Douglass says, “at a time like this, scorching irony, not convincing argument is needed” (lines 138–139). why does he say this, and what is it that he finds ironic?

He says this because he is discussing the Fourth of July -- Independence Day.

Douglass says, however, that not everyone is independent. Some Americans, he reasons, are far from independent -- namely slaves. It is ironic that, on a day Americans celebrate freedom, a large portion of the population is anything but free.