Complex sentence: Hall soon became principal proprietor, but, unable to turn a profit on his own, the London publisher George Virtue purchased into Hall's "Art Union Monthly Journal" in 1848, retaining Hall as editor.
Split into simpler sentences: Hall soon became principal proprietor, but he was unable to turn a profit on his own. The London publisher George Virtue then purchased into Hall's "Art Union Monthly Journal" in 1848, retaining Hall as editor.