"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."