"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."