the sky?” “It gets cooled,” answered Frank. “Yes, and its water-gas gets cooled too. Then we can see it as great masses of water-dust. These masses we call clouds. If these masses of cloud get further cooled, the tiny water particles run together to form great drops—as they did on the cold knife. They are now too large and heavy to float in the air, so they fall to the earth as rain.” A Showery Day. Questions and Exercises.
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