The Shadow of the Dead Hand by Fred M. White

Murdstone, in the insidious disguise of Professor Rogers, walked on in the darkness, knowing exactly what he was going to do and how he was going to do it. He had not been at Kindale Castle on one or two occasions without carefully studying his ground, and committing to memory the exact geography of the place. He knew precisely where the main treasures were to be found, and with any sort of luck he would be able to get away with them in the course of the next hour. All he wanted was some sixty intense minutes without interruption and the big coup would be accomplished.

He knew perfectly what had happened in the pleasant dining room where he had left the elderly earl in the company of Countess Visconti. He knew all about that potent little drug which had been dropped in the second glass of liqueur and the effect it would have upon Kindale. It needed no one to tell him that Kindale was now sleeping peacefully in the dining room and that the countess would see to it that no servant came in until at least an hour had elapsed. Then he calculated that the effects of the drug would have exhausted themselves and it would be an easy matter, afterwards, to rally the old gentleman from his little lapse from the conventions and get him to believe that he had been merely taking what is known as 'forty winks.' In other words, it was necessary to convince Kindale that the professor had not been off the premises at all and that he had been detained rather longer than he had expected at the telephone. The whole thing had been planned to the minutest detail, and all that was necessary now was to get on with the scheme and hide the plunder until Kindale discovered for himself that he had been robbed.