The original text reads:
No man was ever yet deeply convinced of any momentous truth, without feeling in himself the power as well as the desire of communicating it ( Southey, Colloquies on the Progress and Prospects of Society, I, p. 83)
In the original text, in which Southey imagines a conversation with the ghost of Sir Thomas More, the sentiment is expressed by More rather than, as Ruskin implies, by Southey himself.