A precocious Scot, Maclean entered the University of Glasgow in his native city at the age of thirteen, determined to become a surgeon like his father. Under the influence of his friend Charles Macintosh (who later invented waterproof cloth) he joined the Chemical Society, before which he read several papers. After completing his preliminary studies in arts and medicine, he did further work in Edinburgh, London, and Paris; and in 1791, when he was only twenty, Maclean was licensed to practice and was admitted to the faculty of physicians and surgeons of the University of Glasgow.
Attracted by American political ideals, he came to this country in the spring of 1795 and, on the advice of Benjamin Rush, established himself in Princeton in the practice of ``physic'' and surgery. That summer, on President Stanhope Smith's invitation, he delivered a short course of chemistry lectures, which made so favorable an impression that the trustees in October appointed him Professor of Chemistry with the understanding that he would be at liberty to continue the practice of his profession. Two years later, on the death of Walter Minto, Maclean assumed responsibility for instruction in mathematics and natural philosophy as well as in chemistry; he gave up his medical practice, and from that time on devoted himself wholly to the College.
In 1797 he published Two Lectures on Combustion, which helped to overthrow Priestley's phlogiston theory and to clear the way for acceptance of Lavoisier's ``new chemistry,'' to which he had been won over during his studies in Paris.
Maclean helped Benjamin Silliman, later regarded as one of the fathers of science in America, to prepare for his duties as Yale's first chemistry professor, following his appointment in 1802. In his diary Silliman said he considered Maclean his ``earliest master of Chemistry'' and Princeton his ``starting point in that pursuit.''
Maclean died at the age of forty-three in 1814 -- two years after his resignation (which had been requested by a board of trustees more concerned with training ministers than scientists) and two years before the graduation of his eldest son, John Maclean, Jr., who became the tenth president of the College.
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