He dragged Allan this time to one of the huge pendent magnets. A long vacuum tube lay parallel to the wire- coiled bar. At the farther end of the tube, inside, was a screen, with dulled surface. Attached to the nearer end was a leaden, funnel-shaped machine. Sandy thrust down a lever.

The machine whirred, but the tube remained dark. The dull screen, however, burst into a thin perpendicular line of glowing, sparkling pin points.

“The machine is a wave filter,” Dale explained. "It cuts out all wave lengths, allows only projectilelike particles to get through—my own inven¬ tion. They might be electrons, positrons, or neutrons. They’re hitting the fluorescent screen, see, and activating it into light. Now I’ll turn on the magnet.” He thrust another switch. “Look at that!”

The thin line of glittering pin points moved inexorably to the right, almost to the very edge of the screen.

“Positrons, my boy,” he snapped. “That’s what they are. Electrons would have moved to the left, toward the positive pole of the magnet, and neutrons, having no electrical charge, would have remained where they were.”

Allan shrugged. "So that proves there are positrons around. What, if anything, has that do with heavy water ?”