Smith stepping forward and engaging the famous pathologist in conversation, I seized the opportunity to examine Sir Crichton’s body.

The dead man was in evening dress, but wore an old smoking-jacket.  He had been of spare but hardy build, with thin, aquiline features, which now were oddly puffy, as were his clenched hands.  I pushed back his sleeve, and saw the marks of the hypodermic syringe upon his left arm.  Quite mechanically I turned my attention to the right arm.  It was unscarred, but on the back of the hand was a faint red mark, not unlike the imprint of painted lips.  I examined it closely, and even tried to rub it off, but it evidently was caused by some morbid process of local inflammation, if it were not a birthmark.

Turning to a pale young man whom I had understood to be Sir Crichton’s private secretary, I drew his attention to this mark, and inquired if it were constitutional.  “It is not, sir,” answered Dr. Cleeve, overhearing my question.  “I have already made that inquiry.  Does it suggest anything to your mind?  I must confess that it affords me no assistance.”

“Nothing,” I replied.  “It is most curious.”

“Excuse me, Mr. Burboyne,” said Smith, now turning to the secretary, “but Inspector Weymouth will tell you that I act with authority.  I understand that Sir Crichton was—­seized with illness in his study?”

“Yes—­at half-past ten.  I was working here in the library, and he inside, as was our custom.”

“The communicating door was kept closed?”

“Yes, always.  It was open for a minute or less about ten-twenty-five, when a message came for Sir Crichton.  I took it in to him, and he then seemed in his usual health.”

“What was the message?”

“I could not say.  It was brought by a district messenger, and he placed it beside him on the table.  It is there now, no doubt.”

“And at half-past ten?”

“Sir Crichton suddenly burst open the door and threw himself, with a scream, into the library.  I ran to him but he waved me back.  His eyes were glaring horribly.  I had just reached his side when he fell, writhing, upon the floor.  He seemed past speech, but as I raised him and laid him upon the couch, he gasped something that sounded like `The red hand!’ Before I could get to bell or telephone he was dead!”

Mr. Burboyne’s voice shook as he spoke the words, and Smith seemed to find this evidence confusing.

“You do not think he referred to the mark on his own hand?”

“I think not.  From the direction of his last glance, I feel sure he referred to something in the study.”

“What did you do?”

“Having summoned the servants, I ran into the study.  But there was absolutely nothing unusual to be seen.  The windows were closed and fastened.  He worked with closed windows in the hottest weather.  There is no other door, for the study occupies the end of a narrow wing, so that no one could possibly have gained access to it, whilst I was in the library, unseen by me.  Had someone concealed himself in the study earlier in the evening—­and I am convinced that it offers no hiding-place—­ he could only have come out again by passing through here.”