"Or Keppel," said Stone with great emphasis. He leaned over and tapped me on the knee. "The only constant visitor he did have! This is the way I look at it, Blake. I pride myself on being go-ahead. I've had a lot of fun, and got a whole lot of information, about reading this Experimental Psychology. You've got to figure out how a man's mind is going to work.
"Now look at this murder. Under the circumstances, the police must assume — which is what you're all assuming — that it was an inside job. I mean, that somebody who had access to Antrim's place: somebody close at hand: sneaked in and changed the bottles. Therefore the field's limited. Therefore the police don't look any farther. But suppose that's just what the murderer wants you to think?
"Suppose that what Mrs. Antrim served Hogenauer yesterday evening really was honest-to-God bromide and nothing else? All right! — Hogenauer comes home with it. If he had medicine from the doctor, why didn't he take a dose of it last night? That's what I want to know. But maybe he did take it, and it didn't hurt him because it was really bromide. All right!" said Stone growing more excited as he grew more earnest. "But somebody learned he'd got the bromide at Antrim's, and therefore thought of a mighty slick little plan.
"So this person, during the night, goes over to Antrim's and does a little of the Observe-My-Neighbour burglary that seems to be so popular in this neck of the woods. He gets into Antrim's dispensary. There are the bottles on the shelves- perfectly honest bottles; see what I mean? The bromide bottle now has a lot gone out of it, but the bottle of strychnine salts is full: because nobody has touched it. Well, this somebody has in his pocket a big dose of ordinary bromide powder that he could get at any drug-store. So he fills up the real bromide bottle. Then he goes over and steals a hefty dose of strychnine salts out of the other bottle. Next he pushes the strychnine bottle way out of line in the shelf, so it'll be noticed next morning. He sticks some traces of mucilage on each label. Maybe he's even written a couple of fake labels, and crumpled them up and thrown them somewhere that they'll be found later.
"So you can see what a set-up he's got now. It'll be noticed, all right. He'll have proof that, by a change of bottles, Hogenauer got strychnine; and that, later on, somebody put the bottles back in their right places. And in his pocket he's now got a sweet consignment of the poison ready to use on Hogenauer.
"I don't have to tell you how easy it would be, do I?" demanded Stone, sitting back. "All he's got to do is go and pay a visit to Hogenauer next day. He can get Hogenauer out of sight for a couple of minutes, and load the little bromide bottle with strychnine. The next dose Hogenauer takes — whiff! And there's rock-bottom proof Hogenauer must have got the poison at Antrim's. And you told me yourself that the only one who did visit Hogenauer to-day was Keppel.
"How am I doing?" Stone broke off to inquire, with broad complacency.
Again the blast of the whistle rose in mocking flight. In the course of trailing after H.M.'s investigations I have heard some devilishly ingenious explanations of murder-traps, but this one I had to consign to the top order. It was neat. It was simple. It would work.
Evelyn had a wrinkle over one eyebrow. "Yes, I know what you're thinking, Ken," she remarked moodily. "You're remembering the Chateau de I'Ile last year, and all those explanations… d'Andrieu… Auguste. You remember arguing with Auguste as to whether your valise did or didn't have a false bot — " She paused, and looked up with a startled expression at Serpos's bag in the rack over our heads. "Shades of Flamande! No, I don't suppose it could be. I say, Mr. Stone, it's terribly ingenious, and what do you know about Keppel?"
"Nothing," acknowledged Stone, in cherubic complacence. "Never heard his name until to-night."