“I would have returned it to Mademoiselle to-morrow,” I murmured.

“Only one copy of it, I think,” he retorted; “the other you would have sold to whichever spy of the Danish or Russian Governments happened to have employed you in this discreditable business.”

“How did you know?” I said involuntarily.

“Through a very simple process of reasoning, my good M. Ratichon,” he replied blandly.  “You are a very clever man, no doubt, but the cleverest of us is at times apt to make a mistake.  You made two, and I profited by them.  Firstly, after my sister and I left you this afternoon, you never made the slightest pretence of making inquiries or collecting information about the mysterious theft of the document.  I kept an eye on you throughout the evening.  You left your office and strolled for a while on the quays; you had an excellent dinner at the Restaurant des Anglais; then you settled down to your coffee and liqueur.  Well, my good M. Ratichon, obviously you would have been more active in the matter if you had not known exactly where and when and how to lay your hands upon the document, for the recovery of which my sister had offered you ten thousand francs.”

I groaned.  I had not been quite so circumspect as I ought to have been, but who would have thought—­

“I have had something to do with police work in my day,” continued M. Geoffroy blandly, “though not of late years; but my knowledge of their methods is not altogether rusty and my powers of observation are not yet dulled.  During my sister’s visit to you this afternoon I noticed the blouse and cap of a commissionnaire lying in a bundle in a corner of your room.  Now, though M. de Marsan has been in a burning fever since he discovered his loss, he kept just sufficient presence of mind at the moment to say nothing about that loss to any of the Chancellerie officials, but to go straight home to his apartments in the Rue Royale and to send for my sister and for me.  When we came to him he was already partly delirious, but he pointed to a parcel and a letter which he had brought away from his office.  The parcel proved to be an empty box and the letter a blank sheet of paper; but the most casual inquiry of the concierge at the Chancellerie elicited the fact that a commissionaire had brought these things in the course of the morning.  That was your second mistake, my good M. Ratichon; not a very grave one, perhaps, but I have been in the police, and somehow, the moment I caught sight of that blouse and cap in your office, I could not help connecting it with the commissionnaire who had brought a bogus parcel and letter to my future brother-in-law a few minutes before that mysterious and unexplained altercation took place in the corridor.”

Again I groaned.  I felt as a child in the hands of that horrid creature who seemed to be dissecting all the thoughts which had run riot through my mind these past twenty hours.