Illustration from 1908 Chambers's Twentieth Century. <b>J</b>, the tenth letter in our alphabet, developed from I, the initial form <i>j</i> being specialised to denote the consonantal sound, the medial <i>i</i> being retained for the vowel-sound—not universal in English books before the middle of the 17th century.
Rev. Thomas Davidson 1856-1923 (<i>ed.</i>)
Chambers's Twentieth Century Dictionary of the English Language