Eustacia Vye is linked to the imagery of flames, fire and heat throughout the novel. This motif—and the metaphors and similes of fire Hardy uses when it occurs—refers to her inflammatory character. It also gestures to her passionate nature and her tendency to act destructively when out of control. In Book 1, Chapter 7, the narrator comments that:

Assuming that the souls of men and women were visible essences, you could fancy the colour of Eustacia’s soul to be flame-like. The sparks from it that rose into her dark pupils gave the same impression.

The tactile and visual imagery of Eustacia's soul being "flame-like," hot enough to send sparks rising "into her pupils" reinforces the association of her personality with heat. As opposed to other female characters in the novel, Eustacia often publicly expresses anger and discontent. This "heat" is apparently visible in her personal appearance, as well as in her words. Later in Book 1, this image of Eustacia being internally aflame reappears:

‘Ah, my life!’ said Eustacia, with a laugh which unclosed her lips, so that the sun shone into her mouth as into a tulip, and lent it a similar scarlet fire. 

The simile in this passage compares the sun shining into Eustacia's mouth to sunlight breaking through the thin petals of a tulip. Both images evoke delicacy and vivacity for the reader. Like a flame, she's pure energy, but like a tulip, she's fragile. Even when Eustacia is a flower, she's still a fire. This description also emphasizes Eustacia's natural beauty and sensuality. Tulips are often associated with sexual love, which aligns with Eustacia's sensual nature.

It is not just her personal "fire" that Hardy focuses on, however. The other motif with which Eustacia is often linked is that of bonfires. Her affair with Damon Wildeve is delineated by clandestine meetings at night around open flames. Eustacia's association with fire, especially given this, is also a subversion of the "darkness" Hardy ascribes to her. She is a character made up of contrasts, like a bonfire burning on the heath—she is firelight and darkness.