"Well," he said, leaning both elbows upon the table and resting his chin upon his hands, while the tame brown rat ran along the table and scrambled into his pocket, "on the first evening you sought my assistance I knew, from the remote effects which both of us experienced, that the evil influence of that mysterious visitor in black, was due to some unknown neurotic poison. It was for that reason that I was enabled to administer an antidote without making an exact diagnosis. Now, as you are well aware, toxicology is a very strange study. Even common table-salt is a poison, and has caused death. But my own experiments have proved that, although the various narcotic poisons produce but little local change, their remote effects are very remarkable. Certain substances affect certain organs in particular. The remote action of a poison may be said to be due, in every instance, to its absorption into the veins or lymphatics, except when there is a direct continuity of effect traceable from the point where the poison was applied to the point where the remote effect is shown. It is remarkable that the agents which most affect the nervous system do not act at all when applied to the brain or trunks of nerves. Poisonous effects result from absorption of the poisoning body, and absorption implies solution; the more soluble, therefore, the compound is, the more speedy are its effects. Do you follow me?" "Quite clearly." "The rapid, remote effect produced on leaving that room made it plain that I must look for some powerful neurotic poison that may be absorbed through the skin," he went on. "With this object I searched microscopically various objects within and without the room, but for a long time was unsuccessful, when, one morning, I made a discovery that upon the white porcelain handle of the door a little colourless liquid had been applied. Greater part of it had disappeared by constant handling, but there was still some remaining on the shaft of the handle, and the microscope showed distinct prism-shaped crystals. All these I secured, and with them have since been experimenting. I found them to be a more deadly poison than any of the known paralysants or hyposthenisants, with an effect of muscular paralysis very similar to that produced by curare, combined with the stiffness about the neck and inability to move the jaws so apparent in symptoms provoked by strychnia. The unknown substance--a most deadly, secret poison, and, as I have since proved, one of those known to the ancients--had been applied to the door-handle on the inside, so that any person in pulling open the door to go out must absorb it in sufficient quantity to prove fatal. Indeed, had it not been for the antidote of chlorine and the mixed oxides of iron which I fortunately hit upon, death must have ensued in the case of each of us.