ORIGIN OF THE EARTH AND ITS CRUST 115 



The earth is to man, one of the two most important members 

 of a group of celestial bodies held in relation to each other by 

 gravitation, which we call the solar system. The center of 

 this system is the sun, about which revolve the planets with 

 their satellites and the planetoids, and without which as a source 

 of light and heat, no life could exist on earth. 



To explain the origin of the solar system the Nebular hypothesis a 

 was suggested by Swedenborg and Kant and elaborated by La- 

 place. Although not completely proven it is highly plausible, 

 and answers most of the conditions. According to this hypothe- 

 sis our solar system originated as a vast nebula, similar to 

 nebulae which now exist, in the form of an immense volume of 

 incandescent gas rotating in space from west to east, of which 

 the limits extended beyond those of the present solar system 

 which is about 5,500 millions of miles in diameter. 



As this mass slowly parted with its heat and contracted in 

 obedience to physical laws, its velocity of rotation would increase 

 and in the peripheral or outer portion the centrifugal force would 

 overcome the attraction toward the center, causing it to separate 

 from the central portion in the form of a ring. This ring through 

 unequal condensation would subsequently be broken, its frag- 

 ments uniting by gravitation into a body revolving about the 

 nucleus and ultimately forming a planet or in one instance a 

 zone of small planets, that of the planetoids or asteroids. This 

 process is supposed to have continued until the various mem- 

 bers of the system were set free; the remnant of the much dimin- 

 ished but still intensely heated nucleus remaining as our sun 

 which now has a diameter of 860,000 miles. The primary rings 

 after condensing into planets are believed to have formed second- 

 ary rings which subsequently broke and became satellites, except 

 in the case of Saturn which still retains two rings. 



Inasmuch as some of the planets near the sun are denser than 

 those more distant, it has been suggested that in the rotation 

 of the primal nebula its components arranged themselves in lay- 



aSee Young, General Astronomy, p. 515-25. 



