Cullen discerned the promise of his pupil, and carefully abstained from entering upon fields of research in which he expected him to make a mark. Black submitted his treatise on fixed air to Cullen, and dedicated it to him. About this time Cullen made some important discoveries on the evolution of heat in chemical combination, and the cooling of solutions, some of which were not published till 1755, while others remained in manuscript, but suggested to Black important points in his view of latent heat. At the beginning of 1751, by the interest of the Duke of Argyll, Dr. Cullen succeeded Dr. Johnstone as Professor of Medicine in the University of Glasgow, at the same time that Adam Smith was appointed to the Chair of Logic; and a friendship of great intimacy arose between these thoughtful minds. Only a few months afterwards, Adam Smith’s transfer to the Moral Philosophy chair led Dr. Cullen to favour strongly the election of David Hume to the vacant chair, on an occasion when Edmund Burke was also a candidate. Neither was elected, strict orthodoxy carrying the day. At this period the applications of chemistry to arts and manufactures and to agriculture engaged Cullen’s attention considerably, and he proposed to carry out a process for purifying common salt, but it proved too expensive. Cullen, finding that Glasgow did not promise to build up a large medical school at present, and being compelled to take country practice, began to look longingly to Edinburgh, to which also his friends were calling him. He says in a letter to William Hunter, in August 1751, “I am quite tired of my present life; I have a good deal of country practice, which takes up a great deal of time, and hardly even allows me an hour’s leisure. I get but little money for my labour; and indeed by country practice, with our payments, a man cannot make money.” Various circumstances, however, prevented this step being taken, until, in the beginning of 1756, he was appointed to the professorship of chemistry at Edinburgh, and was thus fairly launched on his notable career. In the competition for this chair, Joseph Black had been nominated, but the two friends honourably refused to do anything to prejudice each other, and on appointment indeed Cullen offered Black all the fees if he would assist him. Cullen’s first course at Edinburgh was attended by only 17 students, his second by 59, while it rose later to 145. Practice soon came to him, and freed him from his pecuniary struggles.