That should be a “long s”. From the wiki: The minuscule form ſ, called the long s, developed in the early medieval period, within the Visigothic and Carolingian hands, with predecessors in the half-uncial and cursive scripts of Late Antiquity. It remained standard in western writing throughout the medieval period and was adopted in early printing with movable types. It existed alongside minuscule “round” or “short” s, which were at the time only used at the end of words.