Potts did as bidden.
“No shot was fired in here,” Grover snapped. “When Astrovox was later assaulted. What happened, Roger? Don’t you know? And you exploded a torpedo to call attention to a certain place and away from some other?”
Roger was all at sea for a moment.
“Astrovox was leaving. The other fellow didn’t know that.” Grover had caught some clue or hint, somewhere. He was as active, as alive, as if he had never been a cold, precise, restrained scientist.
“Some one wanted us all to run here. As he produced the summons, Astrovox ran in. The man realized that he was recognized. Poor Astrovox! Well, he will recover. And—see there!”
He pointed to a brown, scorched spot under the far edge of the desk as it had been before.
“A foot, on an explosive, such as your torpedo, Roger. Evidence out of sight. Evidently had no time, later, to remove the burn, but did remove the exploded detonating cap. Rubbed his shoe over it. See the scorch? Test and you will get something like a gunpowder reaction. Maybe you can scrape up dust that would test out with the nitric acid to show the stains of explosive gases.”
Of a sudden he straightened up.
“The acid test!”
Roger, and Potts, gaping, had no way of following the swift deductions which the Mystery Wizard, on the trail at last, made.