At the beginning of the novel, Montag sees fire eating things, blackening and changing them. He sees it as a destroyer, burning to "bring down the tatters and charcoal ruins of history." (pg 3) He feels like he is conducting a symphony, and the smile it gives him never goes away. He enjoys its destructive power.
After Montag leaves the river, he sees a fire and watches it from the cover of some bushes. He felt it was a strange fire because it meant something different to him. It was not a destructive, burning fire. It was a warming fire. He saw men sitting around the fire warming their hands. It had never entered his mind that fire could give something to mankind. He had only seen it as a destructive force. He noticed that even the smell was different. He listened to the crackle, and he even describes the crackle as "warm". (pg 146)