Let us confine ourselves to surface temperatures, which are all that concern us in discussing climate. It has been calculated by Poynting[5] that if a small sphere absorbed and re-radiated all the heat that fell upon it, its temperature at the distance of Mercury from the sun would average about 210 deg.C.; at the distance of Venus, 85 deg.; the earth 27 deg.; Mars -30 deg.; Neptune -219 deg.. A planet much nearer the sun than is Mercury might be heated to a temperature of a thousand, or even several thousand, degrees, while one beyond Neptune would remain almost at absolute zero. It is well within the range of possibility that the temperature of a planet's surface should be anywhere from near -273 deg.C. up to perhaps 5000 deg.C. or more, although the probability of low temperature is much greater than of high. Thus throughout the whole vast range of possibilities extending to perhaps 10,000 deg., the earth claims only 60 deg. at most, or less than 1 per cent. This may be remarkable, but what is far more remarkable is that the earth's range of 60 deg. includes what seem to be the two most critical of all possible temperatures, namely, the freezing point of water, 0 deg.C., and the temperature where water can dissolve an amount of carbon dioxide equal to its own volume. The most remarkable fact of all is that the earth has preserved its temperature within these narrow limits for a hundred million years, or perchance a thousand million. To appreciate the extraordinary significance of this last fact, it is necessary to realize how extremely critical are the temperatures from about 0 deg. to 40 deg.C., and how difficult it is to find any good reason for a relatively uniform temperature through hundreds of millions of years. Since the dawn of geological time the earth's temperature has apparently always included the range from about the freezing point of water up to about the point where protoplasm begins to disintegrate. Henderson, in _The Fitness of the Environment_, rightly says that water is "the most familiar and the most important of all things." In many respects water and carbon dioxide form the most unique pair of substances in the whole realm of chemistry. Water has a greater tendency than any other known substance to remain within certain narrowly defined limits of temperature. Not only does it have a high specific heat, so that much heat is needed to raise its temperature, but on freezing it gives up more heat than any substance except ammonia, while none of the common liquids approach it in the amount of additional heat required for conversion into vapor after the temperature of vaporization has been reached. Again, water substance, as the physicists call all forms of H_{2}O, is unique in that it not only contracts on melting, but continues to contract until a temperature several degrees above its melting point is reached. That fact has a vast importance in helping to keep the earth's surface at a uniform temperature.