He gazed about keenly.  Meanwhile, I had taken the crumpled note from him and was reading it.  Somehow, I had leaned against the wall.  As I turned, Craig happened to glance at me.

“For heaven’s sake, Walter,” I heard him exclaim.  “What have you been up against?”

He fairly leaped at me and I felt him examining my shoulder where I had been leaning on the wall.  Something on the paper had come off and had left a white mark on my shoulder.  Craig looked puzzled from me to the wall.

“Arsenic!” he cried.

He whipped out a pocket lens and looked at the paper.  “This heavy fuzzy paper is fairly loaded with it, powdered,” he reported.

I looked, too.  The powdered arsenic was plainly discernible.  “Yes, here it is,” he continued, standing absorbed in thought.  “But why did it work so effectively?”

He sniffed as he had before.  So did I. There was still the faint smell of garlic.  Kennedy paced the room.  Suddenly, pausing by the register, an idea seemed to strike him.

“Walter,” he whispered, “come down cellar with me.”

“Oh—­be careful,” cried Elaine, anxious for him.

“I will,” he called back.

As he flashed his pocket electric bull’s-eye about, his gaze fell on the electric meter.  He paused before it.  In spite of the fact that it was broad daylight, it was running.  His face puckered.

“They are using no current at present in the house,” he ruminated.  “Yet the meter is running.”

He continued to examine the meter.  Then he began to follow the electric wires along.  At last he discovered a place where they had been tampered with and tapped by other wires.

“The work of the Clutching Hand!” he muttered.

Eagerly he followed the wires to the furnace and around to the back.  There they led right into a little water tank.  Kennedy yanked them out.  As he did so he pulled something with them.

“Two electrodes—­the villain placed there,” he exclaimed, holding them up triumphantly for me to see.

“Y-yes,” I replied dubiously, “but what does it all mean?”

“Why, don’t you see?  Under the influence of the electric current the water was decomposed and gave off oxygen and hydrogen.  The free hydrogen passed up the furnace pipe and combining with the arsenic in the wall paper formed the deadly arseniuretted hydrogen.”

He cast the whole improvised electrolysis apparatus on the floor and dashed up the cellar steps.

“I’ve found it!” he cried, hurrying into Elaine’s room.  “It’s in this room—­a deadly gas—­arseniuretted hydrogen.”

He tore open the windows and threw them all open.  “Have her moved,” he cried to Aunt Josephine.  “Then have a vacuum cleaner go over every inch of wall, carpet and upholstery.”