I. THE BIRTH OF AN IDEA II. THE PARTNERS III. THE BAITING OF HERZOG IV. AN INTERLOPER V. IN THE LABORATORY VI. OXYGEN, KING OF INTOXICATORS VII. A FREAK OF FATE VIII. ONE UNBIDDEN, SHARES GREAT SECRETS IX. DISCHARGED X. A GLIMPSE OF THE PARASITES XI. THE END OF TWO GAMES XII. ON THE GREAT HIGHWAY XIII. CATASTROPHE XIV. THE RESCUE XV. AN HOUR AND A PARTING XVI. TIGER WALDRON "COMES BACK" XVII. THOUGHTS XVIII. FLINT AND WALDRON PLAN XIX. CATHERINE'S DEFIANCE XX. THE BILLIONAIRE'S PLOT XXI. GABRIEL, GOOD SAMARITAN XXII. THE TRAP IS SPRUNG XXIII. THE BEAST GLOATS XXIV. CATHERINE'S SUPREME DECISION XXV. THROUGH STEEL BARS XXVI. "GUILTY" XXVII. BACK IN THE SUNLIGHT XXVIII. IN THE REFUGE XXIX. "APRÈS NOUS LE DÉLUGE!" XXX. TRAPPED! XXXI. ESCAPE! XXXII. OMINOUS DEVELOPMENTS XXXIII. "NOW COMES THE HOUR SUPREME" XXXIV. THE ATTACK XXXV. TERROR AND RETREAT XXXVI. THE STORMING OF THE WORKS XXXVII. DEATH IN THE PIT OF STEEL XXXVIII. VISIONS LIST OF ILLUSTRATIONS "VISIONS!" SHE SAID SOFTLY, "DO YOU BEHOLD THEM TOO?" "CAN'T BE DONE, EH?" SAID FLINT HE GATHERED HER UP AS THOUGH SHE HAD BEEN A CHILD AIMING AT THE BASE OF THE SKULL SHE STRUCK THE SPY'S BODY BURST INTO A SHEAF OF FIRE HIS FINGERS LOST THEIR HOLD--HE DROPPED LIKE A PLUMMET THE AIR TRUST CHAPTER I. THE BIRTH OF AN IDEA. Sunk far back in the huge leather cushions of his morris chair, old Isaac Flint was thinking, thinking hard. Between narrowed lids, his hard, gray eyes were blinking at the morning sunlight that poured into his private office, high up in the great building he had reared on Wall Street. From his thin lips now and then issued a coil of smoke from the costly cigar he was consuming. His bony legs were crossed, and one foot twitched impatiently. Now and again he tugged at his white mustache. A frown creased his hard brow; and, as he pondered, something of the glitter of a snake seemed reflected in his pupils. "Not enough," he muttered, harshly. "It's not enough--there must be more, more, more! Some way must be found. Must be, and shall be!"