In 1801 he was appointed professor of natural philosophy, in the Royal institution, then newly established, and he began to deliver lectures early in the following year. As a lecturer he was not popular, his style being too condensed, and the matter of Ids lec tures unsuited to the miscellaneous audiences of the Royal institution. He published in 1:802 a Syllabus of a Course of Lectures on Natural and Experimental Philosophy, in which, among other things, he first announced his great discovery of the law of the interfer ence of light (see INTERFERENCE), which by itself, as sir John Herschel] has remarked, would have procured him a scientific immortality. It was this discovery which first fairly turned the balance of evidence in favor of the undulatory as against the molecu lar theory of light (see UNDULATORY THEORY OF LIGHT). It is Young's most important contribution to science. He had been elected a fellow of the Royal society as soon as he was 21; in 1802 lie became its foreign secretary, is post which he retained till the end of his life. Ile resigned his professorship on his marriage, in 1804, fearing that his fill ing a chair of science might interfere with his success as a physician. The lectures which he delivered as professor were the foundation of the Course of Lectures on _Araf ura and Jleclutnical Philosophy, which he published in 1807—a great work, embodying a complete system of natural and mechanical philosophy, on which he was engaged for nearly five years. A new edition of these lectures was published in 1845, edited by prof. Kelland of Edinburgh. Young's doctrine of interference was at first unfavora bly received by scientific men in England: it was attacked and ridiculed in the Edin burgh Review, and so little interest was taken in the subject, that of a pamphlet which Young published in answer to the Edinburgh Review, only a single copy was sold. As has often happened, the first recognition of the importance and ingenuity of this and others of Young's speculations came from the scientific men of the continent.
was admitted as a fellow of the college of physicians in 1808, and was elected one of die physicians of St. George's hospital in 1810. He afterward published several medical works, which, though they were little more than compilations, and are now forgotten, show that he was thoroughly versed in the history of diseases and of medical opinion. His hospital practice, too, is said to have been successful, but he had but little
success in getting patients. He retired from practice—retaining, however, his connec tion with St. George's hospital—in 1818, on his appointment to be secretary of the board of longitude. On the dissolution of the board of longitude, he became the sole con ductor of the Nautical Almanac, and afterward, when the system of life-insurance began to be popular, he held, along with this post, the office of scientific adviser of a life in surance company. During the last years of his life, he was a member of a council appointed to advise the admiralty in scientific matters.
Young's greatest achievement after hisgreat discovery of the law of interference, was made in connection with the subject of hieroglyphics (q.v.). He was the first to hit upon the process of investigation by means of which the received interpretation of hieroglyphics has been arrived at. His discovery, originally published in papers written for the of literature, and afterward in the Encyclopadia Britannica, was given to the world in a book in 1823. In his later years much of his attention was given to this and cog nate subjects. He was engaged on an Egyptian dictionary at the time of his death. His miscellaneous writings, contributions to the transactions of learned and scientific bodies, to reviews, and 'the L' ncyclopadia Britannica, were very numerous. Three volumes of them, two consisting of scientific papers, edited by Dean Peacock, the third of hiero glyphical essays and correspondence, edited by John Leitch, were published in 1855.
He died after several months of failing health, and a short period of severe illness, on May 10, 1829. His character seems to have been singularly amiable, and to have endeared him to a multitude of friends, by one of whom, Dr. Peacock, dean of Ely, an ample biography of him was published in 1855. Young was, two years before his death, elected a foreign associate of the academy of sciences at Paris, succeeding to the illustrious Volta.