together, you and M. Halliday?"

She looked a trifle surprised.

"But of his work! His work--and also mine."

"Did he mention to you the theories he had embodied recently in his paper read before the British Association?"

"Certainly he did. It was chiefly of those we spoke."

"His ideas were somewhat fantastic, were they not?" asked Poirot carelessly.

"Some people have thought so. I do not agree."

"You considered them practicable?"

"Perfectly practicable. My own line of research has been somewhat similar, though not undertaken with the

same end in view. I have been investigating the gamma rays emitted by the substance usually known as Radium C., a product of Radium emanation, and in doing

so I have come across some very interesting magnetical

phenomena. Indeed, I have a theory as to the actual

nature of the force we call magnetism, but it is not yet time for my discoveries to be given to the world. Mr.

Halliday's experiments and views were exceedingly interesting to me."

Poirot nodded. Then he asked a question which surprised

me.

"Madame, where did you converse on these topics. In here?"

"No, monsieur. In the laboratory."

"May I see it?"

"Certainly."

52 Agatha Christie

She led the way to the door from which she had

entered. It opened 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.

"Mademoiselle 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.

"Madame, were you alone with M. Halliday during

your interview?"

"Yes, monsieur. My two assistants were in the

smaller room next door."

"Could your conversation be overheard--by them or

any one else?"

Madame reflected, then shook her head.

"I do not think so. I am almost sure it could not. The doors were all shut."

"Could any one have been concealed in the room?"

"There is the big cupboard in the corner--but the

idea is absurd."

"Pas tout d fait, madame. One thing more: did M.

Halliday make any mention of his plans for the evening?"

"He said nothing whatever, monsieur."

"I thank you, madame, and I apologise for disturbing you. Pray do not trouble--we can find our way

out."

THE BIG FOUR 53

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.