In his Outlines of Astronomy (pp. 630-1), Sir John Herschel refersto speculations respecting the rotation of our Sidereal System in the planeof the galactic circle. Dismissing the hypothesis of Mädler that thecentre of rotation is in the Pleiades, he thinks that no opinion can reasonablybe formed whether rotation exists or not, until after some thirty or fortyyears of observations of a special class. In any case, however, the irregularitiesof the Milky Way necessitate the conclusion that there is going on, and mustcontinue to go on, a general change of structure. The greater massivenessof it in the northern than in the southern hemisphere, the cleft form, thebreach of continuity, the branchings, the narrow connecting necks, and theparts that are almost or quite islanded, exclude the idea of equilibrium,whether the system as a whole be stationary or whether it be rotating. In§150, when referring to the fate of nebulous rings, I cited the optionof Sir John Herschel to the effect that a nebulous ring would not break atone place and collapse, but would break at many places and form separatemasses. I joined with it the opinion of Sir G. B. Airy, to whom I put thequestion whether these would remain separate, and who agreed that the massesthus formed, parting more widely at some one place, would eventually collapseinto a single mass. Parallel conclusions respecting changes in the MilkyWay seem legitimate, or rather, indeed, seem necessitated. Separation ofit into parts -- minor Sidereal Systems -- is a result to which its presentaspect points. That such minor sidereal systems could remain permanentlyindependent is not to be supposed. Mutual attraction would cause in somecases the formation of binary sidereal systems, and in other cases coalescence,according to the directions and amounts of their respective proper motions.
The implication is that there may be repeated, on vaster scales, changeslike those described as occurring in star-clusters: local concentrationstaking place within these minor sidereal systems, with resulting evolutionsand dissolutions, at the same time that the minor sidereal systems themselves,progressively uniting, become more condensed, and consequently the scenesof more active changes of like kinds. If, giving imagination the rein, wesuppose this process carried to its limit, and ultimately to present on animmensely larger scale the kind of change which the nubeculae exhibit, therearises the thought of a progressing destruction of the molar motions possessedby the concentrating stars, and a simultaneous diffusion of their substances,which, as the process comes to a close, spreads the matter of the SiderealSystem in its nebulous form throughout the whole of that space which it originallyfilled -- a diffusion reversing the preceding concentration -- a dissolutionthat prepares the way for a new evolution. Reduced to its abstract form,the argument is that the quantity of motion implied by dispersion must beas great as the quantity of motion implied by aggregation, or rather mustbe the same motion, taking now the molar form and now the molecular form;and if we allow ourselves to conceive this as an ultimate result there arisesthe conception not only of local evolutions and dissolutions throughout ourSidereal System but of general evolutions and dissolutions alternating indefinitely.
But we cannot draw such a conclusion without tacitly assuming somethingbeyond the limits of possible knowledge, namely, that the energy containedin our Sidereal System remains undiminished. Continuance of such alterationswithout end presupposes that the quantity of molecular motion radiated byeach star in the course of its formation from diffused matter, shall eithernot escape from our Sidereal System or shall be compensated by an equal quantityof molecular motion radiated into it from other parts of space. If the etherwhich fills the interspaces of our Sidereal System has a boundary somewherebeyond the outermost stars, it is inferable that motion is not lost by radiationbeyond that boundary; and if so the original degree of diffusion may be resumed.
Or if, supposing that the ether is unbounded, the temperature of space isthe same within and without our Sidereal System, then it is inferable thatthe quantity of motion contained within our Sidereal System remaining undiminished,its alternate concentrations and diffusions may continue undiminished. Butwe shall never be able to say whether either condition is fulfilled.
We may indeed dismiss such questions as passing the bounds of rationalspeculation. They have here been touched upon for the purpose of showingthat it is not inferable from the general progress towards equilibrium thata state of universal quiescence or death will be reached; but that if a processof reasoning ends in that conclusion, a further process of reasoning pointsto renewals of activity and life.
Here, however, it is needless for the adequate presentation of the generaldoctrine, that Evolution and Dissolution should be traced in either directionto their ends. In §93 it was said that no actual philosophy can fillout the scheme of an ideal philosophy -- cannot even of a small aggregatetrace the entire history from its appearance to its disappearance, and mustbe immeasurably far from doing this with the all-comprehensive aggregate.
But unable though we must ever remain to give a complete account of thetransformation of things, even in any of its minor parts, and still morein its totality we are able to recognize throughout it the same general law;and may reasonably infer that it holds in those parts of the transformationwhich are beyond the reach of our intelligence as it does in those partswhich are within its reach.
