A simile means that two things are being compared to one another using the word "like" or the word "as." For instance, I could say the house is "solid as a rock" or "solid like a rock," and you would know that the house is not actually a rock, but is very sturdy. In this poem, Whitman uses simile in the line, "I depart as air."

So then, what purpose is achieved by the poet comparing himself to air? How does this simile contribute to the purpose of the poem? Perhaps, just as he claims to be alive in the dirt and grass, and able to shout ("yawp") over the rooftops of the entire world, this simile is just another way of describing how he feels that his soul is infinite, intertwined with the rest of the world and all of nature.