In the next chapter we proceed to take up the planets for study as individuals, beginning with Mercury, the one nearest the sun.
MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS
Mercury, the first of the other worlds that we are going to consider, fascinates by its grotesqueness, like a piece of Chinese ivory carving, so small is it for its kind and so finished in its eccentric details. In a little while we shall see how singular Mercury is in many of the particulars of planetary existence, but first of all let us endeavor to obtain a clear idea of the actual size and mass of this strange little planet. Compared with the earth it is so diminutive that it looks as if it had been cut out on the pattern of a satellite rather than that of an independent planet. Its diameter, 3,000 miles, only exceeds the moon's by less than one half, while both Jupiter and Saturn, among their remarkable collections of moons, have each at least one that is considerably larger than the planet Mercury. But, insignificant though it be in size, it holds the place of honor, nearest to the sun.
It was formerly thought that Mercury possessed a mass greatly in excess of that which its size would seem to imply, and some estimates, based upon the apparent effect of its attraction on comets, made it equal in mean density to lead, or even to the metal mercury. This led to curious speculations concerning its probable metallic composition, and the possible existence of vast quantities of such heavy elements as gold in the frame of the planet. But more recent, and probably more correct, computations place Mercury third in the order of density among the members of the solar system, the earth ranking as first and Venus as second. Mercury's density is now believed to be less than the earth's in the ratio of 85 to 100. Accepting this estimate, we find that the force of gravity upon the surface of Mercury is only one third as great as upon the surface of the earth--i.e., a body weighing 300 pounds on the earth would weigh only 100 pounds on Mercury.
This is an important matter, because not only the weight of bodies, but the density of the atmosphere and even the nature of its gaseous constituents, are affected by the force of gravity, and if we could journey from world to world, in our bodily form, it would make a great difference to us to find gravity considerably greater or less upon other planets than it is upon our own. This alone might suffice to render some of the planets impossible places of abode for us, unless a decided change were effected in our present physical organization.