“What sort of place was it in which the receivers of the detectaphone were located—­do you know?” asked Kennedy quickly.

“Yes, it seems to be a very respectable boardinghouse,” answered Carton.  “She came there with a grip about a week ago and hired a room, saying she was out of town a great deal.  Just about the same time a young man, who posed as a student in electrical engineering at some school uptown, left.  It must have been he who installed the detectaphone—­perhaps with the aid of a waiter in Gastron’s.  At any rate, she seems to have been alone in the boarding-house—­ that is, I mean, not acquainted with any of the other guests—­ during the time when she was taking down the record.  Dorgan traced the wires, outside the two buildings, to her rooms, but she was not there.  In fact there was nothing there but a grip with a few articles that give no clue to anything.  Somehow she must have heard of it, for no one knows anything about her, since then.”

“Perhaps Langhorne is keeping her out of the way so that no one can tamper with her testimony,” I suggested.

“It’s possible,” said Carton in a tone that showed that he did not believe in that explanation.  “How about that safe robbery, Kennedy?  Some of the papers hinted that she might have known something of that.  I had a man down there watching, afterwards, but I had cautioned him to be careful and keep under cover.  One of the elevator boys told him that the robbers had made a hole in the safe.  What did he mean?  Did you see it?”

Rapidly Kennedy sketched what we had done, telling the story of how the dynamometer had at least partly exonerated Betty Blackwell.

When he reached the description of the hole in the safe, Carton was absolutely incredulous.  As for myself, it presented a mystery which I found absolutely inexplicable.  How it was possible in such a short time to make a hole in a safe by any known means, I could not understand.  In fact, if I had not seen it myself, I should have been even more sceptical than Carton.

Kennedy, however, made no reply immediately to our expressions of doubt.  He had found and set apart from the rest a couple of little glass bottles with ground glass stoppers.  Then he took a thick piece of steel and laid it across a couple of blocks of wood, under which was a second steel plate.

Without a word of explanation, he took the glass stopper out of the larger bottle and poured some of the contents on the upper plate of steel.  There it lay, a little mound of reddish powder.  Then he took a little powder of another kind from the other bottle.

He lighted a match and ignited the second pile of powder.

“Stand back—­close to the wall—­shield your eyes,” he called to us.

He had dropped the burning mass on the red powder and in two or three leaps he joined us at the far end of the room.

Almost instantly a dazzling, intense flame broke out.  It seemed to sizzle and crackle.  With bated breath we waited and, as best we could, shielding our eyes from the glare, watched.