391. The development of heat in a body is not an extrusion of a matter adherent, and as it were foreign, to it; but an ascent of the matter itself into heat. The matter does not develop, or give out heat, but _becomes_ heat, namely æther. 392. The loss which a body sustains by the radiation of heat is as slight as the æther is subtile or rare; thus it is infinitely small, where the æther is infinitely light. We cannot speak of the loss of matter, while it is hot; although a true loss is present, if the point in question be philosophically regarded, it can come, however, as little under consideration as the weight of the æther. 393. The fusion of bodies is a diminution of the fixity of their poles, their further evaporation, and thus an approximation to indifference or the apolarity of æther. Heat is the actual retrogression of matter into æther; light is only the efficient of this transition. 394. As heat becomes originally excited, so must it be always excited; two causes for one effect are impossible. The excitation of heat by oxydation takes place in the same way as by light, namely by polarization and separation therefore of the Indifferent. The generation of heat by chemical processes is based upon the same principle. Lastly, the generation also of heat by compression and simple _friction_ is wholly similar to that which is caused by light. In every case they are only polar, and by no means mechanical operations upon matter, whereby the fixity of the poles becomes changed. 395. It is not a change of cohesion, which the friction mechanically effects, but the act is purely dynamical. The essence of friction consists in the constant renewal of polar change, because thereby an infinite number of projecting points or apices are alternately brought into contact. There is verily no smooth body. 396. Heat is the transition of light unto darkness; for it is indifferent æther, only moved. Colours are thus also a conflict of light with heat, and out of this conflict issues the most beautiful, the highest colour, the Red of fire. In fire, the contest between light and darkness has risen to the highest pitch; the æther therefore is also moved to the greatest degree, becomes hot. If the indifference becomes the maximum, the vital tension then relaxes, the fire is extinguished; finally motion ceases, it is cold and dark. 397. In matter also light and heat operate against each other. Light deoxydizes, heat oxydizes. If light appears to oxydize, it is only by evoking heat. 398. Heat is the function of expansion for matter. Every body has a definite degree or amount of expansion, therefore a definite fixity of æther; this æther is _latent_ heat.