PLANETARY EVOLUTION 



are sure evidence of the gaseous condition of 

 the luminous body. Since then several other 

 nebulas have been proved to be gaseous ; so that 

 the question may now be regarded as settled for- 

 ever, and as settled in favour of the nebular hy- 

 pothesis. Henceforth, to the evidence found in 

 the structure of our planetary system, there may 

 be added the weighty argument that masses of 

 matter still exist in space, in the very condition 

 in which our system must have originally ex- 

 isted. 



If the nebular hypothesis was ever to be sub- 

 jected to a hazardous trial, one would suppose 

 that the discovery of spectrum analysis must 

 have furnished the occasion. Here is a discov- 

 ery which has suddenly enlarged our knowledge 

 of the stellar universe in a manner utterly be- 

 yond the power of the greatest and subtlest 

 mind to have predicted twenty years ago, — 

 a discovery which not only reveals to us the 

 actual motions of the stars, but even penetrates 

 into their molecular structure, and discloses the 

 chemical elements of which their surfaces are 

 composed as well as the physical state of ag- 

 gregation of those surfaces. Now if ever, one 

 might think, is the time to find out whether 

 our nebular hypothesis, devised in an era of 

 comparatively scanty astronomical knowledge, 

 is a sound hypothesis or not. If it survives this 

 immense, unprecedented extension of our know- 

 295 



