Then from somewhere above him came such a trill of demoniacal laughter as chilled his blood. The top of the wall was concealed by the overhanging branch of a tree and his light was valueless. "Come down," he shouted, "I've got you covered!" Again came that terrible laugh, half-fear, half-derision, and a voice shrill and harsh came down to him. "Murderer! Murderer! You killed Thornton Lyne, damn you! I've kept this for you--take it!" Something came crashing through the trees, something small and round, a splashing drop, as of water, fell on the back of Tarling's hand and he shook it off with a cry, for it burnt like fire. He heard the mysterious stranger drop from the coping of the wall and the sound of his swift feet. He stooped and picked up the article which had been thrown at him. It was a small bottle bearing a stained chemist's label and the word "Vitriol." CHAPTER XXVIII THE THUMB-PRINT It was ten o'clock in the morning, and Whiteside and Tarling were sitting on a sofa in their shirt-sleeves, sipping their coffee. Tarling was haggard and weary, in contrast to the dapper inspector of police. Though the latter had been aroused from his bed in the early hours of the morning, he at least had enjoyed a good night's sleep. They sat in the room in which Mrs. Rider had been murdered, and the rusty brown stains on the floor where Tarling had found her were eloquent of the tragedy. They sat sipping their coffee, neither man talking, and they maintained this silence for several minutes, each man following his own train of thought. Tarling for reasons of his own had not revealed his own adventure and he had told the other nothing of the mysterious individual (who he was, he pretty well guessed) whom he had chased through the grounds. Presently Whiteside lit a cigarette and threw the match in the grate, and Tarling roused himself from his reverie with a jerk. "What do you make of it?" he asked. Whiteside shook his head. "If there had been property taken, it would have had a simple explanation. But nothing has gone. Poor girl!" Tarling nodded. "Terrible!" he said. "The doctor had to drug her before he could get her to go." "Where is she?" asked Whiteside "I sent her on an ambulance to a nursing-home in London," said Tarling shortly. "This is awful, Whiteside." "It's pretty bad," said the detective-inspector, scratching his chin. "The young lady could supply no information?" "Nothing, absolutely nothing. She had gone up to see her mother and had left the door ajar, intending to return by the same way after she had interviewed Mrs. Rider. As a matter of fact, she was let out by the front door. Somebody was watching and apparently thought that she was coming out by the way she went in, waited for a time, and then as she did not reappear, followed her into the building." "And that somebody was Milburgh?" said Whiteside. Tarling made no reply. He had his own views and for the moment was not prepared to argue.