was fitted a long funnel; through another ran a glass tube,
connecting with a large U-shaped drying-tube filled with calcium
chloride, which in turn connected with a long open tube with an
up-turned end.

Into the flask Craig dropped some pure granulated zinc coated with
platinum. Then he covered it with dilute sulphuric acid through
the funnel tube. "That forms hydrogen gas," he explained, "which
passes through the drying-tube and the ignition-tube. Wait a
moment until all the air is expelled from the tubes."

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