Now that the danger was past for me, and he felt all right, his active mind began at once on the reconstruction of what had happened.

What was it—­man or devil?  Could a human fly have scaled the walls, or an aeroplane have dropped an intruder at the window ledge?  The lock on the door did not seem to have been tampered with.  Nor was there any way by which entrance could have been gained from a fire escape.  It was not illuminating gas.  Every one agreed on that.  No, it was not an accident.  It was an attempt at murder.  Some one was getting close to us.  Every other weapon failing, this was desperation.

I had been made comfortable, and he was engaged in one of his characteristic searches, with more than ordinary eagerness, because this was his own apartment, and it was I who had been the victim.

I followed him languidly as he went over everything, the furniture, the walls, the windows, the carpets—­there looking for finger-prints, there for some trace of the poisonous gas that had filled the room.  But he did not have the air of one who was finding anything.  I was too tired to reason.  This was but another of the baffling mysteries that confronted us.

A low exclamation caused me to open my eyes and try to discover what was the cause.  He was bending over the lock of the door looking at it intently.

“Broken?” I managed to say.

“No—­corroded,” he replied.  “You keep still.  Save your energy.  I’ve got strength enough for two, for a while.”

He came over to the bed and bent over me.  “I won’t hurt you,” he encouraged, “but just let me get a drop of your blood.”

He took a needle and ran it gently into my thumb beside the nail.  A drop or two of blood oozed out and he soaked it up with a piece of sterile gauze.

“Try to sleep,” he said finally.

“And you?” I asked.

“It’s no use.  I’m going over to the laboratory.  I can’t sleep.  There’s a cop down in front of the house.  You’re safe enough.  By George, if this case goes much further we’ll have half the force standing guard.  Here—­drink that.”

I had made up my mind not to go to sleep, if he wouldn’t, but I slipped up when I obeyed him that time.  I thought it was a stimulant but it turned out to be a sedative.

I did not wake up until well along in the morning, but when I did I was surprised to find myself so well.  Before any one could stop me, I was dressed and had reached the door.

A friend of ours who had volunteered to stay with me was dozing on a couch as I came out.

“Too late, Johnson,” I called, trying hard to be gay, though I felt anything but like it.  “Thank you, old man, for staying with me.  But I’m afraid to stop.  You’re stronger than I am this morning—­and besides you can run faster.  I’m afraid you’ll drag me back.”

He did try to do it, but with a great effort of will-power I persuaded him to let me go.  Out in the open air, too, it seemed to do me good.  The policeman who had been stationed before the house gazed at me as though he saw a ghost, then grinned encouragingly.