"Right where you are, facing the laboratory door."
The Thinking Machine glanced back at the window behind him.
"And you?" he asked.
"I sat here facing her."
"You know that she did not enter the laboratory?"
"I know it, yes," replied Professor Dexter promptly. "No one save me has entered that laboratory today. I have taken particular pains to see that no one did. When Mr. Bowen spoke to me I had the radium in my hand. He merely opened the door, handed me her card and went right out. Of course it's impossible that--"
"Nothing is impossible, Mr. Dexter," blazed The Thinking Machine suddenly. "Did you at any time leave Mme. du Chastaigny in this room alone?"
"No, no," declared Dexter emphatically. "I was looking at her every moment she was here; I did not put the radium out of my hand until Mr. Bowen was out of this room and in the hallway there. I then came into this room and met her."
For several minutes The Thinking Machine sat perfectly silent, squinting upward while Professor Dexter gazed into the inscrutable face anxiously.
"I hope," ventured the Professor at last, "that you do not believe it was any fault of mine?"
The Thinking Machine did not say.
"What sort of a voice has Mme. du Chastaigny?" he asked instead.
The Professor blinked a little in bewilderment.
"An ordinary voice-the low voice of a woman of education and refinement," he replied.
"Did she raise it at any time while talking?"
"No."
"Perhaps she sneezed or coughed while talking to you?"
Unadulterated astonishment was written on Professor Dexter's face.
"She coughed, yes, violently," he replied.
"Ah!" exclaimed The Thinking Machine and there was a flash of comprehension in the narrow blue eyes. "Twice, I suppose?"
Professor Dexter was staring at the scientist blankly.
"Yes, twice," he responded.
"Anything else?"
"Well, she laughed I think."
"What was the occasion of her laughter?"
"I stumbled over a suit case she had set down by the laboratory door there."
The Thinking Machine absorbed that without evidence of emotion, then reached for the letter of introduction which Mme. du Chastaigny had given to Professor Dexter and which he still carried crumpled up in his hand. It was a short note, just a few lines in French, explaining that Mme. du Chastaigny desired to see Professor Dexter on a matter of importance.
"Do you happen to know Mme. Curie's handwriting?" asked The Thinking Machine after a cursory examination. "Of course you had some correspondence with her about this work?"
"I know her writing, yes," was the reply. "I think that is genuine, if that's what you mean."
"We'll see after a while," commented The Thinking Machine.
He arose and led the way into the laboratory. There Professor Dexter indicated to him the exact spot on the work table where the radium had been placed. Standing beside it he made some mental calculation as he squinted about the room, at the highly placed windows, the glass roof above, the single door. Then wrinkles grew in his tall brow.
"I presume all the wall windows are kept fastened?"
"Yes, always."
"And those in the glass roof?"
"Yes."
"Then bring me a tall step-ladder please!"
It was produced after a few minutes. Professor Dexter looked on curiously and with a glimmer of understanding as The Thinking Machine examined each catch on every window, and tapped the panes over with a pen-knife. When he had examined the last and found all locked he came down the ladder.
"Dear me!" he exclaimed petulantly. "It's perfectly extraordinary-most extraordinary. If the radium was not stolen through the reception room, then-then--" He glanced around the room again.
Professor Dexter shook his head. He had recovered his self-possession somewhat, but his bewilderment left him helpless.
"Are you sure, Professor Dexter," asked The Thinking Machine at last coldly, "are you sure you placed the radium where you have indicated?"
There was almost an accusation in the tone and Professor Dexter flushed hotly.
"I am positive, yes," he replied.
"And you are absolutely certain that neither Mr. Bowen nor Mme. du Chastaigny entered this room?"
"I am absolutely positive."
The Thinking Machine wandered up and down the long table apparently without any interest, handling the familiar instruments and glittering appliances as a master.
"Did Mme. du Chastaigny happen to mention any children?" he at last asked, irrelevantly.
Professor Dexter blinked again.
"No," he replied.
"Adopted or otherwise?"
"No."