Stone sat up.
"I've got you," Stone said. "He suggests Hogenauer ought to visit the doctor the night before the experiment for a going-over. He suggests Hogenauer ought to ask the doctor about bromide, and take a bromide before the experiment begins-"
"Sure. They were talkin' about Antrim, dye see, in that back parlour with the door shut. And Bowers, comin' in late and hearin' Hogenauer do so much talkie' about Antrim, thinks it's Antrim who's there.
"It's always the cussedest thing that happens. Those bottles really were switched in the surgery, with fake labels pasted on 'em: Charters arranged it in the evening before Hogenauer got there. He could get in easily through the French window. The trap was all ready.
"But a very revealin' question has been asked in this case. Somebody asked: If the murderer really switched the bottles, why was he so fastidious as to switch 'em back again to their right places? And there you got the answer before you. Because Charters's lop-sided conscience was always stingin' him in an unexpected place; it even stung him after it was dead, the way a wasp can. He could quite coolly arrange to poison Hogenauer. Y'know, I got a suspicion Charters has an idea that foreigners are — well, not exactly not human, but at least that poisonin' a foreigner is slightly less reprehensible an offense than poisonin' a countryman. He could kill Hogenauer. But he couldn't stand the thought that somebody else, somebody he wasn't after, might get a dose out of that bottle. Above all, at Madam Antrim's hands."
"So," put in Evelyn, "during the ten or fifteen minutes while Antrim was out for a stroll after Hogenauer had left, he sneaked in — "
"No!" said H. M. sharply. "That's just what I don't mean. Otherwise there'd have been no mix-up about that sash-window. Think back again. Antrim went for a stroll, yes. The house was open and the lights were on. But where did Antrim say he went for a stroll?"
"On the headland just behind the house," said Evelyn.
"Yes. And so, with the lights on, Charters couldn't get in without bein' seen. And afterwards Antrim locked up the place. But Charters had to get in.’
"He did get in late that night, d'ye see. But somethin' was rackin' him all over again. There were holes in his plan. The bottles had been switched and put back into their places, yes. Hogenauer now had the dose of poison, yes. But suppose nobody noticed that there had been a switch of bottles from a mysterious hand: as he'd intended? Suppose it was simply thought Mrs. Antrim had given poison to Hogenauer, out of the strychnine-bottle, with deliberate intent — as Evelyn did think?"