Opinions differ on where Ray Bradbury came up with the temperature of 451 degrees Fahrenheit as the definitive temperature at which paper burns. A scholarly text (cited below) gives the general autoignition temperature of paper to be 450 degrees Celsius, which would be about 840 degrees Fahrenheit; it is possible that Bradbury misremembered Fahrenheit for Celsius. Regardless, the temperature and number are used in the novel to symbolize firemen and their job, which is to burn things rather than extinguishing them. Fireman helmets, jackets, and kerosene canisters are all marked with 451, to show their purpose; the repetition of the number serves to remind firemen of their job, and to show the citizens of the city that they are at the government's mercy, even to the point of non-court-sanctioned killing in the pursuit of burning books.
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