5 i6 THE POPULAR SCIENCE MONTHLY. 



the latter class having been evolved by degrees from the original 

 simple gases of the nebulae. This is a problem well worthy the study 

 of chemists. 



The comets differ from nebulae 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 nebulae. 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 

 nebulae 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 sur- 

 face. Suns like this are to be enumerated by the thousand in the 

 heavens, all of them doubtless the centres of other star-systems, im- 

 parting 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, be- 

 cause 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. 



Transition prom the Spiral to the Annular Form. 



i 

 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 



