Final answer:
The phrase represents a metaphor, where the air's refreshing quality in good health is directly compared to the beneficial properties of a cordial.
Step-by-step explanation:
The phrase "In good health, the air is a cordial of incredible virtue" is an example of metaphor. This figure of speech draws a direct comparison between the refreshing quality of air when one is in good health to a cordial, which is a sweetened and flavored aromatic drink known for its supposed medicinal benefits. Rather than saying that the air is literally a medicinal drink, this metaphor implies that the air possesses restorative qualities similar to a cordial, which is rejuvenating and beneficial to one's well-being.