Poirot nodded, and gently taking the photographs from me locked them up again.

“I suppose,” I said, “that as usual, you are not going to explain?”

“On the contrary. No. 1 were the finger-prints of Monsieur Lawrence. No. 2 were those of Mademoiselle Cynthia. They are not important. I merely obtained them for comparison. No. 3 is a little more complicated.”

“Yes?”

“It is, as you see, highly magnified. You may have noticed a sort of blur extending all across the picture. I will not describe to you the special apparatus, dusting powder, etc., which I used. It is a well-known process to the police, and by means of it you can obtain a photograph of the finger-prints of any object in a very short space of time. Well, my friend, you have seen the finger-marks—it remains to tell you the particular object on which they had been left.”

“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?”

“Why, that there is altogether too much strychnine about this case. This is the third time we run up against it. There was strychnine in Mrs. Inglethorp’s tonic. There is the strychnine sold across the counter at Styles St. Mary by Mace. Now we have more strychnine, handled by one of the household. It is confusing; and, as you know, I do not like confusion.”

Before I could reply, one of the other Belgians opened the door and stuck his head in.

“There is a lady below, asking for Mr Hastings.”

“A lady?”

I jumped up. Poirot followed me down the narrow stairs. Mary Cavendish was standing in the doorway.

“I have been visiting an old woman in the village,” she explained, “and as Lawrence told me you were with Monsieur Poirot I thought I would call for you.”

“Alas, madame,” said Poirot, “I thought you had come to honour me with a visit!”

“I will some day, if you ask me,” she promised him, smiling.

“That is well. If you should need a father confessor, madame”—she started ever so slightly—“remember, Papa Poirot is always at your service.”

She stared at him for a few minutes, as though seeking to read some deeper meaning into his words. Then she turned abruptly away.

“Come, will you not walk back with us too, Monsieur Poirot?”

“Enchanted, madame.”

All the way to Styles, Mary talked fast and feverishly. It struck me that in some way she was nervous of Poirot’s eyes.

The weather had broken, and the sharp wind was almost autumnal in its shrewishness. Mary shivered a little, and buttoned her black sports coat closer. The wind through the trees made a mournful noise, like some great giant sighing.

We walked up to the great door of Styles, and at once the knowledge came to us that something was wrong.

Dorcas came running out to meet us. She was crying and wringing her hands. I was aware of other servants huddled together in the background, all eyes and ears.

“Oh, m’am! Oh, m’am! I don’t know how to tell you——”

“What is it, Dorcas?” I asked impatiently. “Tell us at once.”

“It’s those wicked detectives. They’ve arrested him—they’ve arrested Mr. Cavendish!”

“Arrested Lawrence?” I gasped.