At the beginning of Chapter 33, we read that Elizabeth "was engaged one day as she walked in reperusing Jane's last letter, and dwelling on some passages which proved that Jane had not written in spirits ... ."

I am struck by two things from this passage: (1) Elizabeth was able (or thought she was!) to tell that Jane was not in the best of moods based on her style of writing, and (2) that JA obviously knew that this was possible (probably based on her own correspondence with her sister).