Pathos

Pathos uses emotional appeals to convince an audience. The speaker's tone, word choice, and imagery are all intended to provoke an emotional response.

Douglass aims to provoke several emotions in his listeners, the chief of which is shame. He pretty much says, "Hey, look at all you hypocrites celebrating freedom while your country is built on slavery."

Check it out:

Your high independence only reveals the immeasurable distance between us. The blessings in which you this day rejoice are not enjoyed in common. The rich inheritance of justice, liberty, prosperity, and independence bequeathed by your fathers is shared by you, not by me. The sunlight that brought life and healing to you has brought stripes and death to me. This Fourth of July is yours, not mine. (15-19)

Douglass specifically states that he's in the shame game. He's not going to argue when the arguments are clear:

At a time like this, scorching irony, not convincing argument, is needed. Oh! had I the ability, and could I reach the nation's ear, I would today pour out a fiery stream of biting ridicule, blasting reproach, withering sarcasm, and stern rebuke. (75-76)

He explicitly states that this is an emotional appeal to make Americans feel how wrong slavery is. He's not going to convince their brains; he's going to convince their hearts:

For it is not light that is needed, but fire; it is not the gentle shower, but thunder. We need the storm, the whirlwind, and the earthquake. The feeling of the nation must be quickened; the conscience of the nation must be roused; the propriety of the nation must be startled; the hypocrisy of the nation must be exposed; and its crimes against God and man must be denounced. (77-79)

Yikes. Even more than 150 years later, we still feel horrible about America's history of slavery…which is exactly the response Douglass is looking for.