MLA
Early, John, 1786-1873. Letter from John Early (Philadelphia, [Pennsylvania]) to Robert Paine - July 18, 1855. July 18, 1855. Robert Paine Papers, 1828-1882. Pitts Theology Library. https://digital.library.emory.edu/purl/139x0k6fqf-cor.
APA, 6th edition
Early, John, 1786-1873. (July 18, 1855)[Letter from John Early (Philadelphia, [Pennsylvania]) to Robert Paine - July 18, 1855]. Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/139x0k6fqf-cor.
Chicago
Early, John, 1786-1873, Letter from John Early (Philadelphia, [Pennsylvania]) to Robert Paine - July 18, 1855, July 18, 1855, Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/139x0k6fqf-cor.