Ariel's comment excellently clarifies Matthias's questions. I just want to comment on the sad and unfortunate version of Michel Verne. To adapt the novel to 1577, and to please Jules-Louis Hetzel (Letter of April 9, 1913), Michel had to remove a large number of important literary, musical, historical and scientific references contained in the original novel that takes place in 1877: Hoffman and Poe (in Chaps. I and IX), Victor Hugo (in Chap. VIII); Reclus (in Chapter IV); La Marche transylvanienne [Berliotz], La Marche de Rákóczy [anonymous] and Chant de la haine by Georges Harwegh (Ch. VII); Napoleon (in Ch. II); the Hungarian Revolution of 1848 (in Ch. V); the Orient Express (in Ch. XVI); the Rhumkorf Coil, the Moissan System and the Rœntgen Rays (in Ch. XVIII).
However, as Michel had warned Jules-Louis, about the possibility of not having eliminated "all modern words... Maybe they are still there!", Henri Vidal, who narrates the story in present tense, refers in 1577 to: "The territories where in the 17th century the French and the Turks had to fight so fiercely" (in Chapter II); to the territory called Military Confinements, "an institution from before the reign of María Teresa" [1740-1780] (in Chapter II); and, to “Príncipe Milloch street” [1780-1860] (in Chapter III).
But in my opinion, the interesting thing about all this is that the elimination of the references to the inventions and discoveries of Rhumkorf, Moissan and Rœntgen did the most damage, since it affected Jules Verne's ability to make the narration seem plausible (as recommended by Poe). Michel totally destroyed his father's frequent literary ruse of supporting his explanations on scientific references of his time to make the narrative seem verosimile. Mentioning those inventions and discoveries would no doubt have further strengthened the story of Storitz's invisibility. I think that, wrongly, Jules-Louis or Michel preferred to emphasize an alchemical explanation and that is why they took the narrative back to the 16th century, bringing Storitz's elixir closer and more closely associated with Alchemy which, until the 17th century, was considered a serious science and it would be a precursor, in the 18th century, of modern chemistry.
Was there perhaps another reason to change the chronology?