“One moment more,” Brixton interrupted, still holding them.  “I shall come back to the letters.  That is not the worst.  I’ve had threatening letters before.  Have you noticed this room?”

We had both seen and been impressed by it.

“Let me tell you more about it,” he went on.  “It was designed especially to be, among other things, absolutely soundproof.”

We gazed curiously about the strong room.  It was beautifully decorated and furnished.  On the walls was a sort of heavy, velvety green wall-paper.  Exquisite hangings were draped about, and on the floor were thick rugs.  In all I noticed that the prevailing tint was green.

“I had experiments carried out,” he explained languidly, “with the object of discovering methods and means for rendering walls and ceilings capable of effective resistance to sound transmission.  One of the methods devised involved the use under the ceiling or parallel to the wall, as the case might be, of a network of wire stretched tightly by means of pulleys in the adjacent walls and not touching at any point the surface to be protected against sound.  Upon the wire network is plastered a composition formed of strong glue, plaster of Paris, and granulated cork, so as to make a flat slab, between which and the wall or ceiling is a cushion of confined air.  The method is good in two respects:  the absence of contact between the protective and protected surfaces and the colloid nature of the composition used.  I have gone into the thing at length because it will make all the more remarkable what I am about to tell you.”

Kennedy had been listening attentively.  As Brixton proceeded I had noticed Kennedy’s nostrils dilating almost as if he were a hound and had scented his quarry.  I sniffed, too.  Yes, there was a faint odour, almost as if of garlic in the room.  It was unmistakable.  Craig was looking about curiously, as if to discover a window by which the odour might have entered.  Brixton, with his eyes following keenly every move, noticed him.

“More than that” he added quickly, “I have had the most perfect system of modern ventilation installed in this room, absolutely independent from that in the house.”

Kennedy said nothing.

“A moment ago, Mr. Kennedy, I saw you and Mr. Jameson glancing up at the ceiling.  Sound-proof as this room is, or as I believe it to be, I—­I hear voices, voices from—­not through, you understand, but from—­that very ceiling.  I do not hear them now.  It is only at certain times when I am alone.  They repeat the words in some of these letters—­’You must not take up those bonds.  You must not endanger the peace of the world.  You will never live to get the interest.’  Over and over I have heard such sentences spoken in this very room.  I have rushed out and up the corridor.  There has been no one there.  I have locked the steel door.  Still I have heard the voices.  And it is absolutely impossible that a human being could get close enough to say them without my knowing and finding out where he is.”