There are no such things as marble kisses or ice kisses, or I should say my ecclesiastical cousin's salute belonged to one of these classes; but there may be experiment kisses, and his was an experiment kiss.
Chapter 34 · Narrator
Context
After Diana suggests St. John should kiss Jane goodnight as he does his sisters, he complies. Jane reflects on the cold, clinical nature of the kiss he gives her.
Analysis
Jane invents a taxonomy of kisses (marble, ice, experiment) to classify something that feels categorically wrong—the humor barely masks her discomfort. The phrase 'experiment kiss' is precise: St. John kisses her to observe the result, treating intimacy as data collection. The detail that 'he viewed me to learn the result' confirms he's monitoring her reaction rather than expressing feeling, turning a gesture of affection into a test of her suitability as his wife.
Essay Tip
Use this to argue that Brontë exposes how gestures can be evacuated of their emotional content—St. John performs the forms of intimacy while entirely lacking its spirit, making his courtship a kind of emotional fraud.