So much then for the fact. Let us now consider the design of such an arrangement. When a body contains more caloric than the air, or the other bodies by which it is surrounded, heat is given off from it in all directions, till the equilibrium is restored. Three, and perhaps more, physical operations take place in this case; radiation from the heated substance, reflection and absorption by the surrounding bodies. Now it has been proved, by repeated experiment, that these changes depend, as it regards their extent and rapidity, upon the color of the bodies. The more light-colored the heated substance is, the more slowly will it part with its superfluous caloric. Were it entirely black, the change would take place with more rapidity than in any other case. If the surrounding bodies were of a light color, a large portion of the heat radiated upon them would be reflected, and but little absorbed. Just the contrary would take place were they dark. The caloric would nearly all be absorbed, and but little reflected. Similar to these are the phenomena of light. Bright substances reflect, and dark absorb, the rays from a luminous body. This, however, is hardly a correct method of expressing the fact intended. Philosophers believe that darkness of color is not the cause of the absorption of the luminous rays, but, on the contrary, that this absorption is the cause of the darkness. The fact in question then is this; some bodies are of such a chemical constitution, that they readily absorb light, and, as a consequence, little being reflected to the eye, they appear dark. Others, differently constituted, reflect nearly all the light that is thrown upon them, and, therefore, the lightness of their color bears proportion to such reflection. Let us apply these facts to the explanation of the design of the geographical distinctions of color, of which we are treating. Suppose that the arrangement were different. Suppose, for instance, that the portion of the earth near the equator presented, throughout the year, a white surface to the sun. The rays of heat from that body would nearly all, upon reaching such a surface, be reflected back into the atmosphere, and would heat that part of it immediately bordering the earth, and most exposed to this reflection, to such a degree as to make the climate insupportable. The consequence would be, that a large portion of the earth would be rendered uninhabitable. But, by the existing provision, the rays of caloric pass directly through the air, heating it comparatively little, and are, for the most part, absorbed by the earth. The principle is similar in regard to light. Had the constitution of the covering of the earth in the tropics been such as to reflect the luminous rays, which are far more numerous and brilliant there than at the poles, the overpowering glare of light would alone have been sufficient to render those regions uninhabitable by any known species of animals.