Sentence:  Marianne's joy was almost a degree beyond happiness, so great was the perturbation of her spirits and her impatience to be gone. Her unwillingness to quit her mother was her only <WSD>restorative</WSD> to calmness; and at the moment of parting her grief on that score was excessive. 
Meaning of the word marked with <WSD> tags:  Something with restoring properties.