In the play The Crucible, by Arthur Miller, how does fire work as a symbol?

Fire in The Crucible symbolizes sinfulness, and fire's ability to quickly become out of control is mirrored by the way sin seems to spread and destroy everyone and everything in the town.

After all, the trials in this play begin with two small fires, one literal and one figurative.  Reverend Parris mentions fire in Act One when he describes seeing "Tituba waving her arms over the fire" when he found the girls dancing in the forest.  "She were swaying like a dumb beast over that fire!"  This small fire was at the center of the circle when the girls were conjuring spirits and Abigail drank the charm to kill Elizabeth Proctor.  Calling spirits back from the dead and attempting to murder an innocent woman were activities that their community would certainly have considered to be sinful.  Once Reverend Hale finds out about their activities, he immediately believes the Devil to have infiltrated Salem via the slave, Tituba.  He elicits a confession from her, and when she accuses two women in the town as being in league with the Devil, the girls join in and immediately accuse nine more.

Then, when Abigail and John are speaking alone in Parris' house, she says to him, "I have a sense for heat, John, and yours has drawn me to my window, and I have seen you looking up, burning in your loneliness."  The fire between them is metaphorical, but it is sinful because of John's married status.  He has broken one of the Ten Commandments, and so this "fire" between Abigail and himself is certainly representative of that sin.  Because of this "fire," Abigail accuses Elizabeth Proctor of witchcraft in order to get rid of her, and the sin spreads further and begins to get more unpredictable and out of control.

When Mrs. Putnam grows upset with Rebecca Nurse, insisting that witchcraft must be responsible for the deaths of her seven babies, she says, "There are wheels within wheels in this village, and fires within fires!"  Because enough people are willing to share this belief, the "fire" spreads in Salem as people begin to accuse their neighbors, land-owners, and outcasts alike, all selfishly motivated.

Finally, after Mary Warren turns on Proctor at the end of Act Three, and Danforth accuses him of being in league with the Devil, Proctor says, "A fire, a fire is burning!  I hear the boot of Lucifer, I see his filthy face!  And it is my face, and yours, Danforth!  For them that quail to bring men out of ignorance as I have quailed, and as you quail now when you know in all your black hearts that this be fraud -- God damns our kind especially, and we will burn, we will burn together!"  The "fire," i.e. sin, has now spread so much that it has gotten out of control and will consume them all.  Abigail never meant for John to be accused; she loves him.  But the sin has spread so far and so uncontrollably that she can no longer determine where it reaches.  Proctor says, now, that he and Danforth are also responsible for the spread of this "fire because they had the power to stop it and didn't.  He feels that God will mete out a special punishment for them.