In 1804 he married Eliza, the daughter of James Primrose Maxwell, of Cavendish Square, and this union is said to have been attended with uninterrupted happiness; his wife who survived him left no children. In 1807 appeared his most elaborate and valuable work, 'A Course of Lectures on Natural Philosophy and the Mechanical Arts,' being the embodiment of the sixty lectures delivered while at the Royal Institution, together with the labour of three more years occupied in further arranging and improving them. This work comprises a complete system of natural and mechanical philosophy, drawn from original sources, and is distinguished not only by the extent of its learning and the accuracy of its statements, but by the beauty and originality of the theoretical principles. It also contains a disquisition upon the doctrine of interference in the undulatory theory of light mentioned before, the general law of which he thus enunciates: "When two undulations from different origins coincide, either perfectly or very nearly in direction, their joint effect is a combination of the motions belonging to each."[49] Sir John Herschel, speaking of this discovery, says that it alone "would have sufficed to have placed its author in the highest rank of scientific immortality, even were his other almost innumerable claims to such a distinction disregarded." Amongst other laborious and difficult matters of investigation, Dr. Young made the first and most important steps in reading the Egyptian Hieroglyphics, in which he preceded Champollion; and he afterwards, in 1823, published a work on this subject, under the title of 'An Account of some recent Discoveries in Hieroglyphical Literature and Egyptian Antiquities; including the author's original Alphabet as extended by Mr. Champollion; with a Translation of five unpublished Greek and Egyptian Manuscripts.' In the year 1808 Dr. Young was admitted a fellow of the College of Physicians, and in 1810 was elected physician to St. George's Hospital, a situation which he retained for the remainder of his life. In 1813 he published 'An Introduction to Medical Literature, including a system of practical Nosology intended as a guide to Students and as an Assistant to Practitioners.' In 1816 Dr. Young was appointed Secretary to the Commission empowered to ascertain the length of the second's pendulum, and thereby establish an uniform system of weights and measures. Two years subsequent to this he became secretary to the Board of Longitude, and on the dissolution of that body, became sole conductor of the 'Nautical Almanac.' Dr. Young at various times contributed eighteen articles to the 'Quarterly Review,' of which nine were on scientific subjects--the rest on medicine, languages, and criticism. Between 1816 and 1823 he wrote sixty-three articles for the 'Supplement to the Encyclopædia Britannica,' Sixth Edition, of which forty-six were biographical. In the year 1821 he made a short