A colonist in a black hat is attempting to make friends with one of the Africans, who doesn't speak much English – but this is no hindrance, the white man has taught himself a few words of some African tongue. The slave is very dark, and the arms of the King of Spain are branded into his left shoulder, and so he is probably Angolan. Life has been strange to him: abducted by Africans fiercer than he, chained up in a hole in Luanda, marked with a hot iron to indicate that duty had been paid on him, loaded onto a ship, and sent to a cold place full of pale men. After all of that, you'd think that nothing could possibly surprise him. But he's astonished by whatever this Barker is telling him. The Barker's punching at the air and becoming quite exercised, and not just because he is inarticulate. Assuming that he has been in touch with his brethren in London (and that is a very good assumption), he is probably telling the Angolan that he, and all of the other slaves, are perfectly justified in taking up arms and mounting a violent rebellion.