The oldest extant reference to the problem of an international language appears to be contained in the letter written by Descartes on November 20th, 1629, to his friend Mersenne. The great philosopher here explains the principles which convinced him that it would be possible to construct an artificial language which could be used as an international auxiliary language. As for Leibnitz, who was attracted throughout his whole life by this problem, his language projects have been recently investigated by L. Couturat by means of documents, many of which have never before been published (_La Logique de Leibnitz_ and _Opuscules et Fragments Inédits de Leibnitz_). There may further be mentioned the _Ars signorum Vulgo Charakter Universalis et Lingua Philosophica_ (London, 1661) of George Dalgarno, and the recently discovered memoir of an unknown author entitled _Carpophorophili Novum inveniendæ Scripturæ Œcumenicæ Consilium_ (Leipzig, 1734). The last-mentioned system in particular strikes one as highly modern in principle. It was only, however, at the end of the last century that the era of practical things began with the Volapük of Schleyer. The success of this language was very considerable. It possessed about thirty journals, published in the most different countries, even in Japan, and its literature has been estimated at from 300 to 400 works. The official lists published in 1889 contained the names of 255 local groups belonging to the "Universal Language Society," some of which possessed a very considerable membership. The teaching of the language was highly organised, there being 900 teachers, 200 head teachers, and 50 "professors." This great linguistic experiment was very instructive, and its significance cannot be underrated. Important conclusions concerning the theory and practice of artificial language can be drawn from it, and especially from a consideration of the circumstances which finally led to the downfall of Volapük. It turned out that this was due to the errors which Volapük itself contained, showing us that in these matters, as in others, practical experience is the best teacher. The fate of Volapük was sealed when its supporters, in the year 1889, made the experiment of organising a congress at which Volapük should be spoken. Although a few Volapükists succeeded in speaking the language, it was only too painfully evident that such a goal could not be reached with this system. Almost simultaneously with Volapük another artificial language had been invented. The Russian medical man Dr. Zamenhof published his system in 1887 under the pseudonym of "Doktoro Esperanto." But as Esperanto arrived while Volapük was at its zenith, it failed at first to attract general attention. It found, however, in France, an enthusiastic supporter in the Marquis de Beaufront, who had himself worked out an international language called "Adjuvanto." He gave this up as soon as he came to know about Esperanto, and founded the _Société