Complex sentence: In Kazan, he joined another revolutionary circle, through which he discovered Karl Marx's "Capital" (1867); exerting a strong influence on him, he became increasingly interested in Marxism.
Split into simpler sentences: In Kazan, he joined another revolutionary circle, through which he discovered Karl Marx's "Das Kapital" (1867). It exerted a strong influence on him, and he grew increasingly interested in Marxism.