The adventure of the speckled band
The idea of using a form of poison which could not possibly be discovered by any chemical test was just what a clever and ruthless man who had had an Eastern training would think of. The rapidity with which such a poison would take effect would also, from his point of view, be an advantage. It would be a sharp-eyed policeman, indeed, who could distinguish the two little dark punctures which would show where the poison fangs had done their work. Then I thought of the whistle. Of course he must recall the snake before the morning light revealed it to the victim. He had trained it, probably by the use of the milk which we saw, to return to him when summoned. He would put it through this ventilator at the hour that he thought best, with the certainty that it would crawl down the rope and land on the bed. It might or might not bite the occupant, perhaps she might escape every night for a week, but sooner or later she must fall a victim.
"I had come to these conclusions before ever I had entered his room. An inspection of his chair showed me that he had been in the habit of standing on it, which of course would be necessary in order that he should reach the ventilator. The sight of the safe, the saucer of milk, and the loop which he used to hold the snake were enough to finally dispel any doubts which may have remained. The metallic clang heard by Miss Stoner was obviously caused by her stepfather hastily closing the door of his safe upon its terrible occupant. Having once made up my mind, you know the steps which I took in order to put the matter to the proof. I heard the creature hiss as I have no doubt that you did also, and I instantly lit the light and attacked it."