"Our last chat about vapor," said Fred, "showed us that evaporation is constantly going on everywhere. The air always contains vapor, and yet we cannot always see it. Teacher proved this in a very simple way today. How do you think he did it? He stood a tumbler of cold water on the table.""Please tell me all about it, Fred," said Norah, "for I can"t see how that would prove it.""Well," said Fred, "after he had let the tumbler of water stand some time on the table, he showed us that the outside of the glass was covered with tiny drops of moisture.
"This moisture did not come from the water in the glass. It came from the vapor in the air. That vapor was at first invisible, but when it touched the cold sides of the glass, it became visible as little round drops of water, which settled on the glass itself.""We could not at first make it out," said Willie, "for teacher had a tumbler of warm water on thetable as well as the other one, and there were no drops of moisture on this.""Let me see," said Fred, "whether I can explain it. Take this slate, Norah, and breathe on it. I cannot see your breath, but look at the slate, and you will see some drops of moisture on it.
