XII. The Last Link

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"We are now able to account for the symptoms of strychnine poisoning being so long in making their appearance. A narcotic taken with strychnine will delay the action of the poison for some hours."

Poirot paused. Mary looked up at him, the colour slowly rising in her face.

"All you have said is quite true, Monsieur Poirot. It was the most awful hour of my life. I shall never forget it. But you are wonderful. I understand now----"

"What I meant when I told you that you could safely confess to Papa Poirot, eh? But you would not trust me."

"I see everything now," said Lawrence. "The drugged coco, taken on top of the poisoned coffee, amply accounts for the delay."

"Exactly. But was the coffee poisoned, or was it not? We come to a little difficulty here, since Mrs. Inglethorp never drank it."

"What?" The cry of surprise was universal.

"No. You will remember my speaking of a stain on the carpet in Mrs. Inglethorp's room? There were some peculiar points about that stain. It was still damp, it exhaled a strong odour of coffee, and imbedded in the nap of the carpet I found some little splinters of china. What had happened was plain to me, for not two minutes before I had placed my little case on the table near the window, and the table, tilting up, had deposited it upon the floor on precisely the identical spot. In exactly the same way, Mrs. Inglethorp had laid down her cup of coffee on reaching her room the night before, and the treacherous table had played her the same trick.

"What happened next is mere guess work on my part, but I should say that Mrs. Inglethorp picked up the broken cup and placed it on the table by the bed. Feeling in need of a stimulant of some kind, she heated up her coco, and drank it off then and there. Now we are faced with a new problem. We know the coco contained no strychnine. The coffee was never drunk. Yet the strychnine must have been administered between seven and nine o'clock that evening. What third medium was there--a medium so suitable for disguising the taste of strychnine that it is extraordinary no one has thought of it?" Poirot looked round the room, and then answered himself impressively. "Her medicine!"

"Do you mean that the murderer introduced the strychnine into her tonic?" I cried.

"There was no need to introduce it. It was already there-- in the mixture. The strychnine that killed Mrs. Inglethorp was the identical strychnine prescribed by Dr. Wilkins. To make that clear to you, I will read you an extract from a book on dispensing which I found in the Dispensary of the Red Cross Hospital at Tadminster:

" 'The following prescription has become famous in text books: Strychninae Sulph . . . . . . gr.I Potass Bromide . . . . . . . 3vi Aqua ad . . . . . . . . . . . 3viii Fiat Mistura