“How did you know, Thorndyke?” and as he looked up inquiringly, I added: “I mean, how were you able to make so confident a guess, for, of course, you couldn’t actually know?”
“When do you mean?” he asked.
“I mean that when you applied for a Home Office authority you must have had something to go on beyond a mere guess.”
“Certainly I had,” he replied. “It was not a guess at all. It was a certainty. When I made the application I was able to say that I had positive knowledge that Stella Keene had been poisoned with arsenic. The examination of the poor child’s body was not for my information. I would have avoided it if that had been possible. But it was not. As soon as my declaration was made, the exhumation became inevitable. The Crown could not have prosecuted on a charge of poisoning without an examination of the victim’s body.”
“But, Thorndyke,” I expostulated, “how could you have been certain—I mean certain in a legal sense? Surely it could have been no more than a matter of inference.”
“It was not,” he replied. “It was a matter of demonstrated fact. I could have taken the case into court and proved the fact of arsenical poisoning. But, of course, the jury would have demanded evidence from an examination of the body, and quite properly, too. Every possible corroboration should be obtained in a criminal trial.”
“Certainly,” I agreed. “But still I find your statement incomprehensible. You speak of demonstrated fact. But what means of demonstration had you? There was my diary. I take it that that was the principal source of your information; in fact I can’t think of any other. But the diary could only have yielded documentary evidence, which is quite a different thing from demonstrated fact.”
“Quite,” he agreed. “The diary contributed handsomely to the train of circumstantial evidence that I had constructed. But the demonstration—the final, positive proof—came from another source. A very curious and unexpected source.”
“I suppose,” said I, “as the case is finished and dealt with, there would be no harm in my asking how you arrived at your conclusion?”
“Not at all,” he replied. “The whole investigation is a rather long story, but I will give you a summary of it if you like.”