When Dr. Heidegger's guests arrive, he asks for their help "in one of those little experiments with which [he] amuse[s himself] here in [his] study" and later speaks of "an exceedingly curious experiment" (2, 5). Shortly after, he refers to his guests as "the subjects" of this little trial (17). But what is Dr. Heidegger's experiment? What is he trying to learn, determine, or prove?
The surface interpretation is that Dr. Heidegger has obtained an elixir from a friend in Florida and wants to see if, true to legend, it will in fact restore a person's youth. But if you read "Dr. Heidegger's Experiment" carefully, you start to see that it's not about the science of the elixir. Dr. Heidegger isn't interested in whether or not the elixir works, or how it functions, or the efficacy of its effects. His experiment has much more to do with the behavior of his guests once they are "young again."
As we discuss in "Symbols, Imagery, Allegory" and in "Characters," one of the questions the reader has to decide in interpreting "Dr. Heidegger's Experiment" is whether or not the elixir actually works, or whether the guests, drunk with booze and wishful thinking, have fooled themselves into believing they are young again. Either way, however, the four guests behave as though they are young, and it is this behavior that interests Dr. Heidegger.
Remember that he warns them before they drink the elixir:
"It would be well that, with the experience of a lifetime to direct you, you should draw up a few general rules for your guidance, in passing a second time through the perils of youth. Think what a sin and shame it would be, if, with your peculiar advantages, you should not become patterns of virtue and wisdom to all the young people of the age!" (19)
Later, as the four guests romp about, Dr. Heidegger watches them carefully, with "philosophic coolness" (25). This is an important line. Heidegger's experiment isn't scientific – it's philosophic. He wants to see if, given a second youth, his guests will make the same mistakes the second time around.
The conclusions of Dr. Heidegger's experiment are covered in "What's Up with the Ending?" We'll meet you there.