It was a tough one.  Never much good at answering his conundrums when I was well, I could not even make a guess now.

“The key-hole, of course!” he explained.  “I cut away the entire lock, and have submitted it to these tests which you see.”

“I don’t see it all yet,” I said.

“Some one came to our door in the night, after gaining entrance to the hall—­not a difficult thing to do, we know.  That person found our door locked, knew it would be locked, knew that I always locked it.  Knowing that such was the case, this person came prepared, bringing perhaps, a tank of compressed nitrous oxide, certainly the materials for making the gas expeditiously.”

I began to understand how it had been done.

“Through the keyhole,” he resumed, “a stream of the gas was injected.  It soon rendered you unconscious, and that would have been all, if the person had been satisfied.  A little bit would have been harmless enough.  But the person was not satisfied.  The intention was not to overcome, but to kill.  The stream of gas was kept up until the room was full of it.

“Only my return saved you, for the gas was escaping very slowly.  Even then, you had been under it so long that we had to resort to the wonderful little pulmotor after trying both the Sylvester and Schaefer methods and all other manual means to induce respiration.  At any rate we managed to undo the work of this fiend.”

I looked at him in surprise, I, who didn’t think I had an enemy in the world.

“But who could it have been?” I asked.

“We are pretty close to that criminal,” was the only reply he would give, “providing we do not spread the net in sight of the quarry.”

“Why should he have wanted to get me?” I repeated.

“Don’t flatter yourself,” replied Craig.  “He wanted me, too.  There wasn’t any light in the laboratory last night.  There was a light in our apartment.  What more natural than to think that we were both there?  You were caught in the trap intended for both of us.”

I looked at him, startled.  Surely this was a most desperate criminal.  To cover up one murder—­perhaps two—­he did not hesitate to attempt a third, a double murder.  The attack had been really aimed at Kennedy.  It had struck me alone.  But it had miscarried and Craig had saved my life.

As I reflected bitterly, I had but one satisfaction.  Wretched as I felt, I knew that it had spared Craig from slowing up on the case at just the time when he was needed.

The news of the attempt spread quickly, for it was a police case and got into the papers.

It was not half an hour after I reached the laboratory that the door was pushed open by Inez Mendoza, followed by a boy spilling with fruit and flowers like a cornucopia.

“I drove to the apartment,” she cried, greatly excited and sympathetic, “but they told me you had gone out.  Oh, I was glad to hear it.  Then I knew it wasn’t so serious.  For, somehow, I feel guilty about it.  It never would have happened if you hadn’t met me.”