He had already placed it in a porcelain basin, and in this basin he had poured some liquids. Then he passed the liquids through a fine screen and at last took up a tube containing some of the resulting liquid.
"I have already satisfied myself," he explained, "but for your benefit, seeing that you're the chief sufferer, I'll run over a part of the test. You saw the reaction which showed the gas a moment ago. I have proved chemically as well as microscopically that it is present in your blood. Now if I take this test-tube of liquid derived from my treatment of the lock and then test it as you saw me do with the other, isn't that enough for you? See—it gives the same reaction."
It did, indeed, but my mind did not react with it.
"Nitrous oxide," he continued, "in contact with iron, leaves distinct traces of corrosion, discernible by chemical and microscopic tests quite as well as the marks it leaves in the human blood. Manifestly, if no one could have come in by the windows or doors, the gas must have been administered in some way without any one coming into the room. I found no traces of an intruder."
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.