Sherman imagined himself in the airplane's seat, operating the controls and as well as he could to a strange type of mind, explained how they worked. "But what drives it?" insisted the Lassan. "I do not understand. No, not the queer thing at the front that turns round. We have that principle ourselves. But the thing that makes it turn." For answer, Sherman tried to picture the interior of the engine and show the gasoline exploding and driving it. The mind opposite his became thoughtful at once, and then flashed a question. "Are there many--explosives--in this earth?" Sherman pictured gunpowder, dynamite and all the others he could think of. He at once sensed that the Lassan was both astonished and troubled. Something like a mental curtain which he could not pierce, dropped between them. A moment later the elephant-man rose. "That will be sufficient for the present," he flashed, and came forward to remove the helmet from Sherman's head. * * * * * A few moments later the door was swung open; Sherman saw that one of the cars was waiting for him with the word "EXIT" beckoning him on and he was soon back in his cage. As nearly as he could judge time, he was left alone for quite twenty-four hours before being recalled for further questioning. As soon as he entered the interrogation room he perceived that something serious had engaged the attention of the Lassans. The seat was prepared for him as before, but instead of one of the twisted benches, there were now three. His acquaintance, the old Lassan, occupied the center one; on one side was a chubby elephant-man whose obesity gave a singularly infantile expression to his features and on the other a slender-limbed type, as though by contrast. All three had tubes connected to the helmet which was placed on his head, but he soon recognized that the older Lassan was the only one to ask questions. "We wish to ask you about these explosives," came the message. "Are they all alike?" "No," he answered instantly. "What causes them to explode?" "I am not a chemist. I don't know." The idea of chemistry was slightly unfamiliar to them; it was apparent from their thoughts that chemistry had never occurred to them as the subject of a special study. Then came another question, "Are there many chemists?" An idea struck Sherman. He closed his mind resolutely against the question and flashed back the message that he had come to learn as well as teach. He sensed a certain annoyance among the new auditors, but the old Lassan answered, "That is only just. What do you wish to know?" "What the machines are for." "In the center of this as of every other earth lies the substance of life, as it lies at the heart of every sun. The machines pierce to it and draw it up for our uses." "What is this substance of life?"