See his Kyrill und Method, Prague, 1823. Schlözer considers likewise the Old Slavic as a Bulgarian dialect of the ninth century. See his Northern History, p. 330. In another place he calls it the mother of the other Slavic languages; see his Nestor, I. p. 46.
In his Grammar of the Slavic Language in Carniola, Carinthia, and Stiria.
Jahrbücher der Literatur, Vienna, 1822, Vol. XVII. Grimm is of the same opinion; see the Preface to his translation of Vuk Stephanovitch's Servian Grammar.