He lighted a match and touched it to the open upturned end.  The hydrogen, now escaping freely, was ignited with a pale-blue flame.

Next, he took the little piece of wall-paper I had seen him tear off in the den, scraped off some powder from it, dissolved it, and poured it into the funnel-tube.

Almost immediately the pale, bluish flame turned to bluish white, and white fumes were formed.  In the ignition-tube a sort of metallic deposit appeared.  Quickly he made one test after another.  I sniffed.  There was an unmistakable smell of garlic in the air.

“Arseniureted hydrogen,” commented Craig.  “This is the Marsh test for arsenic.  That wall-paper in Brixton’s den has been loaded down with arsenic, probably Paris green or Schweinfurth green, which is aceto-arsenite of copper.  Every minute he is there he is breathing arseniureted hydrogen.  Some one has contrived to introduce free hydrogen into the intake of his ventilator.  That acts on the arsenic compounds in the wall-paper and hangings and sets free the gas.  I thought I knew the smell the moment I got a whiff of it.  Besides, I could tell by the jaundiced look of his face that he was being poisoned.  His liver was out of order, and arsenic seems to accumulate in the liver.”

“Slowly poisoned by minute quantities of gas,” I repeated in amazement.  “Some one in that Red Brotherhood is a diabolical genius.  Think of it—­poisoned wall-paper!”

It was still early in the forenoon when Kennedy excused himself, and leaving me to my own devices disappeared on one of his excursions into the underworld of the foreign settlements on the East Side.  About the middle of the afternoon he reappeared.  As far as I could learn all that he had found out was that the famous, or rather infamous, Professor Michael Kumanova, one of the leaders of the Red Brotherhood, was known to be somewhere in this country.

We lost no time in returning again to Woodrock late that afternoon.  Craig hastened to warn Brixton of his peril from the contaminated atmosphere of the den, and at once a servant was set to work with a vacuum cleaner.

Carefully Craig reconnoitred the basement where the eavesdropping storeroom was situated.  Finding it deserted, he quickly set to work connecting the two wires of the general household telephone with what looked very much like a seamless iron tube, perhaps six inches long and three inches in diameter.  Then he connected the tube also with the private wire of Brixton in a similar manner.

“This is a special repeating-coil of high efficiency,” he explained in answer to my inquiry.  “It is absolutely balanced as to resistance, number of turns, and everything.  I shall run this third line from the coil into Brixton’s den, and then, if you like, you can accompany me on a little excursion down to the village where I am going to install another similar coil between the two lines at the local telephone central station opposite the railroad.”