To all three came a muffled, booming roar. The windows beside them shivered with the thud of the distant explosion; they had not ceased their trembling before Harkness had switched on the news broadcast. And it was a minute only until the news-gathering system was on the air. "Explosion at the Institute of Physical Science!" it said. "This is Vienna broadcasting. An explosion has just occurred. We are giving a preliminary announcement only. The laboratories of the Scientific Institute of this city are destroyed. A number of lives have been lost. The cause has not been determined. It is reported that the laboratories were beginning analytical work, on the so-called Harkness Dark Moon gas-- "Confirmation has just been radioed to this station. Dark Moon gas exploded on contact with air. The American, Harkness, is either a criminal or a madman; he will be apprehended at once. This confirmation comes from Herr Schwartzmann of Vienna who left the Institute only a few minutes before the explosion occurred--" And, in the quiet of a hospital room, Walter Harkness, drew a long breath and whispered: "Schwartzmann! His hand is everywhere.... And that sample was all I had.... I must leave at once--go back to America." He was halfway to the door--he was almost carrying Diane Delacouer with him--when Chet's quiet tones brought him up short. "I've never seen you afraid," said Chet; and his eyes were regarding the other man curiously; "but you seem to have the wind up, as the old flyers used to say, when it comes to Schwartzmann." * * * * * Harkness looked at the girl he held so tightly, then grinned boyishly at Chet. "I've someone else to be afraid for now," he said. His smile faded and was replaced by a look of deep concern. "I haven't told you about Schwartzmann," he said; "haven't had time. But he's poison, Chet. And he's after our ship." "Where is the ship; where have you hidden it? Tell me--where?" Harkness looked about him before he whispered sharply: "Our old shop--up north!" He seemed to feel that some explanation was due Chet. "In this day it seems absurd to say such things," he added; "but this Schwartzmann is a throw-back--a conscienceless scoundrel. He would put all three of us out of the way in a minute if he could get the ship. _He_ knows we have been to the Dark Moon--no question about that--and he wants the wealth he can imagine is there. "We'll all plan to leave; I'll radio you later. We'll go back to the Dark Moon--" He broke off abruptly as the door opened to admit the nurse. "You'll hear from me later," he repeated; and hurried Diane Delacouer from the room. But he returned in a moment to stand again at the door--the nurse was still in the room. "In case you feel like going for a hop," he told Chet casually, "Diane's leaving her ship here for you. You'll find it up above--private landing stage on the roof."