COSMICAL GEOLOGY. 159 



nebulae were examined in 1869 by Huggins and Miller, and 

 the results indicated the presence of vapour, of water, and in 

 addition an element which, unknown in the earth, has been 

 determined in the sun's spectrum and termed "helium." 



Next to the red stars may be grouped the so-called new 

 and variable stars, sometimes brilliantly luminous, sometimes 

 growing rapidly obscure or quite vanishing from observation. 

 These probably represent bodies in a far-advanced stage of 

 cooling, but which, owing to collision with other bodies in the 

 universe, or to internal changes, temporarily ignite, emit 

 eruptions of glowing gases, and perhaps in some cases 

 also eruptions of molten rock-masses. 



By mathematical calculations astronomers have determined 

 that in addition to luminous stars, there must be completely 

 cooled dark bodies in the vault of heaven. Thus the sidereal 

 world exhibits all phases from the nebulous, incandescent, 

 gaseous, and vaporous states to the cooled and solid condition. 



The further history of a cooled celestial body surrounded 

 by a firm crust is displayed in the various conditions of the 

 planets and satellites of our solar system, and these have 

 therefore a closer interest for geology. The planets move 

 round the sun in slightly elliptical paths at definite distances 

 from it. Of the six planets that were known in early astrology, 

 Mercury is nearest the sun in position, and has itself a diameter 

 of 648 miles; Venus (diam. 1,613 miles) follows Mercury, 

 then the Earth (diam. 1,719 miles), then Mars (diam. 909 

 miles), Jupiter (diam. 19,000 miles), and Saturn (diam. 

 16,675 miles). Herschel in 1780 discovered on the 

 farther side of Saturn the planet Uranus with a diameter 

 of about 8000 miles, and Leverrier in 1846 discovered, by 

 mathematical calculation, the outermost planet, Neptune, with 

 four and a half times the diameter of the earth. 



The paths of Mars and Jupiter are separated by a much 

 greater distance from one another than the- paths of the inner 

 planets. Piazzi in 1801 discovered the small planet Ceres in 

 this gap, and later there have been discovered more than 400 

 small planetoids or asteroids, a number which is continually 

 being added to by new researches. The Earth has one 

 satellite, Mars two, Jupiter five, Uranus four, Saturn eight, 

 Neptune one. Saturn is also further distinguished by the 

 possession of a broad ring freely suspended over the equator 

 and separated into three parts. 



