H. G. Wells 
In the Days of the Comet 

In the Days of the Comet (1906) is a science fiction novel by H. G. Wells in which humanity is "exalted" when a comet causes "the nitrogen of the air, the old azote, " to "change out of itself" and become "a respirable gas, differing indeed from oxygen, but helping and sustaining its action, a bath of strength and healing for nerve and brain." The result: "The great Change has come for evermore, happiness and beauty are our atmosphere, there is peace on earth and good will to all men."
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An unnamed narrator is the author of a prologue ("The Man Who Wrote in the Tower") and an epilogue ("The Window of the Tower"). In these short texts is depicted an encounter with a "happy, active-looking" old man: the protagonist and author of the first-person narrative, writing the story of his life immediately before and after "the Change".
This narrative is divided into three "books": Book I: The Comet; Book II: The Green Vapours; and Book III: The New World.