"That would not surprise me either. But how do you know it?"

"Do you remember that discovery of mine, that I called 'the sleeping
death'?"

"Yes. What has that to do with it?" Marcello's expression changed.

"Corbario stole one of the tablets from the tube in my pocket, while I
was asleep that night."

"What?" Marcello began to grow pale.

"Your mother died asleep," said Kalmon in a very low voice.

Marcello was transfixed with horror, and grasped the arms of his chair.
His face was livid. Kalmon watched him, and continued.

"Yes. Corbario did it. Your mother used to take phenacetine tablets when
she had headaches. They were very like the tablets of my poison in size
and shape. Corbario stole into my room when I was sound asleep, took one
of mine, and dropped in one of hers. Then he put mine amongst the
phenacetine ones. She took it, slept, and died."

Marcello gasped for breath, his eyes starting from his head.

"You see," Kalmon went on, "it was long before I found that my tablets
had been tampered with. There had been seven in the tube. I knew that,
and when I glanced at the tube next day there were seven still. The tube
was of rather thick blue glass, if you remember, so that the very small
difference between the one tablet and the rest could not be seen through
it. I went to Milan almost immediately, and when I got home I locked up
the tube in a strong-box. It was not until long afterwards, when I
wanted to make an experiment, that I opened the tube and emptied the
contents into a glass dish. Then I saw that one tablet was unlike the
rest. I saw that it had been made by a chemist and not by myself. I
analysed it and found five grains of phenacetine."

Marcello leaned back, listening intently, and still deadly pale.

"You did not know that I was trying to find out how you had been hurt,
that I was in communication with the police from the first, that I came
to Rome and visited you in the hospital before you recovered your
memory. The Contessa was very anxious to know the truth about her old
friend's son, and I did what I could. That was natural. Something told
me that Corbario had tried to kill you, and I suspected him, but it is
only lately that I have got all the evidence we need. There is not a
link lacking. Well, when I came to Rome that time, it chanced that I met
Corbario at the station. He had come by the same train, and was looking
dreadfully ill. That increased my suspicion, for I knew that