Watson was elected a Fellow of the Royal Society in 1769, and for some years afterwards contributed many chemical papers to the 'Philosophical Transactions.' In 1771 he published 'An Essay on the Subjects of Chemistry, and their General Divisions.' In 1781 he published two volumes 12mo. of 'Chemical Essays;' a third appeared in 1782; and a fourth in 1786 completed the work, which has often been reprinted, and was long very popular. In connection with his chemical professorship, Watson obtained from Government, by proper representations, a salary of 100_l._ for himself, and for all future professors. He also paid some attention to theoretical and practical anatomy, as having a certain relation to the science of chemistry. In October, 1771, on the death of Dr. Rutherforth, he unexpectedly obtained the lucrative and important office of Regius Professor of Divinity, and in that capacity, held the Rectory of Somersham in Huntingdonshire. At this time he had neither taken his degree of B.D. or D.D., and by his own account, seems to have known little more of theological learning than he did of chemistry seven years before. Yet such was his good fortune, or the reputation that he had established, for carrying an object whenever he took it in hand, that no other candidate appeared for the professorship, while his eloquence and ingenuity supplied the want of deeper erudition, and attracted as numerous audiences to the exercises in the schools at which he presided, as had ever attended his chemical lectures. Watson himself, in the anecdotes of his life, gives the following account of this circumstance:--"I was not, when Dr. Rutherforth died, either Bachelor or Doctor in Divinity, and without being one of them I could not become a candidate for the Professorship. This puzzled me for a moment, I had only seven days to transact the business in, but by hard travelling, and some adroitness, I accomplished my purpose, obtained the King's mandate for a Doctor's degree, and was created Doctor on the day previous to that appointed for the examination of the candidates. Thus did I, by hard and incessant labour for seventeen years, attain at the age of thirty-four, the first office for honour in the University; and, exclusive of the mastership of Trinity College, I have made it the first for profit; I found the Professorship not worth quite 330_l._, and it is now worth 1000_l._ at least."