399. Heat operates spherically in matter or in all directions, not in the linear direction like light. The propagation of heat can only take place slowly, because it is not a polar action, but only the result of such an one, only motion. Heat does not penetrate bodies mechanically, but dynamically like light, yet without decomposing them, as light does. 400. Heat is related as indifferent æther to the matter as to a Polar. This relation imparts the _process of conduction_. Light, however, is itself polar and disturbs matter, while it passes through it. 401. With the exception of their cohesion matters are not directly changed by heat. 402. During every process of decomposition, during every process of light heat must be produced, but not light also during every process of heat. 403. Dense materials must conduct heat because they are most opposed to it. It is only the formed element therefore that can possess capacity for conducting heat. 404. Absence of form is the character of isolators of heat, form that of the conductors, apart from every remaining quality. Solid bodies, which easily pass into the formless condition are isolators. 405. The densest bodies among the solids must be the best conductors. Regard may be first paid to the nature of their constituent parts in the sequel of the present section of this work. 406. The conduction of heat is a continuous excitation from one part striving against or resisting the other; the earths (as metals) are the best conductors. 407. Matters, which are images of heat, do not conduct, because they enter only as minimum into conflict with it, and while they expand at once convert themselves into the same. Such is the air. The Heterogeneous only conducts. The heat expands in the air only by continuous motion of the aerial particles. The air is an isolator. Water ranks in the middle between air and earth. 408. With respect also to conduction, light is opposed to heat. Light is conducted by those very bodies, which isolate heat, and isolated, not admitted to permeate or absorbed, by those which conduct heat. The air conducts light, isolates heat; the metal, the earth conducts heat, isolates light; water holds a mediate relation towards the two, yet towards light that of a conductor, because it is deoxydizable. 409. The conduction of light is likewise a process of deoxydation, or a disintegration of matter. By the conduction of light the bodies are chemically analysed, and finally resolve themselves into their principles; such after all is the case with every glass and crystal. As the process of conduction of light may be called a process of deoxydation, so also may it be called a process of the generation of colour. The conduction is an offuscation of light, a colouring; the deoxydation is a solution of the material bonds, an elevation unto colour. 2. _Functions of the Air._ ELECTRICITY.