Heath went at once and summoned the nurse.
But her evidence was unilluminating. She had left Mrs. Greene reading about eleven o'clock, had gone to her own room to make her toilet for the night, and had returned to Ada's room half an hour later, where she had slept all night, according to Heath's instructions. She had risen at eight, dressed, and gone to the kitchen to fetch Mrs. Greene's tea. As far as she knew, Mrs. Greene had drunk nothing before retiring—certainly she had taken no citrocarbonate up to eleven o'clock. Furthermore, Mrs. Greene never attempted to take it alone.
"You think, then," asked Vance, "that it was given to her by some one else?"
"You can bank on it," the nurse assured him bluntly. "If she'd wanted it, she'd have raised the house before mixing it herself."
"It's quite obvious," Vance observed to Markham, "that some one entered her room after eleven o'clock and prepared the citrocarbonate."
Markham got up and walked anxiously about the room.
"Our immediate problem boils down to finding out who had the opportunity to do it," he said. "You, Miss O'Brien, may return to your room. . . ." Then he went to the bell-cord and rang for Sproot.
During a brief interrogation of the butler the following facts were brought out:
The house had been locked up, and Sproot had retired, at about half past ten.
Sibella had gone to her room immediately after dinner, and had remained there.
Hemming and the cook had lingered in the kitchen until