"Half as much?" asked the reporter. "One-quarter as much," said Teddy. "Or three times away I'll get one-ninth as much, or four times away I'll get one-sixteenth as much. You see? If I want to make the ends of an iron bar hot, and I can only heat the middle, the middle has to be red-hot or white-hot to make the ends even warm. If I have to make the middle of a bar red-hot to have the ends warm, you see in order to make the ends cold the middle would have to be very cold indeed." "Y-yes, I understand." "Well, the professor worked on that principle. He knew the temperature of the edges, and he knew the size of the ice cake. It was easy to figure what the temperature must be in the middle. It worked out to within two degrees of absolute zero!" "What's that?" "There isn't any limit to high temperatures. You can go up two thousand degrees, three thousand, four, or five. Some things almost certainly produce a temperature of as much as eight thousand degrees. But high temperatures are produced by putting more heat in--by stuffing the thing with calories. I make an iron bar red-hot by putting calories in. I make it cold by taking calories out." "Well?" "If you keep that up you reach the point where there aren't any more calories left to take out. When you get to that point you have a temperature of 425° Centigrade, or one thousand and seventy-eight degrees Fahrenheit below zero. That's absolute zero." Teddy spoke quite casually, but the reporter blinked. "Rather chilly, then." "Rather," Teddy agreed. "But our calculations told us that Varrhus had reached and was using a temperature within two degrees of that in the center of his ice cake. And right next to that temperature he had a very high one, as evidenced by the plume of steam." "I can't see how you got anywhere," said the reporter hopelessly. "I'm all mixed up." "It's very simple," said Teddy cheerfully. "On one side of a wall the man had what amounted to a thousand and some odd degrees below zero. On the other he had probably as much above zero. Evelyn--Miss Hawkins, you know--made the suggestion that solved the problem. She showed us this." Teddy picked up what seemed to be a square bit of opaque glass. "Smoked glass?" "Yes, and no." Teddy smiled. "You can't see through it, can you?" "No." "Come around to this side and look." The reporter made an exclamation of astonishment. "It's clear glass!" "It's a piece of glass on which a thin film of platinum has been deposited. It lets light through in one direction, but not in the other. Evelyn suggested that Varrhus had something which did the same thing with heat. It would let heat through in one direction, but not in the other. Of course if it would take all the heat from the air on one side and wouldn't let any come back from the other----" "It would be cold?"