The coward was now a maniac with terror, his face perfectly white, writhen with panic, and with staring eyes that gleamed horribly under the greenish vacuum-lights.

“Back, you!  Get out!” roared Waldron, raising a fist.  “We—­”

A sudden belch of flame, outside, split the night with terrible virescence.  The whole steel building trembled and swayed.  Some of its girders buckled; and the east wall, nearest the oxygen-tanks, caved inward as a mass of many tons was hurled against it.

A stunning concussion flung all three men to the floor; and, as they fell, a withering heat-wave quivered through the place.

“The oxygen-tanks!” gasped Flint.  “They’re blown up—­they’re burning—­God help us!”

Scorching, yet still eager to live, he crawled on hands and knees toward the steel door.  Waldron dragged himself along, half-dead with terror.  Now, dripping gouts of inextinguishable fire were raining on the roof of the building.  A whirlwind of flame was sweeping all its eastern side; and a glare like that of Hell itself seared the eyes of the fugitives.

Quivering, trembling, slavering, the old man and Waldron wrenched the steel door open.

“Me!  Me!  Let me in!  Me!  Save me!” howled Herzog, dragging himself toward them.

They only laughed derisively, with howls of demoniacal scorn.

“You slave!  You cur!” shouted Waldron, and spat at him as he drew the vault door shut.  “You cringing dog—­stay there, now, and face it!”

The great door boomed shut.  In the cool of the winding stairway of steel which led, lighted by electricity, to the trap-door and the ladder down into the tremendous vaults, the world-masters breathed deeply once more, respited from death.

Herzog, screaming like a fiend in torment, clawed at the impenetrable steel door, raved, begged, entreated, and tore his fingers on the lock.

No answer, save the muffled echo of a jeer, from within.

Boom!

What was that?

Mad with terror though he was, he whirled about, and faced the room now quivering with heat.

Even as he looked, a great gap yawned in the western wall, farthest from the flame-belching oxygen-tank that had been struck.

Through this gap, pouring irresistibly as the sea, swept a tide of attackers, storming the inner citadel of the infernal, world-strangling Air Trust.

At the head of this victorious army, this flood triumphant of the embattled proletaire, Herzog’s staring eyes caught a moment’s glimpse of a dreaded face—­the face of Gabriel Armstrong.

Gasping, the coward and tool of the world-masters made one supreme decision.  Close by, a rack of vials stood.  He whirled to it, snatched out a tiny bottle and waiting not even to draw the cork—­craunched the bottle, glass and all, in his fang-like, uneven teeth.

An instant change swept over him.  His staring eyes closed, his head fell forward, his whole body collapsed like an empty sack.  He fell, twitched once or twice, and was dead—­dead ere the attackers could reach the door of steel where his bestial masters had betrayed him.