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"Graphite!" exclaimed Thorndyke. "I have found no graphite, but I have found traces of cocoa—spiral vessels and starch grains—and of hops—one fragment of leaf and several lupulin glands. May I see the graphite?"

I passed him the slide and he examined it with keen interest. "Yes," he said, "this is undoubtedly graphite, and no less than six particles of it. We had better go over the coat systematically. You see the importance of this?"

"I see that this is evidently factory dust and that it may fix a locality, but I don't see that it will carry us any farther."

"Don't forget that we have a touchstone," said he; and, as I raised my eyebrows inquiringly, he added, "The Yale latch-key. If we can narrow the locality down sufficiently, Miller can make a tour of the front doors."

"But can we?" I asked incredulously. "I doubt it."

"We can try," answered Thorndyke. "Evidently some of the substances are distributed over the entire coat, inside and out, while others, such as the graphite, are present only on certain parts. We must locate those parts exactly and then consider what this special distribution means." He rapidly sketched out on a sheet of paper a rough diagram of the coat, marking each part with a distinctive letter, and then, taking a