Since the publication of the anecdote respecting the Man in the Iron Mask, which M. de Voltaire has given us in his “_Siècle de Louis_ XIV.,” I have been always very curious to discover who this prisoner could be; but all my researches had hitherto failed in giving me any information which could content me; chance has placed in my hands a detached number of a work, of which the title is “_Histoire abrégée de l’Europe_,” for the month of August 1687, printed that same year at Leyden, by “_Claude Jordan_.” At the article Mantua, I found the letter, which I have the honour to send you a copy of, translated from the Italian. It appears that this Secretary of the Duke of Mantua, who is there mentioned, might very well be the Man in the Iron Mask, transferred from Pignerol to the Islands of Saint Margaret, and from thence to the Bastille, in 1690, when M. de Saint-Mars was made governor of it. I am the more inclined to believe this, because, as M. de Voltaire, and all those who have made researches on this subject, have remarked, there did not at that time disappear any prince, or person of consequence in any part of Europe. If you, Gentlemen, find any appearance of probability in my remark, and if you think it can interest the public, you are welcome to insert it in your Journal, &c. THE BARON DE HEISS. Formerly Captain of the Regiment of Alsace. Phalsbourg, June 28, 1770. Letter on the subject of the Man in the Iron Mask, announced in the preceding one. GENTLEMEN, One of my friends tells me, that he has read in the “_Histoire abrégée de l’Europe_,” (Vol. ii. p. 33.), that it was said, that the Duke of Mantua had had the intention of selling his capital town; but that the author of this History did not believe it. You are ill-informed; it is certain that this affair was negociated, and that it was much advanced towards a conclusion. The secretary of the Duke, who had much influence with his master, dissuaded him from this design; but the doing so cost him very dear, as you shall soon learn.