A vacuum is formed by pumping the air out of the bell jar. Scholars argue about the nature of Esther's "bell jar" and what it can stand for. The novel has also been compared to Sylvia Plath's "The Bell Jar". In Sylvia Plath's novel The Bell Jar, the protagonist, Esther Greenwood, visits Deer Island Prison. "The Bell Jar" addresses the question of socially acceptable identity. The pupa is hurriedly contained in a bell jar in the hospital's in-house laboratory. Sylvia Plath's "The Bell Jar" refers to insulin shock therapy in chapter 15.