“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?”