“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 glass is cooled, and the ‘water-gas’ which this film of air contains is changed into water drops.
“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.”