“Impossible.  You would not know when you saw them.  They are just rough stones.”

“Oh, yes, I would.”

“No, stay where you are.  Unless I attend to it the diamonds might be ruined.”

There was something peculiar about his insistence, but after he picked out the next diamond I was hardly prepared for Kennedy’s next remark.

“Let me see the palms of your hands.”

Poissan shot an angry glance at Kennedy, but he did not open his hands.

“I merely wish to convince you, ‘Mr. Spencer,’” said Kennedy to me, “that it is no sleight-of-hand trick and that the professor has not several uncut stones palmed in his hand like a prestidigitator.”

The Frenchman faced us, his face livid with rage.  “You call me a prestidigitator, a fraud—­you shall suffer for that!  Sacrebleu!  Ventre du Saint Gris!  No man ever insults the honour of Poissan.  Francois, water on the electrodes!”

The assistant dashed a few drops of water on the electrodes.  The sickish odour increased tremendously.  I felt myself almost going, but with an effort I again roused myself.  I wondered how Craig stood the fumes, for I suffered an intense headache and nausea.

“Stop!” Craig thundered.  “There’s enough cyanogen in this room already.  I know your game—­the water forms acetylene with the carbon, and that uniting with the nitrogen of the air under the terrific heat of the electric arc forms hydrocyanic acid.  Would you poison us, too?  Do you think you can put me unconscious out on the street and have a society doctor diagnose my case as pneumonia?  Or do you think we shall die quietly in some hospital as a certain New York banker did last year after he had watched an alchemist make silver out of apparently nothing!”

The effect on Poissan was terrible.  He advanced toward Kennedy, the veins in his face fairly standing out.  Shaking his forefinger, he shouted:  “You know that, do you?  You are no professor, and this is no banker.  You are spies, spies.  You come from the friends of Morowitch, do you?  You have gone too far with me.”

Kennedy said nothing, but retreated and took his coat and hat off the window ledge.  The hideous penetrating light of the tongues of flame from the furnace played on the ground-glass window.

Poissan laughed a hollow laugh.

“Put down your hat and coat, Mistair Kennedy,” he hissed.  “The door has been locked ever since you have been here.  Those windows are barred, the telephone wire is cut, and it is three hundred feet to the street.  We shall leave you here when the fumes have overcome you.  Francois and I can stand them up to a point, and when we reach that point we are going.”

Instead of being cowed Kennedy grew bolder, though I, for my part, felt so weakened that I feared the outcome of a hand-to-hand encounter with either Poissan or Francois, who appeared as fresh as if nothing had happened.  They were hurriedly preparing to leave us.