Agatha Christie: The Mysterious Affair at Styles

CHAPTER 12. THE LAST LINK (continued)

"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.