Answer:
In the sentence from "Nature", "In good health, the air is a cordial of incredible virtue," Emerson uses a metaphor. He is comparing air with virtue.
Explanation:
In a metaphor we compare two unrelated things that share a common quality. In this text Ralph Waldo Emerson compares air with something as noble and positive as virtue. He says that nature fits well both in a comic or in a mourning piece, that is to say in a hostile or flourishing environment, and here, air is an necessary element that gives life and is enjoyed; especially when one has good health.
