"But the sunset colors, sir?" the author of the article asked. "You said they were due to dust. Just how, sir?"
"Yes, to dust, plain ordinary dust, but dust of the lightest kind," was the reply. "If you could go up in the air a hundred miles, the sky above you in the middle of the day would be jet black and the sun would shine down on you like a great bright-blue ball, without any white glare around it at all."
"Then it's a blue sun that makes a blue sky!" cried Fred.
"Don't go so fast," the Forecaster warned him. "I want you to think of the sky, first. It's a dead black, a hundred miles up. Now, at a hundred miles up, the air is so thin that there's little or no dust, but as you gradually come down and the air becomes denser, it begins to be able to buoy up some dust. Boys," he said, breaking off suddenly, "why does a stick float in water when it falls in air?"
"Because water is denser than air?" guessed Ross.
"Exactly. And why does a bar of iron sink through water and not through earth?"
"Because the particles of earth won't move aside as easily as the particles of water, I suppose."
"Not quite, but something that way. So, you see, as the air gets gradually denser it becomes gradually more able to support particles of dust, light ones at first, then heavier and heavier, until near the earth big pieces of dust can be carried in the air. You know how big some of them are when you happen to get a grain in your eye! Viscosity has a lot to do with it, too.
"The light of the sun is a bluish-white, like some of the blue stars. White, as you remember from the rainbow, is just a mixture of all sorts of colors and the different colors are created by waves of light, some being shorter and others longer. A long wave, like the red, will pass around a tiny piece of dust, but a short wave, like the blue, will be stopped by it, and scattered, sometimes polarized, as it is called, or turned into one plane."
"I don't think I quite see that," said Anton.