Frank was tight at his heels, while Clancy brought up the rear of the little file of pursuers. The noise was not so deafening outside the mill, but the boys were blinded temporarily by their quick transition from the bright glow of the mill to the outer gloom. They stared around them, but could see nothing of the prospector. Ballard, however, heard something or other which gave him a clew. "This way!" he shouted. Frank heard his chum's feet swiftly crunching the sand and gravel, and followed the sound. In a moment or two his vision cleared somewhat and he was able to see several rows of huge wooden tanks. A plank incline led to the top of one row, and Ballard could be distinguished racing up the incline. Beyond Ballard, traveling at speed over a plank gangway that spanned the tanktops, was a burly figure silhouetted against the lighter gloom of the night. With a shout to Clancy, Merriwell hustled after Ballard. Those tanks were part of the cyanide plant, wherein the refuse of the mill was treated with deadly cyanide of potassium for recovering what little gold was left after the refuse, or "tailings," had come from the stamp mill. The cyanide plant, presumably, was familiar ground to Porter, whereas the boys had never seen it before. In the gloom the prospector could navigate across the big vats with something like accuracy, while the boys carried on their pursuit at a tremendous disadvantage. Recklessly Ballard ran on. Merriwell called a warning to him, but Ballard either did not hear it or else paid no attention. The form of the prospector, leaping and plunging onward, sprang from one row of vats to another. Each row was a little lower than the row to the north, so that the tiers took on the form of a flight of giant steps. Porter gained the top tier, and stood for a moment on a plank spanning a vat that was three or four times as large as any of the others. Ballard climbed to the same plank. Porter dropped down with a savage, snarling cry. Clinging for a moment to the edge of the tank, he twisted the plank from under Ballard's feet. Ballard dropped with a splash. "Merciful powers!" yelled a voice in wild alarm. "Get him out, quick! That's the solution tank and is filled with cyanide!" Merriwell's heart almost stopped beating. In a gleam of light from the mill he saw the white, drawn face of Pardo peering toward the spot where Ballard was splashing in the deadly cyanide solution. An instant later he bounded to the rescue. CHAPTER IV. A CLOSE CALL.