The Red Thumbmark by R. Austin Freeman

It was now only a week from the date on which the trial was to open. In eight days the mystery would almost certainly be solved (if it was capable of solution), for the trial promised to be quite a short one, and then Reuben Hornby would be either a convicted felon or a free man, clear of the stigma of the crime.

For several days past, Thorndyke had been in almost constant possession of the laboratory, while his own small room, devoted ordinarily to bacteriology and microscopical work was kept continually locked; a state of things that reduced Polton to a condition of the most extreme nervous irritation, especially when, as he told me indignantly, he met Mr. Anstey emerging from the holy of holies, grinning and rubbing his hands and giving utterance to genial but unparliamentary expressions of amused satisfaction.