"I'll tell you one thing we can and must do," said Billie, after they had returned to the library. "We must keep a watch in this room to-night. Whoever put the box here may return." "Right!" from Don. "You do have occasional lucid intervals, Billie." Billie grinned, but made no reply. "I think we may all stay here for a few hours," suggested Lucia. "Father is so much better that I think we may have a little music. I will play some accompaniments on the guitar and Josie can sing." "That will seem a good deal like being back in the States," declared Adrian. "With the Stars and Stripes flying over my head, a brigade of American troops on guard and an American girl singing, I can almost forget I am on Mexican soil." "How about the accompanist?" queried Billie. "Oh!" laughed Adrian, "we're quite willing to adopt her. Hey, Donald?" "Don't ask me, Ad. Ask Billie." "I am sure we could do no better," was Billie's gallant reply. Lucia's suggestion was carried out and for a couple of hours there was a merry little party under Santiago's hospitable roof. Even the mysterious box was forgotten and the young people were giving themselves up to a jolly good time, when suddenly there came a scream which caused every one to turn their eyes toward the room in which the sick man was supposed to be lying. But there in the doorway he stood, his long night robe reaching nearly to the floor and his thin black hair standing almost on end. "Father!" cried Lucia, rushing toward him "What is it?" He waved her off, but made no reply, while with his long bony finger he pointed at the brass-bound box. "Where did it come from?" he asked in a shrill, querulous tone. "Who brought back my secret casket?" "Yours?" came from every one in the room. "Yes! Mine! Mine!!" he almost screamed. "He's raving!" cried Josie. "Can't some one do something for him?" "No," he replied, and his voice became more calm, "I am not raving. I know whereof I speak. Quick! Let me look within it to see that all is safe." "It is locked, Father," said Lucia, coming to his side, "and we have no key." "I can unlock it," he cried. "I can unlock it. Give it to me. Give it to me." He staggered forward and seized the box in his hands. For several seconds he fumbled with it, turning it first upon one side and then upon another, and at last raised the lid. He thrust in his hand and then stopped as one stupefied. "Empty! Empty!" he gasped in an almost audible whisper. "The plan of Montezuma's mine is gone! Gone!!" A moment he stood and gazed around upon the faces of those in the room and then collapsed upon the floor. Quickly the boys picked him up and carried him to his bed and the attending physician was summoned. Billie picked up the box and examined it curiously. "I wonder how he opened it?" he mused. "There must be some sort of a spring somewhere."