But the havoc they wrought as demolition bombs was neither their only nor their greatest damage. The seventy-five million cubic feet of free oxygen, driven downward and prevented from escaping into the open atmosphere by Radnor's forces, quickly diffused into a killing concentration throughout the Chloran city save inside that one upstanding dome. Almost everywhere else throughout that city the natives died exactly as had died the people of the Valeronian village in the strangling chlorine of the invaders; for oxygen is as lethal to that amœbic race as is their noxious halogen to us. Long before the bombs reached the ground Radnor was probing with his spy ray at the great central dome from within which Klynor Siblin's message had in part been sent. But now he could not get through it; either they had detected Siblin's beam and blocked that entire communication band or else they had already put up additional barriers around their headquarters against his attack, quickly though he had acted. Snapping off the futile visiray, he concentrated his destructive beam into a cylinder of the smallest possible diameter and hurled it against the dome; but even that frightful pencil of annihilation, driven by Radnor's every resource of power, was utterly ineffective against that greenly scintillant hemisphere of force. The point of attack flared into radiant splendor, but showed no sign of overloading or of failure. Knowing now that there was no hope at all of rescuing Siblin and that he himself had only a few minutes left in which to work, Radnor left his beam upon the dome only long enough for his recording photometers to analyze the radiations emanating from the point of contact. Then, full-driven still, but now operating at maximum aperture, he drove it in a dizzying spiral outwardly from the dome, fusing the entire unprotected area of the metropolis into a glassily fluid slag of seething, smoking desolation. But beneath that dome of force there was a mighty fortress indeed. It is true that her offensive weapons had not seen active service for many years; not since the last rebellion of the slaves had been crushed. It is also true that the Chloran officers whose duty it was to operate these weapons had been caught napping--as thoroughly surprised at that fierce counterattack as would be a group of Earthly hunters were the lowly rabbits to turn upon them with repeating rifles in their furry paws.