From Gottingen he went to Vienna, visiting the hospitals and most of the celebrated men in the various universities and capitals through which he passed; after which he proceeded to Italy through the Tyrole, and having seen the hospitals at Milan, resided during the winter at Pisa, in the house of Brugnatelli, the professor of chemistry. He there attended the lectures and hospital practice of Scarpa, whose friendship and correspondence he had ever afterwards the honour of retaining; and also clinical medicine under Joseph Frank, and natural history under Spallanzani. He then made the tour of Italy as far as Naples, remained some time at Rome, and returned by Padua, Venice, and Trieste, to Vienna, where he attended the clinical lectures of John Peter Frank, then at the head of the profession in Germany. From Vienna he returned home, through Prague, Leipsic, Halle, Dresden, and Berlin, remaining in each long enough to see the public institutions and become acquainted with the most celebrated men. During this tour, not only did he acquire a more accurate and more extensive knowledge concerning the medical institutions and the state of medical science abroad than was at that time possessed by other medical men in this country; but he attained a proficiency in foreign languages, and an erudition in literature, which added all the accomplishments of a scholar to his qualifications as a physician. Here, too, in leisure hours snatched from severer studies, he cultivated his taste for the fine arts, more especially for painting and music, in which he ever afterwards found a charm to relieve him from the fatigues he had to encounter in the laborious and anxious discharge of his professional and professorial duties.

On his return to Edinburgh, he assisted his father in editing the Medical Commentaries, which, as we have already stated, extended to twenty volumes, and was succeeded by the Annals of Medicine, on the title page of which the name of Dr Duncan junior, first appeared along with that of his father as joint editors. But at the request of lord Selkirk he was again induced to leave his native city to visit the continent, for the purpose of attending his lordship’s son, who was suffering under ill health. On his arrival, however, he found that this young nobleman had expired; but the attainments of Dr Duncan having attracted considerable notice on the continent, and being already signalized by a portion of the fame he afterwards enjoyed, he was solicited to prolong his stay in Italy, where he was by many invalids professionally consulted, and again enjoyed the opportunity of prosecuting his favourite pursuits. No man, perhaps, was ever more thoroughly imbued with the love of knowledge. It was in him an innate desire, urging him on with increasing restlessness to constant mental activity. He now remained chiefly in Florence and Pisa nine months, where he lived on habits of intimacy with the celebrated Fontana and Fabroni; after which, having visited many places in Switzerland and Germany, which he had not passed through during his former tour, he again returned to Edinburgh. He there settled as a medical practitioner, and was elected a fellow of the royal college of physicians, and shortly afterwards one of the physicians of the royal public dispensary, founded by the exertions of his father, in 1773.

While actively engaged in the practical department of his profession, he did not neglect the application of his erudition and talents to the diffusion and advancement of medical science among his professional brethren. In 1805, he undertook the chief editorship of the Edinburgh Medical and Surgical Journal, which has for twenty-seven years sustained the high reputation of being one of the most valuable and influential medical journals in Europe. He acted from the commencement as the chief editor, although for some time he was assisted by Dr Kellie of Leith, Dr Balteman of London, Dr Reeve of Norwich, and afterwards by Dr Craigie. But his chief and most valuable contribution to medical science was the Edinburgh Dispensatory, the first edition of which appeared in 1803. A similar work had been published by Dr Lewis in London, in 1753, under the title of the New Dispensatory, but the advancement of chemistry and pharmacy since that period, had rendered a complete revision of it absolutely necessary. This task, which required no ordinary extent and variety of knowledge, and no slight assiduity, he executed with so much skill, judgment, and fidelity, that his work, immediately on publication, commanded the most extensive popularity, and became a standard authority in every medical school in Europe. Notwithstanding, indeed, that it has had to encounter the rivalship of other meritorious works on pharmaceutic chemistry and materia medica it still maintains its pre-eminence. By Sir James Wylie it was made great use of in his Pharmacopia Castrensis Russica, published at Petersburg in 1808, for the use of the Russian army. It has been since translated into German by Eschenbach, with a preface by professor Kuhn; into French by Couverchel, and has been several times republished by different editors in America.

He next conferred an essential service not only on the university, but on the general interests of the community, by calling, in a strong and emphatic manner, attention to that branch of science, denominated by the Germans, state medicine, which comprehends the principles of the evidence afforded by the different branches of medicine, in elucidating and determining questions in courts of law. This study, to which the more appropriate term of medical jurisprudence was applied, had been chiefly confined to the Germans, nor had the advantages resulting from their labours been sufficiently communicated to other countries. This Dr Duncan fully perceived; he laid before the profession the substance of the few medico-legal works which had then been published on the continent; he pointed out, and advocated ably, the necessity of this department of medical science being systematically studied in this country; and, after combating many prejudices and overcoming many difficulties, succeeded in the cause he defended, and was rewarded by seeing the chair of medical jurisprudence instituted in the university. To his exertions, the profession—we should rather say the public—is indebted for the institution of this important professorship, and when we look at the current of public events, and the numerous complex and momentous cases that are continually agitated in our justiciary and civil courts, often implicating the liberty, fortunes, and even lives of our fellow-creatures, we cannot remain insensible of the great good he has achieved. The chair of medical jurisprudence and police was instituted in the Edinburgh university in 1807, and Dr Duncan was considered the most proper person to discharge its duties, he was therefore appointed the professor, and commenced his lectures the following session. He soon, by the lectures he delivered, and the numerous papers he published in his journal, impressed on the public mind the importance of the science he taught; and the interest he excited in its cultivation, both among his pupils, and medical practitioners generally, gave, in this country, the first impetus to the progress of medical jurisprudence.

