Sentence:  Supported by an impregnable sense of justice but still dangerously fuming, Lucia went back to her garden-room, to <WSD>tranquillize</WSD> herself with an hour's practice on the new piano. 
Meaning of the word marked with <WSD> tags:  To make (something or someone) tranquil.<ref>(Thomas Blount), "Glossographia," London: George Sawbridge, 1661: “"Tranquillize" (...) to make quiet, still or calm, to cause tranquility.”[http://name.umdl.umich.edu/A28464.0001.001]</ref>