The various facts concerning the radiation of caloric present a very interesting subject of inquiry, and various theories have been formed, with a view to their explanation. The principal difficulty has been to explain the facts of the apparent radiation of cold. The most simple theory with respect to these, is that proposed by M. Prevost of Geneva. He supposes, that all bodies are always radiating caloric in proportion to their temperature, and that those surfaces, which radiate least, make up for the deficiency, by reflecting most; so that the combination of reflection and radiation from any surface, when in æquilibrio with the surrounding bodies, is the same. When a body is heated, its radiation is increased, and therefore, when placed near a thermometer, it radiates more to the thermometer than it receives from it, and therefore elevates its temperature. When, on the other hand, a body is cooled, it intercepts from the thermometer near it part of the radiation of the surrounding objects, and it radiates less than it receives. The thermometer must therefore sink.