"I'd rather," replied Fox promptly. "Come on, Henrietta. We're going to town." He looked at his watch. "Train goes in fourteen minutes, and that's the train we take. Step lively, now." Henrietta giggled and Sally smiled; and they stepped lively and got to the station with two minutes to spare. Fox occupied that two minutes with a rattle of airy nothings which kept Sally busy and her mind off her errand; which may have been Fox's object or it may not. For Sally had not told her errand yet, and how could Fox Sanderson have known it? When they got into the car, Sally was a little disappointed because she had not been able to tell him. She had meant to--distinctly meant to during that two minutes. She had no chance to tell him in the train. The cars made such a noise that she would have had to shout it in his ear and, besides, he talked steadily. "I'll tell you what," he said, at the end of a stream of talk of which Sally had not heard half. "Let's get your father, Sally, and take him with us while you do your errands, whatever they are. He'll be through in the laboratory, and we'll just about catch him." "All right," Sally murmured; and she sank back in her seat contentedly. She had been sitting bolt upright. She felt that it was all right now, and she would not need to tell Fox or anybody. She felt very grateful to him, somehow. She felt still more grateful to him when he let the conductor take all their fares from her ticket without a protest. Fox was looking out of the window. "It looks as if we might have some snow," he remarked. "Or it may be rain. I hope it will wait until we get home." When they got to the laboratory, they found one of the cleaners just unlocking the door. She didn't know whether the professor had gone or not. He always kept the door locked after hours; but would they go in? They would and did, but could not find Professor Ladue. Fox found, on his desk, a beaker with a few drops of a liquid in it. He took this up and smelt of it. The beaker still held a trace of warmth. "He has just this minute gone," he said. "If we hurry I think we can catch him. I know the way he has probably gone." "How do you know he has just gone?" asked Sally, looking at him soberly and with her customary directness. "How can you tell?" "Sherlock Holmes," he answered. "You didn't know that I was a detective, did you, Sally?" "No," said Sally. "Are you?" "Seem to be," Fox returned. "Come on, or we'll lose him." So they hurried, twisting and winding through streets that Sally did not know. They seemed to be highly respectable streets. Sally wondered where they were going. She wanted to ask Fox, but, evidently, he didn't want to take the time to talk. Henrietta's eyes were brighter than usual and she looked from Fox to Sally with a curiosity which she could not conceal; but Sally, at least, did not notice, and Henrietta said nothing. "There he is," said Fox, at last.