About noon on the following day Falconer called at the Mapleton’s house in El Pardo as though upon a casual visit. As soon as he met Sylvia, the girl called him aside, and whispered:
“I’m so glad you are here, Geoffrey. Mrs. Mapleton had another attack last night, but is better now. She is in the habit of eating but very little dinner, and taking some patent invalid food just before going to bed. I managed to save a little of it before Martin cleared it away. I’ve got it in a bottle upstairs.”
“Excellent. I will take it at once to Professor Barrera,” replied Falconer. “He will analyse it, and see whether it has been doctored.”
The young Englishman remained to luncheon, and then, without telling anyone of his journey to San Sebastian, he went back to Madrid, and there saw the Professor, who had already been warned by the police.
Next day, when Falconer called upon Professor Barrera, he was told that into the invalid food had been introduced the juice of a certain poisonous mushroom which produced the exact symptoms of acute indigestion, and which, when absorbed by the human body, was almost impossible to detect. It was one of the most subtle and dangerous poisons known to modern toxicologists.
“The mushroom is a large dark-grey fungus with scarlet spots and grows on the mountains. It is found often in the Guadarrama,” he said. “Whoever is using it must be an expert poisoner.”
With that knowledge, and the other knowledge he now possessed, Falconer waited until evening, and then returned to El Pardo, where he was asked to remain to dinner, and to sleep, as a motor excursion had been arranged for the following day.
He dined, but though he went to his room, he could not sleep. The night was moonlit, and from his window he had a good view of the white road outside. Instead of undressing, he watched that road through the night hours until the first streak of dawn. It slowly became light at about four o’clock, when suddenly he saw the figure of a man going out upon a brisk walk.
Without a second’s hesitation he took his hat, and creeping silently down the stairs, let himself out.
By that time the man, whose figure he had recognised as Martin’s, was far ahead. The morning mist was thick as, leaving the highway, he ascended the steep hill-path, Geoffrey, whose rubber-soled boots made but little noise, following swiftly.