"Go on--I am really excited."
"Eh bien! Photo No. 3 represents the highly magnified surface of a tiny bottle in the top poison cupboard of the dispensary in the Red Cross Hospital at Tadminster--which sounds like the house that Jack built!"
"Good heavens!" I exclaimed. "But what were Lawrence Cavendish's finger-marks doing on it? He never went near the poison cupboard the day we were there!"
"Oh, yes, he did!"
"Impossible! We were all together the whole time."
Poirot shook his head.
"No, my friend, there was a moment when you were not all together. There was a moment when you could not have been all together, or it would not have been necessary to call to Monsieur Lawrence to come and join you on the balcony."
"I'd forgotten that," I admitted. "But it was only for a moment."
"Long enough."
"Long enough for what?"
Poirot's smile became rather enigmatical.
"Long enough for a gentleman who had once studied medicine to gratify a very natural interest and curiosity."
Our eyes met. Poirot's were pleasantly vague. He got up and hummed a little tune. I watched him suspiciously.
"Poirot," I said, "what was in this particular little bottle?"
Poirot looked out of the window.
"Hydro-chloride of strychnine," he said, over his shoulder, continuing to hum.
"Good heavens!" I said it quite quietly. I was not surprised. I had expected that answer.
"They use the pure hydro-chloride of strychnine very little-- only occasionally for pills. It is the official solution, Liq. Strychnine Hydro-clor. that is used in most medicines. That is why the finger-marks have remained undisturbed since then."
"How did you manage to take this photograph?"
"I dropped my hat from the balcony," explained Poirot simply. "Visitors were not permitted below at that hour, so, in spite of my many apologies, Mademoiselle Cynthia's colleague had to go down and fetch it for me."
"Then you knew what you were going to find?"
"No, not at all. I merely realized that it was possible, from your story, for Monsieur Lawrence to go to the poison cupboard. The possibility had to be confirmed, or eliminated."
"Poirot," I said, "your gaiety does not deceive me. This is a very important discovery."
"I do not know," said Poirot. "But one thing does strike me. No doubt it has struck you too."
"What is that?"