David sat very thoughtfully for some little time.

"The case must have been changed," he said. "It is very difficult to say
how, but there is no other logical solution of the matter. At about
half-past twelve on that eventful night you placed on my doorstep a
gun-metal cigar-case, mounted in diamonds, that you had purchased from
Lockhart's?"

"Yes, and the very one that you admired. Of that I am certain."

"Very well. I take that case with me to 218, Brunswick Square, and I
bring it back again. Did I take it with me or not? Anyhow, it was found
on the floor beside the body. It never passed out of my possession to my
knowledge. Next day I leave it at the office of Messrs. Mossa and Mack,
and it gets into the hands of the police."

"Was it not possibly changed there, David?"

"No, because of the initials I had scratched inside it. And beyond all
question that case--the same case, mind you, that I picked up on my
doorstep--was purchased by the man now lying in the hospital here from
Walen's, in West Street. Now, how was the change made?"

"If I could only see my way to help you!"

"The change was made the day you bought the case. By the way, what
time was it?"

"I can't tell you the exact time," Ruth replied. "It was on the morning
of the night of your adventure."