He assisted John Pinkerton in his "Essay on Medals" (1784). Collections were made by him for a ‘History of the Saxons and Danes in England,’ illustrated by coins, but the work was not completed. Southgate's books and prints were sold by Leigh & Sotheby in 2,599 lots on 27 April 1795 and eleven following days. His coins and medals were announced for sale in eight days, but, according to John Nichols, they passed by private contract to Samuel Tyssen. The shells and natural curiosities were sold on 12 and 13 May 1795. Each catalogue was printed separately, and the whole was bound up, with life prefixed by Charles Combe, as "Museum Southgatianum". The frontispiece was a medallion portrait of him at the age of fifty-five. "Sermons preached to Parochial Congregations" by Southgate were published in 1798 (2 vols.), with a biographical preface by George Gaskin which was mainly borrowed from Combe.