In "What's Up with the Title?", we make the argument that Heidegger's "experiment" has nothing to do with the elixir and everything to do with the behavior of his guests once they transform – or believe they have transformed – into their younger selves. Heidegger has a hypothesis at the start: if given a second chance at youth, people will make the same foolish mistakes again. What does the ending of "Dr. Heidegger's Experiment" suggest?
That Heidegger was right. The doctor's four guests behave exactly as he warned them not to (but secretly believed they would). Each guest falls back into the foolish behavior that characterized his/her youth: Colonel Killigrew, who wasted his youth on drinking and partying, sings drunken songs at the top of his lungs; Mr. Medbourne, who lost his money speculating, can think of nothing but dollars and cents; the Widow, who ruined her reputation by being loose and vain, flaunts her good looks and encourages the men to flirt with her; and Mr. Gascoigne, the politician, is caught up in the political rhetoric of his younger days. To top it all off, the three men fight over the Widow exactly the same way they did when they were young.
Dr. Heidegger, is all, "I told you so!" Except that this is Hawthorne, so instead he says: "Lo! the Water of Youth is all lavished on the ground. Well – I bemoan it not; for if the fountain gushed at my very doorstep, I would not stoop to bathe my lips in it – no, though its delirium were for years instead of moments. Such is the lesson ye have taught me!'' (51).
Of course, with "Dr. Heidegger's Experiment" being moralistic and all, Hawthorne hopes that you, too, are yelling "Lo!" and thinking of the lessons you have learned. Namely, that man is a hopeless, sinful, foolish creature and probably always will be, but never so much as when he is a young man (or lady – Hawthorne is an equal-opportunity condemner).