“Potassium carbonate has no particular properties of its own,” replied the doctor. “But, if a piece of caustic potash had been applied to the puncture, say an hour or two ago, it would by now have been converted into potassium carbonate. You see what this suggests. Caustic substances are employed to burn out poisoned surfaces. In this case, caustic potash may have been employed in an attempt to counteract the poison. Of course, it would be ineffectual, as it had been injected far too deeply. But that it has been so applied I am pretty certain. You can see for yourself that the skin shows traces of burning under the incrustation. Whether the same hand that injected the poison applied this ineffectual antidote, I cannot say.” Whyland nodded, and then a sudden thought struck him. “But look here, if he injected the poison himself, the syringe ought to be lying about somewhere!” he exclaimed. “How long would it take for the poison to act, doctor?” The doctor shook his head. “I can’t say, since I do not yet know its nature,” he replied. “If it was the same as was employed in Colburn’s case, we can make a rough guess, however. A very small quantity, applied to a scratch in his tongue, caused death in two hours and a half. We may assume that very much larger quantity would be contained in a syringe, and it was driven well into the tissues. Death might well have occurred within a few minutes. But in any case, there would have been time to dispose of the syringe.” The doctor turned and pointed to the window. “That’s open at the top, as you see, and the curtains do not meet by a couple of feet or more. He could have thrown it out there without the slightest difficulty.” Whyland turned to the three men, who had been standing by the door. “Slip down below, Waters, and search the pavement and roadway outside this window for a hypodermic syringe,” he said. “Sharp, now! Mr. Ludgrove, come to this window for a moment; you know this street better than I do. That’s your place opposite, isn’t it? What’s behind that window over the shop?” “A room I use for storing herbs in,” replied Mr. Ludgrove. “As it happens, I have been in there once or twice this evening.” “You didn’t see anything of what was happening in here, I suppose?” asked Whyland quickly. “No, I did not,” replied Mr. Ludgrove. “I was only in the room for a moment or two, selecting bundles of herbs to take downstairs. I did happen to notice about eleven o’clock that there was a light in here, but that was all.” “What about the windows to the right and left of your place?” “I believe that they belong to offices occupied only in the day-time. I do not think it at all likely that anyone could have overlooked this room from them at night.” “No,” agreed Whyland. “We’re not likely to be lucky to find an actual witness. All right, Mr. Ludgrove, thank you. Now, young man, do you know if your father possessed a hypodermic syringe?”