"When the footman told me in German that the word was 'Freedom,' I knew that I should have to answer the challenge of the sentry in German. I did not know that he would challenge in Spanish, and if I had not understood him, or had replied in any other language but German, he would have shot us both down without saying another word, and no one would ever have known what had become of us. You will be exempt from this condition, because you will always come with me. I am, in fact, responsible for you."
"H'm, there doesn't seem much chance of any one getting through on false pretences," replied Arnold, with an irrepressible shudder. "Has any one ever tried?"
"Yes, once. The two gentlemen whose disappearance made the famous 'Clapham Mystery' of about twelve months ago. They were two of the smartest detectives in the French service, and the only two men who ever guessed the true nature of this house. They are buried under the floor on which you are standing at this moment."
The words were spoken with a cruel inflexible coldness, which struck Arnold like a blast of frozen air. He shivered, and was about to reply when Colston caught him by the arm again, and said hurriedly—
"H'st! We are going in. Remember what I said, and don't speak again till some one asks you to do so."
As he spoke a door opened in the wall of the dark chamber in which they had been standing for the last few minutes, and a flood of soft light flowed in upon their dazzled eyes. At the same moment a man's voice said from the room beyond in Russian—
"Who stands there?"
"Maurice Colston and the Master of the Air," replied Colston in the same language.
"You are welcome," was the reply, and then Colston, taking Arnold by the arm, led him into the room.