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 disturb it.
In beginning upon such an inquiry, we must start with a consideration of the nebulæ themselves. And, at the outset, their varieties of form, and the visible changes which they undergo, offer strong suggestions of processes of evolution actually going on. The spiral nebulæ 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 nebulæ 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 systems 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 nebulæ may be divided into three classes: First, those which give spectra consisting only of bright lines. Secondly, nebulæ whose spectra are continuous. And, in the third place, the nebulæ described by Lieutenant Herschel, which are apparently intermediate between the other two classes, and furnish spectra of bright lines upon a continuous background.
The nebulæ 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 circumstances of temperature and pressure; and the spectrum of a nebula indicates that the gases of which it is composed are in a highly-rarefied condition, and at a temperature considerably lower than that of our sun! Of this we are tolerably sure, though perhaps not absolutely certain.
The nebulæ whose spectra are continuous speak to us with less certainty. Lord Oxmantown has shown that the resolved nebulæ—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 inferred that all the nebulæ of this class probably belong to the resolvable order; but here is where a slight doubt may arise: gases, under great pressure and at a high temperature, give continuous spectra; possibly, then, some of these nebulæ may consist of gases under just such conditions. Here is a problem yet to be solved. The third class of nebulæ may, perhaps, strengthen this latter view. Their spectra are intermediate between those of the other classes. It