ings. You see I knew that whether McMurtrie
stayed on at the Russell or not you were bound to see him again, and
there was nothing to be gained by asking questions which the porter
would probably repeat to him. It would only have helped to put him on
his guard--wouldn't it?"

"My dear Joyce," I said, "I think you did splendidly. Sherlock Holmes
couldn't have done better." I got up and walked to the end of the
cockpit. "But good Lord!" I added, "this does complicate matters.
You're absolutely certain it was McMurtrie you saw at Marks's flat?"

"Absolutely," repeated Joyce with emphasis. "I should remember his
face if I lived to be a hundred."

I clenched my fists in a sudden spasm of anger. "There's some damned
villainy underneath all this, Joyce," I said. "If McMurtrie was there
that afternoon the odds are that he knows who committed the murder."

"He did it himself," said Joyce calmly. "I'm as sure of it as I am
that I'm sitting here."

"But why?" I demanded--"why? Who on earth _was_ Marks? Nobody in
Chelsea seemed to know anything about him, and nothing came out at the
trial. Why should any one have wanted to kill him except me?"

Joyce shook her head. "I don't know," she said stubbornly; "but I'm
quite certain it was McMurtrie. I feel it inside me."

"And in any case," I continued, "what the devil is he doing messing
about with George? I'm the only connecting-link between them, and he
can't possibly mean to betray me--at all events, until he's got the
secret of the powder. He knows George would give me up tomorrow."

Joyce made a gesture of perplexity. "I know," she said. "It's an
absolute mystery to me too. I've been puzzling and puzzling over it
till my head aches, and I can't see any sort of explanation at all."

"The only thing that's quite plain," I said, "is the fact that
McMurtrie and Savaroff have been lying to me from the start. They are
no more powder-merchants than you are. They want to get hold of my
invention for some reason--to make money out of it, I suppose--and
then they're prepared to clear out and leave me to George and the
police. At least, that's what it's beginning to look like."

"Well, anyhow," said Joyce, "you're not tied to them any longer by
your promise."

"No," I said; "it takes two to keep a bargain. Besides," I added
rather bitterly, "I can afford the privilege of breaking my word. It's
only what you'd expect from a convict."

Joyce got up, and coming to where I was sittin