THE NEBULAR HYPOTHESIS.

[_First published in_ The Westminster Review _for July,_ 1858. _In

explanation of sundry passages, it seems needful to state that this

essay was written in defence of the Nebular Hypothesis at a time

when it had fallen into disrepute. Hence there are some opinions

spoken of as current which are no longer current._]

Inquiring into the pedigree of an idea is not a bad means of roughly

estimating its value. To have come of respectable ancestry, is _prima

facie_ evidence of worth in a belief as in a person; while to be

descended from a discreditable stock is, in the one case as in the

other, an unfavourable index. The analogy is not a mere fancy. Beliefs,

together with those who hold them, are modified little by little in

successive generations; and as the modifications which successive

generations of the holders undergo do not destroy the original type, but

only disguise and refine it, so the accompanying alterations of belief,

however much they purify, leave behind the essence of the original

belief.

Considered genealogically, the received theory respecting the creation

of the Solar System is unmistakably of low origin. You may clearly trace

it back to primitive mythologies. Its remotest ancestor is the doctrine

that the celestial bodies are personages who originally lived on the

Earth--a doctrine still held by some of the negroes Livingstone visited.

Science having divested the sun and planets of their divine

personalities, this old idea was succeeded by the idea which even Kepler

entertained, that the planets are guided in their courses by presiding

spirits: no longer themselves gods, they are still severally kept in

their orbits by gods. And when gravitation came to dispense with these

celestial steersmen, there was begotten a belief, less gross than its

parent, but partaking of the same essential nature, that the planets

were originally launched into their orbits by the Creator's hand.

Evidently, though much refined, the anthropomorphism of the current

hypothesis is inherited from the aboriginal anthropomorphism, which

described gods as a stronger order of men.

There is an antagonist hypothesis which does not propose to honour the

Unknown Power manifested in the Universe, by such titles as "The

Master-Builder," or "The Great Artificer;" but which regards this

Unknown Power as probably working after a method quite different from

that of human mechanics. And the genealogy of this hypothesis is as high

as that of the other is low. It is begotten by that ever-enlarging and

ever-strengthening belief in the presence of Law, which accumulated

experiences have gradually produced in the human mind. From generation

to generation Science has been proving uniformities of relation among

phenomena which were before thought either fortuitous or supernatural in

their origin--has been showing an established order and a constant

causation where ignorance had assumed irregularity and arbitrariness.

Each further discovery of Law has increased the presumption that Law is

everywhere conformed to. And hence, among other beliefs, has arisen the

belief that the Solar System originated, not by _manufacture_ but by

_evolution_. Besides its abstract parentage in those grand general

conceptions which Science has generated, this hypothesis has a concrete

parentage of the highest character. Based as it is on the law of

universal gravitation, it may claim for its remote progenitor the great

thinker who established that law. It was first suggested by one who

ranks high among philosophers. The man who collected evidence indicating

that stars result from the aggregation of diffused matter, was the most

diligent, careful, and original astronomical observer of modern times.

And the world has not seen a more learned mathematician than the man

who, setting out with this conception of diffused matter concentrating

towards its centre of gravity, pointed out the way in which there would

arise, in the course of its concentration, a balanced group of sun,

planets, and satellites, like that of which the Earth is a member.

Thus, even were there but little direct evidence assignable for the

Nebular Hypothesis, the probability of its truth would be strong. Its

own high derivation and the low derivation of the antagonist hypothesis,

would together form a weighty reason for accepting it--at any rate,

provisionally. But the direct evidence assignable for the Nebular

Hypothesis is by no means little. It is far greater in quantity, and

more varied in kind, than is commonly supposed. Much has been said here

and there on this or that class of evidences; but nowhere, so far as we

know, have all the evidences been fully stated. We propose here to do

something towards supplying the deficiency: believing that, joined with

the _a priori_ reasons given above, the array of _a posteriori_ reasons

will leave little doubt in the mind of any candid inquirer.

And first, let us address ourselves to those recent discoveries in

stellar astronomy which have been supposed to conflict with this

celebrated speculation.

