"I used to sit where I could command the hall," said Darrow, "and, therefore, I was aware that Monsieur X never left his room. To make the matter certain, I powdered the sill of the door with talcum, which I renewed every day after the cleaners. You remember we got to talking very earnestly in the hall, so earnestly that I, for one, forgot to watch. When I realized my remissness, I saw that the powder on the sill had been disturbed, that Monsieur X had gone out.
"My first thought then was to warn the people. To that end I was on my way to the Despatch office when sheer chance switched me into the City Hall tragedy. I possessed myself of the apparatus—"
"That was the square black bag!" cried Jack.
"Of course—and hustled back to the Atlas Building. You can bet I was relieved when I found that Monsieur X had returned to his lair."
"Talcum disturbed again?" asked Jack.
"Precisely."
"And the black bag?"
"Contained merely a model wireless apparatus with a clockwork arrangement set to close the circuit at a certain time. That is why Monsieur X was not involved in his own catastrophe."
"I see!"
"Then all I had to do was to sit still and wait for him to become dangerous."