After Dr. Heidegger demonstrates the effects of the rejuvenating water upon a welted rose that comes back to its original freshness, he invites the doubtful guests to imbibe the water and see the results.
The guests at first still wonder about this experiment but, according to the narrator, they are the four people who would need this infusion the most. The text then describes that their reactions were quite emotional. Their faces immediately had the refreshed looks of a "cheerful sunshine" falling upon them.
There was a healthful suffusion on their cheeks, instead of the ashen hue that had made them look so corpse-like
They immediately felt happy. Moreover, this glow made their skins begin to change, filling up the gaps that their wrinkles formed, and restoring them to a youthful appearance.
They gazed at one another, and fancied that some magic power had really begun to smooth away the deep and sad inscriptions which Father Time had been so long engraving on their brows. The Widow Wycherly adjusted her cap, for she felt almost like a woman again.
Shortly after, they were asking Dr. Heidegger for more of the water because although they look younger, they were still "old" by their standards. So they drank more, and they became younger and younger. However, with the youth came immaturity in behavior. They flirted and danced but with their youth came quarrels over rivalries as to who would get "the girl" which in this case is the old widow.
Basically the restoration of the youth in these characters not only brought out the best of their physical traits, but it also brought out their vanities, their insecurities, and their disdain for aging which is a natural process. All this was the eye-opener that Dr. Heidegger wished to elicit from their behaviors.