* * * * *

When Sir William Herschel, directing his great reflector to various

nebulous spots, found them resolvable into clusters of stars, he

inferred, and for a time maintained, that all nebulous spots are

clusters of stars exceedingly remote from us. But after years of

conscientious investigation, he concluded that "there were nebulosities

which are not of a starry nature;" and on this conclusion was based his

hypothesis of a diffused luminous fluid which, by its eventual

aggregation, produced stars. A telescopic power much exceeding that used

by Herschel, has enabled Lord Rosse to resolve some of the nebulae

previously unresolved; and, returning to the conclusion which Herschel

first formed on similar grounds but afterwards rejected, many

astronomers have assumed that, under sufficiently high powers, every

nebula would be decomposed into stars--that the irresolvability is due

solely to distance. The hypothesis now commonly entertained is, that all

nebulae are galaxies more or less like in nature to that immediately

surrounding us; but that they are so inconceivably remote as to look,

through ordinary telescopes, like small faint spots. And not a few have

drawn the corollary, that by the discoveries of Lord Rosse the Nebular

Hypothesis has been disproved.

Now, even supposing that these inferences respecting the distances and

natures of the nebulae are valid, they leave the Nebular Hypothesis

substantially as it was. Admitting that each of these faint spots is a

sidereal system, so far removed that its countless stars give less light

than one small star of our own sidereal system; the admission is in no

way inconsistent with the belief that stars, and their attendant

planets, have been formed by the aggregation of nebulous matter. Though,

doubtless, if the existence of nebulous matter now in course of

concentration be disproved, one of the evidences of the Nebular

Hypothesis is destroyed, yet the remaining evidences remain. It is a

tenable position that though nebular condensation is now nowhere to be

seen in progress, yet it was once going on universally. And, indeed, it

might be argued that the still-continued existence of diffused nebulous

matter is scarcely to be expected; seeing that the causes which have

resulted in the aggregation of one mass, must have been acting on all

masses, and that hence the existence of masses not aggregated would be a

fact calling for explanation. Thus, granting the immediate conclusions

suggested by these recent disclosures of the six-feet reflector, the

corollary which many have drawn is inadmissible.

But these conclusions may be successfully contested. Receiving them

though we have been, for years past, as established truths, a critical

examination of the facts has convinced us that they are quite

unwarrantable. They involve so many manifest incongruities, that we have

been astonished to find men of science entertaining them, even as

probable. Let us consider these incongruities.

* * * * *

In the first place, mark what is inferable from the distribution of

nebulae.

"The spaces which precede or which follow simple nebulae," says

Arago, "and _a fortiori_, groups of nebulae, contain generally few

stars. Herschel found this rule to be invariable. Thus every time

that during a short interval no star approached in virtue of the

diurnal motion, to place itself in the field of his motionless

telescope, he was accustomed to say to the secretary who assisted

him,--'Prepare to write; nebulae are about to arrive.'"

How does this fact consist with the hypothesis that nebulae are remote

galaxies? If there were but one nebula, it would be a curious

coincidence were this one nebula so placed in the distant regions of

space, as to agree in direction with a starless spot in our own sidereal

system. If there were but two nebulae, and both were so placed, the

coincidence would be excessively strange. What, then, shall we say on

finding that there are thousands of nebulae so placed? Shall we believe

that in thousands of cases these far-removed galaxies happen to agree in

their visible positions with the thin places in our own galaxy? Such a

belief is impossible.

Still more manifest does the impossibility of it become when we consider

the general distribution of nebulae. Besides again showing itself in the

fact that "the poorest regions in stars are near the richest in nebulae,"

the law above specified applies to the heavens as a whole. In that zone

of celestial space where stars are excessively abundant, nebulae are

rare; while in the two opposite celestial spaces that are furthest

removed from this zone, nebulae are abundant. Scarcely any nebulae lie

near the galactic circle (or plane of the Milky Way); and the great

mass of them lie round the galactic poles. Can this also be mere

coincidence? When to the fact that the general mass of nebulae are

antithetical in position to the general mass of stars, we add the fact

that local regions of nebulae are regions where stars are scarce, and the

further fact that single nebulae are habitually found in comparatively

starless spots; does not the proof of a physical connexion become

overwhelming? Should it not require an infinity of evidence to show that

nebulae are not parts of our sidereal system? Let us see whether any such

infinity of evidence is assignable. Let us see whether there is even a

single alleged proof which will bear examination.

