The Devil's Advocate by Fred M. White

Farrell looked disturbed and uneasy. He had all the air of a man who has made a discovery that has shaken him off his balance. At the same time he was inwardly congratulating himself upon the artistic way in which he had led up to the discovery, and he was telling himself that once this thing was public property Macrae was as good as doomed. He knew perfectly well that Blinn was not in the least likely to keep this to himself, and that before very long it must come to the ears of Inspector Dent. And once this had happened Dent would lose no time in arresting Macrae for the murder of Joseph Baines. Indeed, in the circumstances he could do very little else, for here was an actual motive for the crime, a thing that had been lacking before, which probably accounted for the fact that no steps had been taken against Macrae.

"This is very terrible," Farrell said. "I would give a great deal if I could have prevented it. And it's all the worse, because I have blundered upon this evidence at a time when I am acting as Macrae's legal adviser. Of course, if I had found it out myself I might have been justified in suppressing it, but I cannot do that now without asking your assistance, and I don't see that I can do so, because it might get you into trouble."