On Fire is an elaboration of the energetic model of matter.  Kant argues there that all bodies, solid, liquid, and gaseous types, consist of dynamic particles or molecules (moleculae, 1:372.24 and passim). He contends that the particles cohere in an elastic medium. This medium, the ether, permeates the molecular interstices of bodies (prop. 3, 1: 372). He calls this ether “fire-matter” and identifies it as the carrier of warmth and light. Heat results from wave-like vibrations of this materia ignis among the molecules (prop. 8, 1:377). As it is known today, heat is a symptom of molecular vibration, which in turn depends on the energy-state of a body.