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we need not at present attempt to pass back beyond the earliest stage of which we have any clear information.

Passing from the irregular nebulæ, in which we see chaotic masses of gaseous matter occupying millions of millions of cubic miles and scattered as wildly through space as clouds are scattered in a storm-swept air, we come to various orders of nebulæ in which we seem to find clear evidence of a process of evolution. We see first the traces of a central aggregation. This aggregation becomes more and more clearly defined, until there is no possibility of mistaking its nature as a centre having power (by virtue of the quantity of matter contained in it) to influence the motions of the matter belonging to the rest of the nebula. Then, still passing be it remembered from nebula to nebula, and only inferring, not actually witnessing, the changes described,-we see a subordinate aggregation, wherein, after a while, the greater portion of the mass of the nebula outside the central aggregation becomes gathered, even as Jupiter contains the greater portion of the mass of the solar system outside the central sun. Next we see a second subordinate aggregation, inferior to the first, but comprising, if we judge from its appearance, by far the greater portion of what remained after the first aggregation had been formed,-even as Saturn's mass far exceeds the combined mass of all the planets less than himself, and so comprises far the greater portion of the solar system after account has been taken

*

* The mass of Jupiter exceeds, in the proportion of five to two, the

of Jupiter and the sun.* And we may infer that the other parts of nebulæ contain smaller aggregations not perceptible to us, out of which the smaller planets of the developing system are hereafter to be formed.

Side views of some of these nebulæ indicate a flatness of figure agreeing well with the general tendency of the members of the solar system towards the medial plane of that system. For the solar system may be described as flat, and if the nebulæ I have been dealing with (the spiral nebulæ with aggregations) were globular we could not recognise in them the true analogues of our solar system in the earlier stages of its history. But the telescope reveals nebulæ manifestly corresponding in appearance to the great whirlpool nebula of Lord Rosse, as it would appear if it is a somewhat flattened spiral and could be viewed nearly edgewise.

And here I may pause to note that, although, in thus inferring progressive changes where in reality we have but various forms of nebulæ, I have been adopting an assumption and one which no one can hope either to verify or to disprove, yet it must be remembered that these nebulæ by their very figure indicate that they are not at rest. If they consist of matter possessing the attribute of gravitation,—and it would be infinitely more daring to assert that they do not than that they do,-then they must be undergoing processes of change. Nor can we conceive that discrete gaseous masses in whorls spirally

The mass of Saturn exceeds, in the proportion of nearly three to one, the combined mass of all the planets smaller than himself.

we need not at present attempt to pass back beyond the earliest stage of which we have any clear information.

Passing from the irregular nebulæ, in which we see chaotic masses of gaseous matter occupying millions of millions of cubic miles and scattered as wildly through space as clouds are scattered in a storm-swept air, we come to various orders of nebulæ in which we seem to find clear evidence of a process of evolution. We see first the traces of a central aggregation. This aggregation becomes more and more clearly defined, until there is no possibility of mistaking its nature as a centre having power (by virtue of the quantity of matter contained in it) to influence the motions of the matter belonging to the rest of the nebula. Then, still passing be it remembered from nebula to nebula, and only inferring, not actually witnessing, the changes described,-we see a subordinate aggregation, wherein, after a while, the greater portion of the mass of the nebula outside the central aggregation becomes gathered, even as Jupiter contains the greater portion of the mass of the solar system outside the central sun. Next we see a second subordinate aggregation, inferior to the first, but comprising, if we judge from its appearance, by far the greater portion of what remained after the first aggregation had been formed,-even as Saturn's mass far exceeds the combined mass of all the planets less than himself, and so comprises far the greater portion of the solar system after account has been taken

*

* The mass of Jupiter exceeds, in the proportion of five to two, the