The Man Called Gilray by Fred M. White

In a dim kind of way Temple was beginning to discern the connection between his own evidence and the information that had come into Sparrow's possession. They had started at opposite points of the compass, but it looked as if they were likely to come together now, and not before long. Temple walked along thoughtfully until he came to the shop of Negretti, the famous cigarette maker. He took from his pocket the fragment of charred tobacco he had collected from the table in Silverdale's library.

"Have you ever seen anything like that before?" he asked the tobacconist.