"As seen through colossal telescopes," says Humboldt, "the

contemplation of these nebulous masses leads us into regions from

whence a ray of light, according to an assumption not wholly

improbable, requires millions of years to reach our earth--to

distances for whose measurement the dimensions (the distance of

Sirius, or the calculated distances of the binary stars in Cygnus

and the Centaur) of our nearest stratum of fixed stars scarcely

suffice."

In this confused sentence there is implied a belief, that the distances

of the nebulae from our galaxy of stars as much transcend the distances

of our stars from one another, as these interstellar distances transcend

the dimensions of our planetary system. Just as the diameter of the

Earth's orbit, is a mere point when compared with the distance of our

Sun from Sirius; so is the distance of our Sun from Sirius, a mere point

when compared with the distance of our galaxy from those far-removed

galaxies constituting nebulae. Observe the consequences of this

assumption.

If one of these supposed galaxies is so remote that its distance dwarfs

our interstellar spaces into points, and therefore makes the dimensions

of our whole sidereal system relatively insignificant; does it not

inevitably follow that the telescopic power required to resolve this

remote galaxy into stars, must be incomparably greater than the

telescopic power required to resolve the whole of our own galaxy into

stars? Is it not certain that an instrument which can just exhibit with

clearness the most distant stars of our own cluster, must be utterly

unable to separate one of these remote clusters into stars? What, then,

are we to think when we find that the same instrument which decomposes

hosts of nebulae into stars, _fails_ to resolve completely our own Milky

Way? Take a homely comparison. Suppose a man who was surrounded by a

swarm of bees, extending, as they sometimes do, so high in the air as to

render some of the individual bees almost invisible, were to declare

that a certain spot on the horizon was a swarm of bees; and that he knew

it because he could see the bees as separate specks. Incredible as the

assertion would be, it would not exceed in incredibility this which we

are criticising. Reduce the dimensions to figures, and the absurdity

becomes still more palpable. In round numbers, the distance of Sirius

from the Earth is half a million times the distance of the Earth from

the Sun; and, according to the hypothesis, the distance of a nebula is

something like half a million times the distance of Sirius. Now, our own

"starry island, or nebula," as Humboldt calls it, "forms a lens-shaped,

flattened, and everywhere detached stratum, whose major axis is

estimated at seven or eight hundred, and its minor axis at a hundred and

fifty times the distance of Sirius from the Earth."[11] And since it is

concluded that the Solar System is near the centre of this aggregation,

it follows that our distance from the remotest parts of it is some four

hundred distances of Sirius. But the stars forming these remotest parts

are not individually visible, even through telescopes of the highest

power. How, then, can such telescopes make individually visible the

stars of a nebula which is half a million times the distance of Sirius?

The implication is, that a star rendered invisible by distance becomes

visible if taken twelve hundred times further off! Shall we accept this

implication? or shall we not rather conclude that the nebulae are _not_

remote galaxies? Shall we not infer that, be their nature what it may,

they must be at least as near to us as the extremities of our own

sidereal system?

Throughout the above argument, it is tacitly assumed that differences of

apparent magnitude among the stars, result mainly from differences of

distance. On this assumption the current doctrines respecting the nebulae

are founded; and this assumption is, for the nonce, admitted in each of

the foregoing criticisms. From the time, however, when it was first made

by Sir W. Herschel, this assumption has been purely gratuitous; and it

now proves to be inadmissible. But, awkwardly enough, its truth and its

untruth are alike fatal to the conclusions of those who argue after the

manner of Humboldt. Note the alternatives.

