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THE POPULAR SCIENCE MONTHLY.

the latter class having been evolved by degrees from the original simple gases of the nebulæ. This is a problem well worthy the study of chemists.

The comets differ from nebulæ by possessing a bright, star-like nucleus, apparently more solid than the surrounding coma or brush trailing behind. The spectroscope indicates that the entire material of comets is similar to the gaseous nebulæ. Possibly their nuclei are centres of attraction around which the heavier atoms are gradually falling, "granulating into star-dust," in the process of transition from nebulæ to suns.

Foremost among the worlds comparable with our planet, when in the condition of igneous fluidity, is the centre of our own solar system. Though fourteen hundred million times larger than the earth, the sun possesses only one-fourth the density of our world, being a trifle heavier than water. The hourly radiation of heat from each square foot of his surface is equal to the combustion of 130,000 pounds of bituminous coal. This is abundantly adequate to heat and illuminate all bodies in space within hundreds of millions of miles from his surface. Suns like this are to be enumerated by the thousand in the heavens, all of them doubtless the centres of other star-systems, imparting light and heat to numberless planets.

It is less easy to determine the character of worlds in the former condition of ours, just incrusted after igneous fluidity, no longer a sun, but shining by borrowed light reflected from some greater sphere, because they are wrapped in an opaque envelope. The moon, whose proximity enables us to inspect her hills, craters, and valleys, appears to have been thoroughly cooled from fusion. She is solid to the core, and has approximated to that final period of barren desolation not yet attained by the earth. Most of the outer planets, Jupiter, Saturn,

Fig. 3.
PSM V04 D534 Spiral to annular form transition.jpg
Transition from the Spiral to the Annular Form.

Uranus, and Neptune, have a small specific gravity; but we cannot tell whether they have advanced beyond us in the cycle of progress, or whether they are yet immature. The adjacent planets, Venus and