Sentence:  ... I feel my heart glow with an enthusiasm which elevates me to heaven, for nothing contributes so much to <WSD>tranquillize</WSD> the mind as a steady purpose,—a point on which the soul may fix its intellectual eye. 
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>