Dr. Heidegger's Experiment
How we cite our quotes:
"That is certainly a very pretty deception,'' said the doctor's friends; carelessly, however, for they had witnessed greater miracles at a conjurer's show; "pray how was it effected?'' (12)
The guests' skepticism reminds the reader that such transformations could be tricks of the eye; we then have to doubt the reality of the next transformation we see (that of the guests themselves).
The liquor, if it really possessed such virtues as Dr. Heidegger imputed to it, could not have been bestowed on four human beings who needed it more woefully. (22)
The narrator keeps the nature of the guests' transformation ambiguous with lines like this one. We don't know for sure if the liquor really does possess the virtues that Dr. Heidegger claims, and the narrator won't tell us one way or the other.
Assuredly there was an almost immediate improvement in the aspect of the party, not unlike what might have been produced by a glass of generous wine, together with a sudden glow of cheerful sunshine brightening over all their visages at once.
This line – along with the description of the bubbling water – suggests that the liquid might actually be alcohol. This is an explanation for the evening's events which keeps the story out of the realm of the supernatural.