"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."

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 anyone else?"Madame reflected, then shook her head. "I do not think so. I am almost sure it could 

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