"When we know the truth--as I hope we may very soon--then you will be staggered," I assured him. "At present you do not know the whole of the amazing story. For certain private reasons I have been unable to reveal it to you. But slowly, piece by piece, I have been steadily working upon the mystery of certain amazing occurrences at De Gex's house in Stretton Street. By slow degrees, and after travelling up and down Europe, I have at last succeeded in finding just a streak of daylight through the impenetrable barrier so cleverly contrived in order to mystify and mislead me. If you desire to ascertain the great ramifications of the desperate plots conceived by De Gex and his friends, and take steps to combat them, it will be best to allow his accomplice Sanz further liberty. Keep vigilant watch, but do not allow him to suspect," I urged. "He will no doubt go to Stretton Street again. Sanz, though a hired assassin as was his friend Despujol, should not be arrested yet, for the longer he remains at liberty the more extensive will be our information against the arch-schemer of Europe, Oswald De Gex." Rivero spent the evening with me. We dined at the Clarendon, across Hammersmith Bridge, and afterwards we idled in one of the foreign cafés near Piccadilly Circus. He was in London with a warrant for the arrest of Mateo Sanz in his pocket. But at my suggestion he stayed his hand. Meanwhile Sanz, all unsuspecting, was being carefully watched, not only by two detective-sergeants from Scotland Yard, but also by two Spanish detectives whom Rivero had brought to London with him. Two days later, in response to a message from Rivero, I called at the Hotel Cecil on leaving the office. He met me in the marble-paved entrance hall, and I noticed at once a grave expression upon his face. "Come up to my room," he said in French. "We can talk quietly there." In surprise I went with him up in the lift to the third floor where, in a bedroom which overlooked the Embankment and the Thames beyond, he turned suddenly to me and exclaimed, still in French: "I am very troubled and mystified, Monsieur Garfield. When you made those curious allegations against Monsieur De Gex I confess that I laughed them to scorn, but I have to-day learned several facts which put an entirely fresh complexion upon the present circumstances. Last night Mateo Sanz visited De Gex again. The financier gave a musical evening, but after the departure of all the guests, Sanz called and was at once admitted to De Gex's library." "Ah!" I exclaimed. "I know that room. I have sad cause to remember it!" "He remained there till nearly two o'clock in the morning. Then he returned on foot to his hotel. My information is that on his walk back he was whistling to himself, as though in high spirits." "But that is surely no extraordinary circumstance!" I remarked. "Did I not tell you that De Gex is as friendly with Sanz as he was with Despujol?"