He repeatedly, during this time, was called upon to assist his father in officiating as physician in the clinical wards, and occasionally delivered clinical lectures. He also had at times the charge of the fever hospital at Queensberry house; to which, on the resignation of Dr Spens, he was elected physician. But his introduction into the university, brought on him an accumulation of labours, for he was shortly afterwards appointed secretary and also librarian; offices, the duties of which required at that period no ordinary exertions to discharge. Already it may have been gathered from the lives of Drs Cullen and Duncan, senior, that the Edinburgh university was at this time only just emerging from that original infantine state which must precede the maturer glory of all institutions, on however grand a scale; and although Pitcairn, M’Laurin, the Monroes, Plummer, St Clair, Alston, and Cullen, had thrown over it a lustre which was recognized by men of science throughout Europe, yet its internal state and economy required the most assiduous attention and careful management. The library, which from the charter of the college, was entitled to every published work, was at this time, as may readily be supposed, a mass of confusion, which to reduce to any thing like order was little less than an Hercolean task. Added to this, the building of the university was yet unfinished, and every possible inconvenience opposed the duties of the librarian. Still the labours of Dr Duncan were incessant. He was then appointed one of the commissioners for superintending the completion of the building of the college; and the services which in both capacities he rendered to the public, cannot be too highly estimated.

Having officiated for his father and Dr Rutherford in the clinical wards of the royal infirmary during the winter of 1817-18 and the summer of 1818, he published at the end of that year reports of his practice, for the purpose of preserving a faithful record of the epidemic, which at that time spread its ravages through Edinburgh. His labours did not go unrewarded. In 1819, the patrons of the university appointed him joint professor with his father in the chair of the theory of medicine. His skill as a lecturer on physiology was duly estimated by his pupils; but he did not retain this office long, for in 1821, Dr Home being translated to the chair of the practice of physic, he was elected in his place professor of materia medica and pharmacy. It is worthy of observation, that so highly were the qualifications of Dr Duncan appreciated, and so obviously did they entitle him to this honour, that when it was understood that he had come forward as a candidate, no person ventured to compete with him for the vacated chair. He commenced his lectures at considerable disadvantage, being at the time in ill health, owing to an accident he had recently met with; but his abilities as a lecturer, and his profound knowledge of materia medica, with all its collateral branches being well known, attracted crowds to his class, among whom no individual can fail to remember how amply his expectations were redeemed. In the discharge of his duties as a professor, he laboured most conscientiously, sacrificing his own comforts and health for the instruction of his pupils. During this season and indeed ever after, says one who had every opportunity of knowing his domestic habits, "he was often seated at his desk at three in the morning, for his lectures underwent a continual course of additions and improvements." When, by the tender solicitude of his own relatives, he was often entreated to relax his incessant toils, and told that surely his task must be finished, he would reply, that to medical knowledge there was no end, and that his labours must be therefore infinite; and so truly they were, for it was one of the peculiar traits of his character to be ever investigating, which he did with unwearied patience, every new improvement and every new discovery that was announced in this country or on the continent. His lectures on materia medica were most comprehensive and profound, and attracted so great a number of students to his class that the expectations which had been formed of the good which the university would derive from his promotion were amply fulfilled. He discharged the duties of this professorship with unwearied zeal and assiduity for eleven years. We have now arrived at the saddest period of his life. His constitution was never strong. It was constantly preyed upon by the exertions of an over-active mind, which allowed itself no repose. Had he been less solicitous about the discharge of his duties and less zealous in the pursuit of science, his health might have been invigorated and his life prolonged. But there was that disparity between the powers and energies of his mind, and the limited rigour of his body, which generally proves fatal to men of superior attainments. He had for years toiled incessantly, bearing up against the consciousness of ill health and physical suffering. His anxiety to discharge his duties, indeed, absorbed every other consideration, and prompted him to endure until endurance itself could no longer obey its own high resolves. His strength, which had been severely impaired by an attack of fever in 1827, which was contracted in the discharge of his hospital duties, gradually declined. After persevering in delivering his lectures until nearly the end of the session, he took to his bed in April 1832, and having endured a lingering illness, during which he displayed all that patience and moral courage which are characteristic of a highly-gifted mind, he died on the 13th of the following May, in the 58th year of his age. His funeral, according to his own directions, was intended to be strictly private; but the members of numerous institutions, anxious to show their affection for his memory, met in the burial ground to attend the obsequies of their lamented friend.

Great energy and activity of mind, a universality of genius that made every subject, from the most abstruse to the most trivial, alike familiar to him, and a devoted love of science, which often led him to prefer its advancement to the establishment of his own fame, were his distinguishing traits. So well was he known and appreciated on the continent, that he received, unsolicited on his part, honorary degrees and other distinctions from the most famous universities; and few foreigners of distinction visited Edinburgh without bringing introductions to him. He had the honour of being in the habit of correspondence with many of the most distinguished persons in Europe, whether celebrated for high rank, or superior mental endowments. He had a great taste for the fine arts in general, and for music in particular; and from his extensive knowledge of languages, was well versed in the literature of many nations. His manners were free from pedantry or affectation, and were remarkable for that unobtrusiveness which is often the peculiar characteristic of superior genius. He possessed a delicacy of feeling and a sense of honour and integrity amounting, in the estimation of many, to fastidiousness, but which were the elements of his moral character. He was indeed as much an ornament to private as to public life.