In his "Emancipation und Mission" Cassel endeavored to show that the Jews would obtain permanent relief from persecution not by civil enfranchisement, but through evangelization. In later years, however, he frankly receded from this view. De le Roi, the historian of Christian propaganda among the Jews, says that Cassel was animated by "a very decided Jewish spirit." In 1860 Cassel published "Die Geschichte des Jüdischen Volkes seit der Zerstörung Jerusalems" (History of the Jewish people since the destruction of Jerusalem). He exerted himself in the interest of proselytism among Jews. He stated his views on missionary work among them in the pamphlet entitled "Wie ich über Judenmission denke", 1886; see also "Nathanael. Zeitschrift für die Arbeit der Evangelischen Kirche an Israel", edited by Strack, 1897. From 1875 to 1889 he edited the weekly "Sunem, ein Berliner Wochenblatt für christliches Leben und Wissen" (16 vols.). "Hallelujah! Einhundertundachtundachtzig geistliche Lieder" is a collection of hymns reprinted from this journal. In 1847 Cassel wrote an earnest though somewhat fantastic study of Hungarian archaeology, "Magyarische Alterthümer", which long remained of value. Of especial interest is his translation (with notes) of the reply to Chisdai ben Isaac of Joseph, king of the Khazars (pp. 183 et seq.).