“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
This solution deposits in a few hours the greater part of the strychnine salt as an insoluble bromide in transparent crystals. A lady in England lost her life by taking a similar mixture: the precipitated strychnine collected at the bottom, and in taking the last dose she swallowed nearly all of it!”
“Now there was, of course, no bromide in Dr. Wilkins’ prescription, but you will remember that I mentioned an empty box of bromide powders. One or two of those powders introduced into the full bottle of medicine would effectually precipitate the strychnine, as the book describes, and cause it to be taken in the last dose. You will learn later that the person who usually poured out Mrs. Inglethorp’s medicine was always extremely careful not to shake the bottle, but to leave the sediment at the bottom of it undisturbed.
“Throughout the case, there have been evidences that the tragedy was intended to take place on Monday evening. On that day, Mrs. Inglethorp’s bell wire was neatly cut, and on Monday evening Mademoiselle Cynthia was spending the night with friends, so that Mrs. Inglethorp would have been quite alone in the right wing, completely shut off from help of any kind, and would have died, in all probability, before medical aid could have been summoned. But in her hurry to be in time for the village entertainment Mrs. Inglethorp forgot to take her medicine, and the next day she lunched away from home, so that the last–and fatal–dose was actually taken twenty-four hours later than had been anticipated by the murderer; and it is owing to that delay that the final proof–the last link of the chain–is now in my hands.”
Amid breathless excitement, he held out three thin strips of paper.
“A letter in the murderer’s own hand-writing, mes amis! Had it been a little clearer in its terms, it is possible that Mrs. Inglethorp, warned in time, would have escaped. As it was, she realized her danger, but not the manner of it.”
In the deathly silence, Poirot pieced together the slips of paper and, clearing his throat, read:
“‘Dearest Evelyn:
‘You will be anxious at hearing nothing. It is all right–only it will be to-night instead of last night. You understand. There’s a good time coming once the old woman is dead and out of the way. No one can possibly bring home the crime to me. That idea of yours about the bromides was a stroke of genius! But we must be very circumspect. A false step—-‘
“Here, my friends, the letter breaks off. Doubtless the writer was interrupted; but there can be no question as to his identity. We all know this hand-writing and—-”
A howl that was almost a scream broke the silence.
“You devil! How did you get it?”