“That action,” he went on, looking from one of us to another, “may be therapeutic, as in the cure for lead poisoning by removing the lead, or it may be toxic—­as in the case of actually introducing such a poison as strychnine into the body by the same forces that will remove the lead.”

He paused a moment, to enforce the point which had already been suggested.  I glanced about hastily.  If anyone in his little audience was guilty, no one betrayed it, for all were following him, fascinated.  Yet in the wildly throbbing brain of some one of them the guilty knowledge must be seared indelibly.  Would the mere accusation be enough to dissociate the truth from, that brain or would Kennedy have to resort to other means?

“Some one,” he went on, in a low, tense voice, leaning forward, “some one who knew this effect placed strychnine salts on one of the electrodes of the bath which Owen Minturn was to use.”

He did not pause.  Evidently he was planning to let the force of his exposure be cumulative, until from its sheer momentum it carried everything before it.

“Walter,” he ordered quickly.  “Lend me a hand.”

Together we moved the laboratory table as he directed.

There, in the floor, concealed by the shadow, he had placed the same apparatus which I had seen him bury in the path between the Pearcy and Minturn estates at Stratfield.

We scarcely breathed.

“This,” he explained rapidly, “is what is known as a kinograph—­ the invention of Professor HeleShaw of London.  It enables me to identify a person by his or her walk.  Each of you as you entered this room has passed over this apparatus and has left a different mark on the paper which registers.”

For a moment he stopped, as if gathering strength for the final assault.

“Until late this afternoon I had this kinograph secreted at a certain place in Stratfield.  Some one had tampered with the leaden water pipes and the electric light cable.  Fearful that the lead poisoning brought on by electrolysis might not produce its result in the intended victim, that person took advantage of the new discoveries in electrolysis to complete that work by introducing the deadly strychnine during the very process of cure of the lead poisoning.”

He slapped down a copy of a newspaper.  “In the news this morning I told just enough of what I had discovered and colored it in such a way that I was sure I would arouse apprehension.  I did it because I wanted to make the criminal revisit the real scene of the crime.  There was a double motive now—­to remove the evidence and to check the spread of the poisoning.”

He reached over, tore off the paper with a quick, decisive motion, and laid it beside another strip, a little discolored by moisture, as though the damp earth had touched it.

“That person, alarmed lest something in the cleverly laid plot, might be discovered, went to a certain spot to remove the traces of the diabolical work which were hidden there.  My kinograph shows the footsteps, shows as plainly as if I had been present, the exact person who tried to obliterate the evidence,”