On the one hand, what follows from the untruth of the assumption? If

apparent largeness of stars is not due to comparative nearness, and

their successively smaller sizes to their greater and greater degrees of

remoteness, what becomes of the inferences respecting the dimensions of

our sidereal system and the distances of nebulae? If, as has lately been

shown, the almost invisible star 61 Cygni has a greater parallax than

[Greek: a] Cygni, though, according to an estimate based on Sir W.

Herschel's assumption, it should be about twelve times more distant--if,

as it turns out, there exist telescopic stars which are nearer to us

than Sirius; of what worth is the conclusion that the nebulae are very

remote, because their component luminous masses are made visible only by

high telescopic powers? Clearly, if the most brilliant star in the

heavens and a star that cannot be seen by the naked eye, prove to be

equidistant, relative distances cannot be in the least inferred from

relative visibilities. And if so, nebulae may be comparatively near,

though the starlets of which they are made up appear extremely minute.

On the other hand, what follows if the truth of the assumption be

granted? The arguments used to justify this assumption in the case of

the stars, equally justify it in the case of the nebulae. It cannot be

contended that, on the average, the _apparent_ sizes of the stars

indicate their distances, without its being admitted that, on the

average, the _apparent_ sizes of the nebulae indicate their

distances--that, generally speaking, the larger are the nearer and the

smaller are the more distant. Mark, now, the necessary inference

respecting their resolvability. The largest or nearest nebulae will be

most easily resolved into stars; the successively smaller will be

successively more difficult of resolution; and the irresolvable ones

will be the smallest ones. This, however, is exactly the reverse of the

fact. The largest nebulae are either wholly irresolvable, or but

partially resolvable under the highest telescopic powers; while large

numbers of quite small nebulae are easily resolved by far less powerful

telescopes. An instrument through which the great nebula in Andromeda,

two and a half degrees long and one degree broad, appears merely as a

diffused light, decomposes a nebula of fifteen minutes diameter into

twenty thousand starry points. At the same time that the individual

stars of a nebula eight minutes in diameter are so clearly seen as to

allow of their number being estimated, a nebula covering an area five

hundred times as great shows no stars at all! What possible explanation

of this can be given on the current hypothesis?

Yet a further difficulty remains--one which is, perhaps, still more

obviously fatal than the foregoing. This difficulty is presented by the

phenomena of the Magellanic clouds. Describing the larger of these, Sir

John Herschel says:--

"The Nubecula Major, like the Minor, consists partly of large

tracts and ill-defined patches of irresolvable nebula, and of

nebulosity in every stage of resolution, up to perfectly resolved

stars like the Milky Way, as also of regular and irregular nebulae

properly so called, of globular clusters in every stage of

resolvability, and of clustering groups sufficiently insulated and

condensed to come under the designation of 'clusters of

stars.'"--_Cape Observations_, p. 146.

In his _Outlines of Astronomy_, Sir John Herschel, after repeating this

description in other words, goes on to remark that--

"This combination of characters, rightly considered, is in a high

degree instructive, affording an insight into the probable

comparative distance of _stars_ and _nebulae_, and the real

brightness of individual stars as compared with one another. Taking

the apparent semidiameter of the nubecula major at three degrees,

and regarding its solid form as, roughly speaking, spherical, its

nearest and most remote parts differ in their distance from us by a

little more than a tenth part of our distance from its center. The

brightness of objects situated in its nearer portions, therefore,

cannot be _much_ exaggerated, nor that of its remoter _much_

enfeebled, by their difference of distance; yet within this

globular space, we have collected upwards of six hundred stars of

the seventh, eighth, ninth, and tenth magnitudes, nearly three

hundred nebulae, and globular and other clusters, _of all degrees of

resolvability_, and smaller scattered stars innumerable of every

inferior magnitude, from the tenth to such as by their multitude

and minuteness constitute irresolvable nebulosity, extending over

tracts of many square degrees. Were there but one such object, it

might be maintained without utter improbability that its apparent

sphericity is only an effect of foreshortening, and that in reality

a much greater proportional difference of distance between its

nearer and more remote parts exists. But such an adjustment,

improbable enough in one case, must be rejected as too much so for

fair argument in two. It must, therefore, be taken as a

demonstrated fact, that stars of the seventh or eighth magnitude

and irresolvable nebula may co-exist within limits of distance not

differing in proportion more than as nine to ten."--_Outlines of

Astronomy_ (10th Ed.), pp. 656-57.

