at the time of any formation the geologists can reach would not noticeably approach the enormous difference that separates the condition of our earth from that of either Jupiter or Saturn. It is possible that they now represent stages which our earth passed through in remote times, and they may be undergoing changes that are approximating
Fig. 2.—Spots on Jupiter observed and drawn by Mr. E. L. Trouvelot.—Observation of December 23, 1878, with the shadow of the third satellite.
them to our present condition. It is, however, probable that, while there are analogies and resemblances in the life-histories of all the heavenly bodies, there are also individual peculiarities and diversities not less important or less striking.
Jupiter's diameter is about eleven times that of our earth, and his mean density is about a quarter that of the earth, or about a third more than water. Now, a bulky body may be composed of heavy materials, and still, as a whole, be light, like an iron ship or a lump of pumice-stone, that will float in water. The pumice-lump is light on account of its vesicular formation, so that the mass consists of heavy feldspathic material and the air it contains. Extract the air, and the pumice loses its floating power, though still far from heavy in proportion to its bulk. Most of the earth's crust is formed of solids much heavier than water. Granites are more than two and a half times heavier than water, slaty rocks much about the same, and so are ordinary limestones, the variations of all being from about 2∙5 to 2·9. The ironstone group contains denser minerals; red hematite has a specific gravity of 4·5; magnetic ironstone, 4·5 to 5·2, etc.; and many other ores are heavy.
