Craig rose and put on his hat and coat, thrusting a pair of opera glasses into his pocket, in case we should want to observe the place at a distance. I followed him excitedly. The trail was hot.
Kennedy and I came at last to the place on the West Side where the crooked streets curved off.
Instead of keeping on until he came to the place we sought, he turned and quickly slipped behind the shelter of a fence. There was a broken board in the fence and he bent down, gazing through with the opera glasses.
Across the lot was the new headquarters, a somewhat dilapidated old-fashioned brick house of several generations back. Through the glass we could see an evil-countenanced crook slinking along. He mounted the steps and rang the bell, turning as he waited.
From a small aperture in the doorway looked out another face, equally evil. Under cover, the crook made the sign of the clutching hand twice and was admitted.
"That's the place, all right," whispered Kennedy with satisfaction.
He hurried to a telephone booth where he called several numbers. Then we returned to the laboratory, while Kennedy quickly figured out a plan of action. I knew Chase was expected there soon.
From the table he picked up the small coil over which I had seen him working, and attached it to the bell and some batteries. He replaced it on the table, while I watched curiously.
"A selenium cell," he explained. "Only when light falls on it does it become a good conductor of electricity. Then the bell will ring."
Just before making the connection he placed his hat over the cell. Then he lifted the hat. The light fell on it and the bell rang. He replaced the hat and the bell stopped. It was evidently a very peculiar property of the substance, selenium.