This supplies yet another _reductio ad absurdum_ of the doctrine we are

combating. It gives us the choice of two incredibilities. If we are to

believe that one of these included nebulae is so remote that its hundred

thousand stars look like a milky spot, invisible to the naked eye; we

must also believe that there are single stars so enormous that though

removed to this same distance they remain visible. If we accept the

other alternative, and say that many nebulae are no further off than our

own stars of the eighth magnitude; then it is requisite to say that at

a distance not greater than that at which a single star is still

faintly visible to the naked eye, there may exist a group of a hundred

thousand stars which is invisible to the naked eye. Neither of these

suppositions can be entertained. What, then, is the conclusion that

remains? This only:--that the nebulae are not further from us than parts

of our own sidereal system, of which they must be considered members;

and that when they are resolvable into discrete masses, these masses

cannot be considered as stars in anything like the ordinary sense of

that word.[12]

And now, having seen the untenability of this idea, rashly espoused by

sundry astronomers, that the nebulae are extremely remote galaxies; let

us consider whether the various appearances they present are not

reconcilable with the Nebular Hypothesis.

* * * * *

Given a rare and widely-diffused mass of nebulous matter, having a

diameter, say, of one hundred times that of the Solar System,[13] what

are the successive changes that may be expected to take place in it?

Mutual gravitation will approximate its atoms or its molecules; but

their approximation will be opposed by that atomic motion the resultant

of which we recognize as repulsion, and the overcoming of which implies

the evolution of heat. As fast as this heat partially escapes by

radiation, further approximation will take place, attended by further

evolution of heat, and so on continuously: the processes not occurring

separately as here described, but simultaneously, uninterruptedly, and

with increasing activity. When the nebulous mass has reached a

particular stage of condensation--when its internally-situated atoms

have approached to within certain distances, have generated a certain

amount of heat, and are subject to a certain mutual pressure,

combinations may be anticipated. Whether the molecules produced be of

kinds such as we know, which is possible, or whether they be of kinds

simpler than any we know, which is more probable, matters not to the

argument. It suffices that molecular unions, either between atoms of the

same kind or between atoms of different kinds, will finally take place.

When they do take place, they will be accompanied by a sudden and great

disengagement of heat; and until this excess of heat has escaped, the

newly-formed molecules will remain uniformly diffused, or, as it were,

dissolved in the pre-existing nebulous medium.

But now what may be expected by and by to happen? When radiation has

adequately lowered the temperature, these molecules will precipitate;

and, having precipitated, they will not remain uniformly diffused, but

will aggregate into flocculi; just as water, precipitated from air,

collects into clouds. Concluding, thus, that a nebulous mass will, in

course of time, resolve itself into flocculi of precipitated denser

matter, floating in the rarer medium from which they were precipitated,

let us inquire what are the mechanical results to be inferred. Of

clustered bodies in empty space, each will move along a line which is

the resultant of the tractive forces exercised by all the rest, modified

from moment to moment by the acquired motion; and the aggregation of

such clustered bodies, if it eventually results at all, can result only

from collision, dissipation, and the formation of a resisting medium.

