Excerpted from De ignis proprietatibus

Fire sometimes expresses the spirit of god, sometimes the Holy Spirit, sometimes charity, sometimes desire, sometimes malice, sometimes understanding, sometimes distress, sometimes anger, and sometimes pleasure. It indicates the will of God when it is said: God is a devouring fire [Dt. 4: 24, Hebr. 12: 29]; The Holy Spirit when it is said: God descended in fiery tongues onto the apostles, when he enlightened them with the light of wisdom. It signals charity, like that which is in the Gospel: I have come to send fire onto the earth, and would that it were burning [Lc 12: 49]. Desire, like in Exodus [35: 3]: You will not kindle fire in any of your houses during the day of the sabbath; that is, he forbids the fire of desire to rise in human hearts and in all their assemblies. It signals malice through a crookedness of the soul towards lesser things, about this fire it is said: fire will consume the enemy. For, as fire rouses an amorous mind, so does fire envelop a malicious mind.

Original Latin

Est et ignis triplex: Alius vitalis et fovens, alius nocivus non tamen plene corrumpens, tertius vero destructivus corrumpens. Est etiam ignis ens non urens, et est lucens et urens, et est urens et non lucens, ut ignis gehennae. Ignis quoque iste communis semper in motu, et non rectus est sed acutus, quasi linqua circumstancia lambens.

Consumit autem iste ignis non id quod est, sed id quod non est,13 peccatum scilicet, cum econtra ignis concupiscientiae id quod est consumat, quia ab esse ad minus esse hominem inflammando trahit.

Occurrunt etiam isti ignes pesti, quae provenit ex solis obscuratione. Obscuratio solis est per interpositionem lunae, et hoc est cum mundialium mutabilitas inter Christum, solem iustitiae, et animam dividit. Ex qua divisione nascuntur pestes innumerae, quibus praedictos ignes manifestum est auxiliari.

Sic homo totus, hoc quod conditus est, configuralis est his ignibus spiritualibus, nec remanet cinis nisi in quantum aglutinat sibi per concupiscentiam haec terrena quae alienae sunt naturae.14 Ignis est species elementorum; sic et caritas forma virtutum.