On his return to Edinburgh, Jackson married a lady of fortune, the daughter of Dr. Stephenson, and niece of his old friend Colonel Francis Shelley, of the 71st regiment; and was enabled by this accession to his means once again to visit Paris, where he not only resumed his medical studies, but acquired the mastery of several languages, Arabic amongst the rest. Having graduated M. D. at Leyden, he came back again to England, and commenced practice at Stockton-upon-Tees, in Durham. Although his reputation speedily became considerable, especially in cases of fever, he seems scarcely to have liked his new avocation. He found solace, however, in his favorite study of language, which he pursued with unremitting ardor--constantly reading through the Greek and Latin classics, and not only rendering himself familiar with the best works of the modern continental authors, but also with the literature of the Arabic, Persian, Hebrew, and Gaelic tongues. The _Bostan_ of Saadi is said to have been one of his most favorite poems.
On the war breaking out in 1793, Dr. Jackson--who, in 1791, had published a valuable work on the fevers of Jamaica and continental America--applied for employment as army-physician; but Mr. Hunter, the director-general of the medical department of the army, considering none eligible for such employment who had not served as staff or regimental surgeon, or apothecary to the forces, Jackson agreed to accept, in the first instance, the surgeoncy of the 3d Buffs, on the understanding, that at a future time, he should be nominated physician as he desired. Mr. Hunter, however, ever, died soon after this; and his promise was not fulfilled by the Board which succeeded him in the medical direction of the army, and which appears to have pursued Dr. Jackson with uniform hostility.
Returning to England with the troops, it was offered to him to accompany, in the capacity of chief medical officer, Sir Ralph Abercromby's expedition against some of the West India islands; and although no employment could possibly have been more agreeable to his taste, he, much to Sir Ralph's chagrin, declined the flattering proposal, on the grounds, that lower terms had been offered to him than to another professional man. Nothing but a sense of professional delicacy, it is plain, governed him in this transaction, for he immediately afterwards embarked (April, 1796) as _second_ medical officer in another expedition to San Domingo. During his abode in this island, he was unwearied in enlarging his acquaintance with tropical diseases--observing the rule he had followed in Holland of noting down by the patient's bedside the minutest particulars of every case he attended, the effects of the treatment pursued, and whatever else might shed light on the intricacies of pathological science. He also gave a larger practical operation to the scheme he had years before devised of amending the dietaries of military hospitals.