The tone of "Rappaccini's Daughter" is solemn, mysterious, and elegiac, especially when the narrator shares their own commentary on Giovanni's decision-making. When Giovanni's infatuation with Beatrice has reached such heights of intensity that he barely notices the effects of her poison on his body, the narrator observes:
Oh, how stubbornly does love,—or even that cunning semblance of love which flourishes in the imagination, but strikes no depth of root into the heart,—how stubbornly does it hold its faith until the moment comes when it is doomed to vanish into thin mist!
In this passage, the narrator offers a seemingly universal truth about human nature: that love, or infatuation, can be intense enough to blind us, but can also disappear in the blink of an eye. The narrator's lamentation here contributes to the mournful tone of the story by foreshadowing the moment when Giovanni's love, too, will "vanish into thin mist".
As the tragic consequences of Rappaccini's experiment unfold, the tone of the story turns mournful. This intensifies as the narrator emphasizes the way in which Rappaccini's poisoning alienates Giovanni and Beatrice from society:
They stood, as it were, in an utter solitude, which would be made none the less solitary by the densest throng of human life.
The use of "utter," as well as the notion that even the presence of a crowd could not make remedy Giovanni and Beatrice 's estrangement, heightens our sense of the narrator's lamentation. In this way, the tone of "Rappaccini's Daughter" becomes increasingly mournful and despairing.
