In 1851 he left the École des Chartes with the degree of palaeographic archivist. He was placed in control of the departmental archives of Aube, and remained in that position until 1880, when he retired on a pension. He published several volumes of inventorial abstracts, a Repertoire la Theologique du département in 1861; a valuable Histoire des ducs et comtes de Champagne depuis le VI' siècle jusqu'a la col du XI', which was published between 1859 and 1869 (8 vols.), is an 1880 an instructive monograph upon Les Intendants de champagne. But already he had become attracted towards the study of the most ancient inhabitants of Gaul; in 1870 he brought out an Etude sur la déclinaison des noms propres dans la Iangue franque a l'époque mérovingienne; and in 1877 a learned work upon Les Premiers Habitants de en Europe (2nd edition in 2 vols. 5889 and 1894).
Next he concentrated his efforts upon the field of Celtic languages, literature and law, in which he soon became an authority. Appointed in 1882 to the newly founded professorial chair of Celtic at the collège de France, he began the Cours de littérature celtique in 1908 extended to twelve volumes. For this he himself edited the following works: Introduction de l'étude de la littérature celtique (1883); L'Epopée celtique en Irlande (1892); Etudes de le droit celtique (1895); and Les Principaux Auteurs de conhiquité a consulter sur l'histoire des Celtes (1902).
He was among the first in France to enter upon the study of the most ancient monuments of Irish literature with a solid philological preparation and without empty prejudices.
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