It is possible that on those parts of Mercury where the sunlight and darkness are unstable there may be something resembling a tolerable temperature. They are something more than a thousand miles in breadth, and perhaps near the center of them the sun may give heat sufficient to enliven and yet not burn. More than likely, they are alternately scorched and frozen. For it takes more than the mere presence of sunlight to make a climate tolerable. Atmosphere is what is necessary, and we have seen that Mercury has probably lost practically all his atmosphere long, long ago. An atmosphere absorbs much of the radiant energy that comes from the sun before it reaches the more solid parts of a planet, and it also acts as a blanket in preventing the too rapid escape of such heat as the planet may have acquired. Thus it has the doubly beneficent office of tempering the rays that would otherwise be scorching and of hindering a radiation that would leave the planet stiffened and frozen. Stiffened and frozen is what the dark side of Mercury undoubtedly is. The sun has never shone upon it since Mercury became a solid body. All the inherent heat it had has long since passed off into space, and its temperature must be somewhere near the absolute zero. The absolute zero is the point in temperature where all known substances become solid. It is more than 450° below the Fahrenheit zero, or more than 350° lower than any temperature recorded in our arctic regions--a degree of cold unthinkable to any but the scientist. On the other side of Mercury the heat is beyond anything we have any notion of. With an equal atmosphere it would receive from the sun six thousand times as much light and heat as Neptune on an equal space, and, on an average, seven times as much as the earth. At Mercury’s distance from the sun his hot side would be more than 300° above zero, if there were absolutely no atmospheric protection. Even though tempered by a thin atmosphere, as it may be, the heat on this side is still probably enough to boil away any water that might be there and to change some other substances from what we regard as their normal state. Stability, at least, is a quality of the hot and the cold side of Mercury. Scorched and seared and desolate of life, as we know it, the one side lies under a blazing, dazzling sun. Cold and hard and bleak, and no less desolate, the other side turns its face toward the darkness of space. Thus they will remain until the end of time. And let us hope that, when the final catastrophe occurs and a new nebula is formed, the matter composing Mercury may find a place in a larger mass, and in its new incarnation have a fuller and larger life.