Later, a long stay in Sweden furnished him with valuable documents for a political and social history of Sweden and France at the end of the 18th century.In 1864 and 1865 he published in the Revue des deux Mondes a series of articles on Gustav III of Sweden and the French court, which were republished in book form in 1867.
To the second volume he appended a critical study on Marie Antoinette et Louis XVI apocryphes, in which he proved, by evidence drawn from documents in the private archives of the emperor of Austria, that the letters published by Feuillet de Conches (Louis XVI, Marie Antoinette et Madame Elisabeth, 1864–1873) and Hunolstein (Corresp.
He was then appointed to superintend the opening of the French school of archaeology at Rome, and drew up two useful reports (1877 and 1884) on its origin and early work.
His last works were an Essai sur la formation des collections d'antiques de la Suède and Des institutions et des mœurs du paganisme scandinave: l'Islande avant le Christianisme, both published posthumously.