The recurring motif of bonfires in this novel represents passion, secrecy, danger, and the primal instincts of the characters. Bonfires are bright, intense, and have an inherent potential for destruction.  Hardy depicts them in lavish detail, employing visual imagery, metaphors, and similes when describing them and the events they point toward. The vivid visual language associated with fire creates a strong impression on the reader. For example, in this passage from Book 1, Chapter 3, Hardy describes a night-time scene on the heath as the Guy Fawkes Night bonfires are set ablaze:

Red suns and tufts of fire one by one began to arise, flecking the whole country round. They were the bonfires of other parishes and hamlets that were engaged in the same sort of commemoration. Some were distant, and stood in a dense atmosphere, so that bundles of pale straw-like beams radiated around them in the shape of a fan. Some were large and near, glowing scarlet-red from the shade, like wounds in a black hide. [...] These tinctured the silent bosom of the clouds above them and lit up their ephemeral caves, which seemed thenceforth to become scalding caldrons.

All of the visual imagery in this passage invokes a starkly contrasted picture of black night and glowing red flame. The bonfires are so intense that they even tint or “tincture” the “caves” of the clouds in the sky, overtaking the entire scene. Hardy evokes the primal with this metaphor of the clouds as "caves." The denizens of the heath have, in a sense, reverted to an ancient state of shelter and flame. The bonfires are also made to seem like an inherent part of the scenery. They sprout from the ground and the sky organically like “red suns and tufts of fire.” Although they've been made by humans, they are also a natural part of the heath's visual landscape.

Hardy furthers this description with the use of visual language in two similes. The fires make the interior of the clouds seem like “scalding caldrons,” and the nearby ones are so bright that they slash through the night like “wounds in a black hide.” The scale of the scene is enormous, as it stretches from “distant” villages to the “bosom” of the sky. The bonfires, which happen regularly throughout the novel, mirror the passionate, often destructive relationships among the characters. They "wound" and "scald" the night as Hardy's characters wound and scald one another.

Hardy associates bonfires in particular with the affair between Eustacia and Wildeve. This pair signal each other for their secret meetings with these fires. The darkness of the heath, the heat of the flames, and the nature of their love affair all become tangled up in this web of images, metaphors and similes. All is hot, dark, primal, and painful in this relationship. For example, in Book 1, Chapter 6, Wildeve and Eustacia meet by a fire:

‘I have come,’ said the man, who was no other than Wildeve. ‘You give me no peace. Why do you not leave me alone? I have seen your bonfire all the evening.’ The words were not without emotion, and retained their level tone as if by a careful equipoise between imminent extremes.

Eustacia and Wildeve cannot help but seek one another out. Wildeve can have “no peace” when Eustacia is around, even though he knows their relationship is immoral. The danger of the bonfire—which always threatens to spread and damage its surroundings—is reflected in this pairing.