Several paragraphs into Book 1, chapter 15 ("What Had Happened in Surrey"), the Black Smoke is described thus: "Save that an unknown element giving a group of four lines in the blue of the spectrum is concerned, we are still entirely ignorant of the nature of this substance."

But in the Epilogue (book 2, chapter 10) there is this description: "Spectrum analysis of the black powder [left behind when the Black Smoke mingles with water] points unmistakably to the presence of an unknown element with a brilliant group of three lines in the green".

I wouldn't be so sure of that "unmistakably" if I were you, Mr. Wells. :-)