406 A Short History of Astronomy [CH. xm. 



in their spectra ( 312) form another link connecting 

 nebulae with stars. 



A good many converging lines of evidence thus point 

 to a greater variety in the arrangement, size, and structure 

 of the bodies with which the telescope makes us acquainted 

 than seemed probable when sidereal astronomy was first 

 seriously studied ; they also indicate the probability that 

 these bodies should be regarded as belonging to a 

 single system, even if it be of almost inconceivable 

 complexity, rather than to a number of perfectly distinct 

 systems of a simpler type. 



318. Laplace's nebular hypothesis (chapter XL, 250) 

 was published a little more than a century ago (1796), and 

 has been greatly affected by progress in various depart- 

 ments of astronomical knowledge. Subsequent discoveries 

 of planets and satellites ( 294, 295) have marred to some 

 extent the uniformity and symmetry of the motions of the 

 solar system on which Laplace laid so much stress ; but it 

 is not impossible to give reasonable explanations of the 

 backward motions of the satellites of the two most distant 

 planets, and of the large eccentricity and inclination of the 

 paths of some of the minor planets, while apart from these 

 exceptions the number of bodies the motions of which 

 have the characteristics which Laplace pointed out has 

 been considerably increased. The case for some sort of 

 common origin of the bodies of the solar system has per- 

 haps in this way gained as much as it has lost. Again, the 

 telescopic evidence which Herschel adduced (chapter xn., 

 261) in favour of the existence of certain processes of 

 condensation in nebulae has been strengthened by later 

 evidence of a similar character, and by the various pieces 

 of evidence already referred to which connect nebulae with 

 single stars and with clusters. The differences in the 

 spectra of stars also receive their most satisfactory explana- 

 tion as representing different stages of condensation of 

 bodies of the same general character. 



319. An entirely new contribution to the problem has 

 resulted from certain discoveries as to the nature of heat, 

 culminating in the recognition (about 1840-50) of heat as 

 only one form of what physicists now call energy, which 

 manifests itself also in the motion of bodies, in the 



