a 'hot-and-cold' expert. Refrigeration!

"Bats are hibernating creatures. And hibernation is not merely a matter of custom, tradition, desire to sleep—it is a physical reflex which cannot be avoided when the conditions are made suitable.

"Bats, like many other hibernating mammals, are automatically forced into slumber when the temperature drops below 46°F. Knowing this, and realizing that was the reason the Harpies—bat-like in form and habit—kept their underground chambers superheated I applied an elemental principle of refrigeration to cool their city below that point!"

Dorothy said, "The—the ammonia—?"

"Exactly. The set-up was perfect. Our apparatus was, perforce, crude, but we had all the elements of a refrigerating unit. Ammoniated water, running in a constant stream, capsules of condensed and concentrated heat from our needle-guns—a small room which was connected, by ventilating ducts, with the rest of the underground city.

"The principle of the absorption process depends on the fact that vapors of low boiling point are readily absorbed in water and can be separated again by the application of heat. At 60°F, water will absorb about 760 times its own volume of ammonia vapor, and this produces evaporation, which, in turn, gives off vapor at a low temperature, thereby becoming a refrigerator abstracting heat from any surrounding body. In this case—the rooms above!

"It—" Tim grinned. "It's as simple as that!"

Captain Lane groaned.

"Simple!" he echoed weakly. "The man says 'simple'! I don't understand a word of it, but—it worked, son! And that's the pay-off."

"No, sir," said Tim promptly.

"What? What's that?"

"The pay-off," persisted Tim, "comes later. When we get back to civilization. You said something about removing your objections to our marriage, remember?"

Captain Jonathan growled and stood up. "Confound it, do you think of everything? Well—all right, then. I'm a man of my word. But when we get back to civilization may be a long time yet."

"I can wait," grinned Tim. "But I've got a feeling I won't have to wait long. Maybe I'm psychic all of a sudden. I don't know. But somehow I've got a hunch that when we get to the cave-mouth, 

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