The Big Four by Agatha Christie

interesting to me.”
    Poirot nodded. Then he asked a question which surprised me.
    â€œMadame, where did you converse on these topics? In here?”
    â€œNo, monsieur. In the laboratory.”
    â€œMay I see it?”
    â€œCertainly.”
    She led the way to the door from which she had entered. Itopened on a small passage. We passed through two doors and found ourselves in the big laboratory, with its array of beakers and crucibles and a hundred appliances of which I did not even know the names. There were two occupants, both busy with some experiment. Madame Olivier introduced them.
    â€œMademoiselle Claude, one of my assistants.” A tall, serious-faced young girl bowed to us. “Monsieur Henri, an old and trusted friend.”
    The young man, short and dark, bowed jerkily.
    Poirot looked round him. There were two other doors besides the one by which we had entered. One, madame explained, led into the garden, the other into a smaller chamber also devoted to research. Poirot took all this in, then declared himself ready to return to the salon.
    â€œMadame, were you alone with M. Halliday during your interview?”
    â€œYes, monsieur. My two assistants were in the smaller room next door.”
    â€œCould your conversation be overheard—by them or anyone else?”
    Madame reflected, then shook her head.
    â€œI do not think so. I am almost sure it could not. The doors were all shut.”
    â€œCould anyone have been concealed in the room?”
    â€œThere is the big cupboard in the corner—but the idea is absurd.”
    â€œ Pas tout à fait, madame. One thing more: did M. Halliday make any mention of his plans for the evening?”
    â€œHe said nothing whatever, monsieur.”
    â€œI thank you, madame, and I apologize for disturbing you. Pray do not trouble—we can find our way out.”
    We stepped out into the hall. A lady was just entering the front door as we did so. She ran quickly up the stairs, and I was left with an impression of the heavy mourning that denotes a French widow.
    â€œA most unusual type of woman, that,” remarked Poirot, as we walked away.
    â€œMadame Olivier? Yes, she—”
    â€œ Mais non, not Madame Olivier. Cela va sans dire! There are not many geniuses of her stamp in the world. No, I referred to the other lady—the lady on the stairs.”
    â€œI didn’t see her face,” I said, staring. “And I hardly see how you could have done. She never looked at us.”
    â€œThat is why I said she was an unusual type,” said Poirot placidly. “A woman who enters her home—for I presume that it is her home since she enters with a key—and runs straight upstairs without even looking at two strange visitors in the hall to see who they are, is a very unusual type of woman—quite unnatural, in fact. Mille tonnerres! what is that?”
    He dragged me back—just in time. A tree had crashed down on to the sidewalk, just missing us. Poirot stared at it, pale and upset.
    â€œIt was a near thing that! But clumsy, all the same—for I had no suspicion—at least hardly any suspicion. Yes, but for my quick eyes, the eyes of a cat, Hercule Poirot might now be crushed out of existence—a terrible calamity for the world. And you, too, mon ami —though that would not be such a national catastrophe.”
    â€œThank you,” I said coldly. “And what are we going to do now?”
    â€œDo?” cried Poirot. “We are going to think. Yes, here andnow, we are going to exercise our little grey cells. This M. Halliday now, was he really in Paris? Yes, for Professor Bourgoneau, who knows him, saw and spoke to him.”
    â€œWhat on earth are you driving at?” I cried.
    â€œThat was Friday morning. He was last seen at eleven Friday night—but was he seen then?”
    â€œThe porter—”
    â€œA night porter—who had not previously seen Halliday. A man comes