322 THE POPULAR SCIENCE MONTHLY. 



hope to find existing to-day, among the heavenly bodies, examples of 

 all the stages of evolution through which matter must pass in forming 

 solid globes from shapeless clouds of incandescent vapor. The task 

 will be a difficult one, but not hopeless. We have much material to 

 begin upon, and can safely look to the spectroscope to furnish us with 

 an abundance in the future. If the work can be done, the nebular 

 hypothesis will become so well grounded that we are scarcely able 

 to conceive of any possible arguments which could afterward dis- 

 turb it. 



In beginning upon such an inquiry, we must start with a considera- 

 tion of the nebulae themselves. And, at the outset, their varieties of 

 form, and the visible changes which they undergo, offer strong sugges- 

 tions of processes of evolution actually going on. The spiral nebulae 

 hint of rotary motion, and some annular forms speak to us of rings of 

 vapor from which planets are yet to grow. In the double nebulas we 

 see future pairs of suns, companion stars ; and in every true nebula are 

 signs of condensation in the brighter portions. The nuclei which are 

 so common may be the germs of central luminaries, around which sys- 

 tems like our own are yet to revolve. But all these observations are 

 due to the telescope. We have to consider what the spectroscope has 

 done. 



Now, as regards spectroscopic work, the nebulae may be divided 

 into three classes : First, those which give spectra consisting only of 

 bright lines. Secondly, nebulae whose spectra are continuous. And, 

 in the third place, the nebulae described by Lieutenant Herschel, 

 which are apparently intermediate between the other two classes, and 

 furnish spectra of bright lines upon a continuous background. 



The nebulae of the first class I have partly described. They consist 

 mainly, if not wholly, of two common gases, nitrogen and hydrogen. 

 But gases give somewhat different spectra under different circum- 

 stances of temperature and pressure ; and the spectrum of a nebula in- 

 dicates that the gases of which it is composed are in a highly-rare- 

 fied condition, and at a temperature considerably lower than that of 

 our sun ! Of this we are tolerably sure, though perhaps not abso- 

 lutely certain. 



The nebulae whose spectra are continuous speak to us with less cer- 

 tainty. Lord Oxmantown has shown that the resolved nebulae those 

 which are known to be mere star-clusters give this kind of spectrum, 

 as do also most of those which appear to be resolvable. Accordingly, 

 it is reasonably infei-red that all the nebulae of this class probably be- 

 long to the resolvable order ; but here is where a slight doubt may 

 arise : gases, under great pressure and at a high temperature, give con- 

 tinuous spectra ; possibly, then, some of these nebulae may consist of 

 gases under just such conditions. Here is a problem yet to be solved. 

 The third class of nebulae may, perhaps, strengthen this latter view. 

 Their spectra are intermediate between those of the other classes. It 



