"Oh, yes! What with Johnny and you and uncle Frank, I'm getting quite learned, I guess. Can't you begin to call me professor, girls?"
"Not till you know more than that," said Sue; "for I know that."
"Felix has got the idea about the fire under water, I guess," said Johnny. "You see, I mix some nitrate of potash, powdered charcoal, sulphur, and nitrate of strontium, and put it into a long narrow paper case, tightly closed with gum. I set it on fire at the bottom, and sink it in the water by a piece of lead hung to it."
"And sometimes the fire is red," said Sue.
"Yes; but I have a different mixture for the red fire. You see, Ruth, the reason water puts out fire is because it shuts out the oxygen by shutting out the air. If you can furnish oxygen to a combustible material under water, you can burn it under water. As I put something in my mixture to furnish oxygen, I have a fire without any trouble. But I have seen a larger laboratory than the one you saw, Julia."
"Where?"
"Oh! I've seen one in everyplace I ever was in.—That's a riddle for you. See who'll guess it first."
"No, you haven't seen one in every place you were ever in; for you haven't seen one here at the beach?" replied Julia.
"Yes, I have."
"It's some nonsense," said Felix. "We'll give it up. There's no fun in trying to guess riddles that have a catch in them: you never can guess them."