“With this train of reasoning in my head I naturally looked about in Mortimer Tregennis’s room to find some remains of this substance. The obvious place to look was the talc shelf or smoke-guard of the lamp. There, sure enough, I perceived a number of flaky ashes, and round the edges a fringe of brownish powder, which had not yet been consumed. Half of this I took, as you saw, and I placed it in an envelope.”

“Why half, Holmes?”