“I took a healthy rabbit, of the sort we use for experiment, and painted—just painted, mind you—the tender area behind the creature’s ear with an undiluted dose of the stuff. Remember, this was not an internal injection. It was merely a painting of the skin. It would have to be absorbed through the dermis before it reached the blood-stream. I watched the rabbit for an hour—and after that I didn’t have to watch him any more. He was as dead as any dead rabbit I ever saw.” “That doesn’t seem so powerful to me, Doctor,” protested the Inspector. “It doesn’t, eh? Well, take my word for it that it’s extraordinary. For a mere daubing of whole, healthy skin—I tell you, I was astounded. If the skin had an incision of some sort, or if the poison were administered internally, that would be a different story. You can imagine, therefore, what happened to Field’s insides when he _swallowed_ the stuff—and he swallowed plenty!” Ellery’s brow was wrinkled in thought. He began to polish the lenses of his pince-nez. “And that isn’t all,” resumed Dr. Jones. “As far as I know—and I have been in the service of the city for God knows how many years, and I’ve not kept uninformed about the progress of my science in other parts of the world, either—as far as I know, tetra ethyl lead has never before been used for criminal purposes!” The Inspector drew up, startled. “That’s saying something, Doctor!” he muttered. “Are you sure?” “Positive. That’s why I’m so keenly interested.” “Just how long would it take for this poison to kill a man, Doctor?” asked Ellery slowly. Dr. Jones grimaced. “That’s something I can’t answer definitely, for the very good reason that to my knowledge no human being has ever died of its effects before. But I can make a fairly good guess. I can’t conceive of Field having lived more than from fifteen to twenty minutes at the utmost after having taken the poison internally.” The silence that followed was broken by a cough from Queen. “On the other hand, Doctor, this very strangeness of the poison should make it fairly easy to trace. What, would you say, is its commonest source? Where does it come from? How would I go about getting it if I wanted some for a criminal purpose and didn’t want to leave a trail?” A gaunt smile lit up the features of the toxicologist. “The job of tracing this stuff, Inspector,” he said fervently, “I’ll leave to you. You can have it. Tetra ethyl lead, as far as I’ve been able to determine—remember, it is almost entirely new to us—occurs most commonly in certain petroleum products. I tinkered around quite a bit before I found the easiest way of making it in quantity. You’ll never guess how it’s done. It can be extracted from common, ordinary, everyday gasoline!” The two Queens exclaimed under their breaths. “Gasoline!” cried the Inspector. “Why—how on earth could a man trace that?”