"she had read about alchemy"

Alchemy is most often thought of as a means of attempting to turn base metal into gold, but it was also a philosophy and discipline, aimed at transforming the soul and attaining immortality. Carl Gustav Jung, the famous Swiss psychiatrist and one-time colleague of Freud wrote about it as such, and the author of "Miss Garnet" worked as a Jungian analytical psychologist. Perhaps the atheist, repressed Miss Garnet is unnerved by a subject that both hints at  the supernatural and suggests examination of the soul, and that is why she hurries past the window.