He ran to the crystal and slapped at it. It hurt. With a glad cry, Dave pried at it with his fingers. The clamping electrodes held it firm--and he could not touch them, for they were as thin and tenuous as the concrete wall through which he had walked. Only the crystal was solid both _there_ and _here_. Dave smiled sourly. If he was dead, then this was a fine psychological hell. Here he was watching friends and a loved one marching into deadly danger, listening to their grief and their dangerous plans, while he was completely helpless to guide them. He felt the crystal move slightly under his straining fingers. Wrapping a handkerchief about his fist, Dave punched at the crystal. It gave--or on the other side, the clamping electrodes gave. At any rate, it was loose. He hit it again and jarred it. "The crystal!" cried Jane. "It's moving!" "Blow-up!" yelled DeLieb. But this time there was no panic. Howes cut the energizing power with a flick of his hands across the toggle switches. DeLieb clamped down on the electrodes with a hand and spun the wingnuts that held it with the other. Jane Nolan grabbed at the crystal as it came free and turned to the pneumatic delivery tube. But Dave reached out a hand and snatched it from her. Jane cried in pain and fear, and watched the crystal make three long swoops towards the concrete wall--Dave had grabbed it and started to run outside. The crystal was wrenched from his fingers as he went through the wall. It fell to the floor, and all three physicists swooped down upon it. Jane came up with it and popped it into the pneumatic tube. It rattled thrice and was gone, racing down the tube end over end, visible to Dave as it raced out of reach. "It wants physicists," breathed DeLieb. "But it's gone now." "And so is Dave," cried Jane. "Dammit," snapped Dave. Then he gave up, because he knew the utter futility of trying to make them hear him. But there was a way! The crystal extended through both worlds. All Dave had to do was to get the crystal and use it as a stylus against some surface in the other world. He turned to follow the pneumatic tube towards the place where the crystal had gone. He was not more than a hundred yards down the length of the tube when the sky blinded him from a couple of miles away, and then the air roared, and then when vision returned he could see a pillar of white smoke billowing skyward. _They_ had destroyed the crystal! * * * * * Dave stopped to think. Clearly, the exploding crystal was as dangerous on this side as it was on the other. That meant that no one could stand close by and watch the thing to be sure of which physicists they got--unless they used some sort of television hookup.