Still, I was glad that the laboratory was only a few blocks away, for I was all in by the time I got there, and hadn’t even energy enough to reply to Kennedy’s scolding.

He was working over a microscope, while by his side stood in racks, innumerable test-tubes of various liquids.  On the table before him lay the lock of our door which he had cut out after he gave me the sleeping draught.

“What was it?” I asked.  “I feel as if I had been on a bust, without the recollection of a thing.”

He shook his head as if to discourage conversation, without taking his eyes off the microscope through which he was squinting.  His lips were moving as if he were counting.  I waited in impatient silence until he seemed to have finished.

Then, still without a word, he took up a test-tube and dropped into it a little liquid from a bottle on a shelf above the table.  His face lighted up, and he regarded the reaction attentively for some time.  Then he turned to me, still holding the tube.

“You have been on a bust,” he said with a smile as if the remark of a few minutes before were still fresh.  “Only it was a laughing gas jag—­nitrous oxide.”

“Nitrous oxide?” I repeated.  “How—­what do you mean?”

“I mean simply that a test of your blood shows that you were poisoned by nitrous oxide gas.  You remember the sample of blood which I squeezed from your thumb?  I took it because I knew that a gas—­and it has proved to be nitrous oxide—­is absorbed through the lungs into the circulation and its presence can be told for a considerable period after administration.”

He paused a moment, then went on:  “To be specific in this case I found by microscopic examination that the number of corpuscles in your blood was vastly above the normal, something like between seven and eight million to a drop that should have had somewhat more than only half that number.  You were poisoned by gas that—­”

“Yes,” I interrupted, “but how, with all the doors locked?”

“I was coming to that,” he said quietly, picking up the lock and looking at it thoughtfully.

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.”