This textbook is a course on properties of heat. The first seven lectures of the course deal with "thermometric heat" and its generation and consumption in mechanical processes; the determination of the mechanical equivalent of heat; the conception of heat as molecular motion; the application of this conception to the solid, liquid, and gaseous forms of matter; expansion and combustion; specific and latent heat; and calorific conduction. The remaining five lectures treat of "radiant heat"; the interstellar medium, and the propagation of motion through this medium; the relations of radiant heat to ordinary matter in its several states of aggregation; terrestrial, lunar, and solar radiation; the constitution of the sun; the possible sources of his energy; and the relation of this energy to terrestrial forces, and to vegetable and animal life.