The Salt Of The Earth by Fred M. White

After the little group of conspirators had broken up, Adela retreated to her room, locked the door, and sat down to think the matter out. She had a fair notion of their plot, and her duty gradually became clear. Of course, she never supposed that Denne was actuated by any criminal motive when he tried to get Callader into his power. Beyond all doubt, his suspicions about him were correct. Mark had taken advantage of his position to remove some of the finest pictures and treasures from the Castle, and replace them with copies. Moving in the set she did, Adela was well aware that Callader's lavish expenditure had formed the subject of much idle gossip. She began to see where this money had come from.

But why was Denne so interested? Why had he taken these enormous pains, and incurred this great expense simply to prove that Callader was a thief? Although she was alone, Adela felt her features suffused with a glow of pleasurable warmth.