154 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



have become more rapid, and have tended to make it discoid. 

 From time to time masses became separated at the circum- 

 ference of this disc under the influence of the increasing 

 centrifugal force. These masses again assumed the form of 

 rotating nebular balls, and either simply condensed as 

 planets, or during condensation also gave off in turn peripheral 

 masses which became satellites or remained, in the case of 

 Saturn, as a connected ring. In another case, the mass which 

 separated at the periphery of the main nebula broke up into a 

 number of nebular fragments, and gave origin to the swarm of 

 small planets between Mars and Jupiter. It has been deter- 

 mined more recently that this process of condensation of 

 loosely composed bodies is still continuing, although in less 

 degree." 



A new field of research was opened for astronomy in 1859, 

 when the spectroscope was discovered by Kirchhoff and 

 Bunsen. It was then rendered possible to learn something 

 definite about the materials composing the stars and the sun. 

 By the use of the spectroscope it has been ascertained that all 

 matter has essentially the same constitution throughout the 

 universe, the same substances taking part in the composition 

 of the earth, the sun, the fixed stars, and the planetary 

 nebula. 



The mechanical theory of heat, together with the principle 

 of conservation of energy founded by Robert Mayer and by 

 Helmholtz, afforded an exact explanation of the high tempera- 

 ture of self-luminous cosmical bodies, since an enormous supply 

 of heat must be absorbed during the processes of condensation 

 of gases and differentiation of atoms. According to Helm- 

 holtz, the supply of heat which the sun has accumulated during 

 its condensation is sufficient, if calculated on the basis of its 

 present expenditure of heat, to have extended over an interval 

 of time in the past equivalent to twenty-two million years. 

 And as the sun is still in process of condensation, it may yet 

 continue for many millions of years to radiate and to impart 

 its animating sunshine to the planets. 



Thus, in respect of the unity of matter and the temperature 

 of solar and planetary bodies, the nebular theory of Kant and 

 Laplace was confirmed by spectroscopical research and by the 

 mechanical theory of heat. But it encountered serious diffi- 

 culty when astronomers discovered that the rotation of the 

 satellites of Uranus and Neptune takes place from east to west, 



