Strychnine

He was so quaintly humorous that I was forced to laugh; and we went together to the drawing-room, where the coffee-cups and tray remained undisturbed as we had left them.

Poirot made me recapitulate the scene of the night before, listening very carefully, and verifying the position of the various cups.

"So Mrs. Cavendish stood by the tray--and poured out. Yes. Then she came across to the window where you sat with Mademoiselle Cynthia. Yes. Here are the three cups. And the cup on the mantel-piece, half drunk, that would be Mr. Lawrence Cavendish's. And the one on the tray?"

"John Cavendish's. I saw him put it down there."

"Good. One, two, three, four, five--but where, then, is the cup of Mr. Inglethorp?"

"He does not take coffee."

"Then all are accounted for. One moment, my friend."

With infinite care, he took a drop or two from the grounds in each cup, sealing them up in separate test tubes, tasting each in turn as he did so. His physiognomy underwent a curious change. An expression gathered there that I can only describe as half puzzled, and half relieved.

"Bien!" he said at last. "It is evident! I had an idea--but clearly I was mistaken. Yes, altogether I was mistaken. Yet it is strange. But no matter!"

And, with a characteristic shrug, he dismissed whatever it was that was worrying him from his mind. I could have told him from the beginning that this obsession of his over the coffee was bound to end in a blind alley, but I restrained my tongue. After all, though he was old, Poirot had been a great man in his day.