About the date of Black’s arrival at this college, it happened that the celebrated Cullen――he who influenced the career of Dr. Hunter――made his first public appearance at that seat of learning, in the capacity of Lecturer on Chemistry; his reputation speedily crept abroad, and the attendance at his class became large. The clever and acute French student was brought under the notice of Cullen, who, being frank and generous to his pupils, almost to a fault, made himself perfectly accessible at all hours, and treated them with much respect. He immediately perceived the bent of Black’s genius; and not only recommended, but strongly urged, him to apply himself with determination to cultivate the science of chemistry, and gave him every assistance in doing so. Cullen was not, perhaps, a first-rate chemist himself, but he had an admirable method of imparting instruction; and his gifted pupil’s preference for the study became so apparent, that he was ere long employed to assist his friend and teacher in the experiments of the classroom; and, when thus occupied, exhibited so much address and dexterity as contributed in no small degree to the success and fame of the lectures. Black was still engaged in medical studies, and in order to complete them under advantageous circumstances he repaired, in 1751, to Edinburgh, where he stayed in the house of a cousin, who held one of the professorships. Having, during three sessions, attended all the requisite classes, he duly took the degree of Doctor of Medicine. On that occasion he chose for his theme a chemical topic――the acid arising from food and magnesia alba. Next year he, “still achieving, still pursuing,” communicated his further ideas on the subject to a scientific society, in a paper which was then read by him, and afterward published in the second volume of “Essays, Physical and Literary,” and gave an account of a most important chemical discovery. This was the existence of an aërial fluid, which he called fixed air, the presence of which gives mildness, as its absence gives causticity, to alkalies and calcareous earths.