"The frequency output depends upon the type of minerals used, and it is completely random so far as any consistent pattern goes. Some elemental minerals are no good, some are excellent."

"You've made determinant charts?"

"Naturally. But there's no determinant. After I said elemental minerals, I should have said that this was the original premise. Now we have a detector working with helium gas surrounding a block of lead bromide.

"Lead and helium are no good, helium and bromine equally poor. Lead and bromide are no good—as long as it lasts. Now don't ask me if the combination of the elements interferes. One good detector operates so wonderfully all the time, that a bit of yellow phosphorus is forming phosphorus pentoxid because it is suspended in an atmosphere of pure oxygen."

"No apparent determining factors, hey?"

"None. You might as well pick out the elements with six-letter names. The periodic chart looks like the scatter-pattern of an open-choke shotgun. Water works fine when it is contained in a glass vessel, but in anything else we know of—no dice."

"You seem to have covered a multitude of things," said Dr. Pollard approvingly.

"We've had a corps of brilliant, imaginative technicians working on the theory and practise for thirty years. Every one of them has come up with a number of elemental detecting combinations. We're now working on four and five element permutations.

"With and without plain and complex electrostatic and magnetic fields—and mixtures of both. We've gone logically as far as we can under a system that demands that we try everything. In each set of permutations, we cover all. You know our motto."

Majors finished with a slight laugh. He pointed to the end of the conference room, where, lettered on the wall above the blackboard was—

CONTENTS

LEAVE NO TURN UNSTONED!

"Where does it come from?" asked Pollard innocently.

"Take a fifteen-degree angle from the middle of Boötes. Maybe Arcturus for all we know. Somewhere within fifteen degrees of an arbitrary point up there. A total conic solid angle of thirty degrees will encompass all but wisps of the stuff that filter through once in a year or so."

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