In Chapter 4, the narrator uses an idiom to describe Hepzibah's fantasies of suddenly becoming a wealthy landowning aristocrat:
These were some of the fantasies which she had long dreamed about; and, aided by these, Uncle Venner’s casual attempt at encouragement kindled a strange festal glory in the poor, bare, melancholy chambers of her brain, as if that inner world were suddenly lighted up with gas. But either he knew nothing of her castles in the air—as how should he?—or else her earnest scowl disturbed his recollection, as it might a more courageous man’s.
The narrator states that Uncle Venner doesn't seem to guess or be interested in Hepzibah's "castles in the air." This phrase is an idiom because it is not immediately evident what it means without extra context. The phrase has long been used to describe wild and unrealistic fantasies that have a foundation of "air." The idea is that these fantasies will crumble as soon as the person starts trying to bring them to life.
Hepzibah's "castles in the air" are especially tragic because they are both figurative and literal castles with no foundation in reality. Hepzibah imagines discovering the deed to the Pyncheon land in Maine and building a palace there. In her palace, she will sit in a tower and look down on her domain. She imagines distant relatives, rich off of their own plantation in Virginia, sending her vast sums of money that will allow her to live in perpetuity as a real "lady" who does not have to get her hands dirty with work. Both of these scenarios involve land and "castle" (or property) ownership, and the exploitation of other people's labor so that Hepzibah can live out her fantasy. As of now, there is no distant land where she can build her own palace. There are no other laborers to save her from laboring in the cent shop. She must be self-reliant.
But Hepzibah can't help but get confused about whether or not her "castles in the air" are achievable. She is currently living almost as she desires; she doesn't have distant relatives sending her enough money to be rich for the rest of her life, but her cousin is providing her with the House of the Seven Gables to live in until she dies. A glamorous, work-free life is a close enough possibility (even if it will never actually be possible) that Hepzibah can't rid herself of her "castles in the air." When Uncle Venner encourages her while she is daydreaming, her "inner world" catches fire, as though it has been full of "gas" or air waiting to be ignited. Hawthorne thus suggests that the American idea that riches and leisure are just around the corner is dangerous: that air was probably never going to turn into a real foundation and can only blow up in Hepzibah's face.
