Popa focuses on the role of the two basic active dunameis, the hot and the cold, in Aristotle, Meteorology IV. There, the dispositional properties of the homoeomers are often defined and explained in virtue of bodies’ reactions to heat and cold: some homogeneous bodies are solidifiable by heat, others by cold. Popa argues that Meteor. IV aims to account for the coming about and persistence of uniform stuffs by appealing to what he calls “thermic equilibrium” (summetria, logos). The main purpose of this chapter is to clarify the ways in which Aristotle puts this notion to work in his study of homogeneous materials (especially in Meteor. IV) and, through this clarification, to contribute to a better understanding of the role played by proper or internal natural heat in his scientific works.