But with clustered bodies already immersed in a resisting medium, and

especially if such bodies are of small densities, such as those we are

considering, the process of concentration will begin forthwith: two

factors conspiring to produce it. The flocculi described, irregular in

their shapes and presenting, as they must in nearly all cases,

unsymmetrical faces to their lines of motion, will be deflected from

those courses which mutual gravitation, if uninterfered with, would

produce among them; and this will militate against that balancing of

movements which permanence of the cluster pre-supposes. If it be said,

as it may truly be said, that this is too trifling a cause of

derangement to produce much effect, then there comes the more important

cause with which it co-operates. The medium from which the flocculi have

been precipitated, and through which they are moving, must, by

gravitation, be rendered denser in its central parts than in its

peripheral parts. Hence the flocculi, none of them moving in straight

lines to the common centre of gravity, but having courses made to

diverge to one or other side of it (in small degrees by the cause just

assigned, and in much greater degrees by the tractive forces of other

flocculi) will, in moving towards the central region, meet with greater

resistances on their inner sides than on their outer sides; and will be

thus made to diverge outwardly from their courses more than they would

otherwise do. Hence a tendency which, apart from other tendencies, will

cause them severally to go on one or other side of the centre of

gravity, and, approaching it, to get motions more and more tangential.

Observe, however, that their respective motions will be deflected, not

towards one side of the common centre of gravity, but towards various

sides. How then can there result a movement common to them all? Very

simply. Each flocculus, in describing its course, must give motion to

the medium through which it is moving. But the probabilities are

infinity to one against all the respective motions thus impressed on

this medium, exactly balancing one another. And if they do not balance

one another the result must be rotation of the whole mass of the medium

in one direction. But preponderating momentum in one direction, having

caused rotation of the medium in that direction, the rotating medium

must in its turn gradually arrest such flocculi as are moving in

opposition, and impress its own motion upon them; and thus there will

ultimately be formed a rotating medium with suspended flocculi partaking

of its motion, while they move in converging spirals towards the common

centre of gravity.[14]

Before comparing these conclusions with facts, let us pursue the

reasoning a little further, and observe certain subordinate actions. The

respective flocculi must be drawn not towards their common centre of

gravity only, but also towards neighbouring flocculi. Hence the whole

assemblage of flocculi will break up into groups: each group

concentrating towards its local centre of gravity, and in so doing

acquiring a vortical movement like that subsequently acquired by the

whole nebula. According to circumstances, and chiefly according to the

size of the original nebulous mass, this process of local aggregation

will produce various results. If the whole nebula is but small, the

local groups of flocculi may be drawn into the common centre of gravity

before their constituent masses have coalesced with one another. In a

larger nebula, these local aggregations may have concentrated into

rotating spheroids of vapour, while yet they have made but little

approach towards the general focus of the system. In a still larger

nebula, where the local aggregations are both greater and more remote

from the common centre of gravity, they may have condensed into masses

of molten matter before the general distribution of them has greatly

altered. In short, as the conditions in each case determine, the

discrete masses produced may vary indefinitely in number, in size, in

density, in motion, in distribution.

And now let us return to the visible characters of nebulae, as observed

through modern telescopes. Take first the description of those nebulae

which, by the hypothesis, must be in an early stage of evolution.

Among the "_irregular nebulae_," says Sir John Herschel, "may be

comprehended all which, to _a want of complete and in most

instances even of partial resolvability_ by the power of the

20-feet reflector, unite such a deviation from the circular or

elliptic form, or such a want of symmetry (with that form) as

preclude their being placed in class 1, or that of Regular Nebulae.

This second class comprises many of the most remarkable and

interesting objects in the heavens, _as well as the most extensive

in respect of the area they occupy_."

And, referring to this same order of objects, M. Arago says:--"The forms

of very large diffuse nebulae do not appear to admit of definition; they

have no regular outline."

This coexistence of largeness, irregularity, and indefiniteness of

outline, with irresolvability, is extremely significant. The fact that

the largest nebulae are either irresolvable or very difficult to resolve,

might have been inferred _a priori_; seeing that irresolvability,

implying that the aggregation of precipitated matter has gone on to but

a small extent, will be found in nebulae of wide diffusion. Again, the

irregularity of these large, irresolvable nebulae, might also have been

expected; seeing that their outlines, compared by Arago with "the

fantastic figures which characterize clouds carried away and tossed

about by violent and often contrary winds," are similarly characteristic

of a mass not yet gathered together by the mutual attraction of its

parts. And once more, the fact that these large, irregular, irresolvable

nebulae have indefinite outlines--outlines that fade off insensibly into

surrounding darkness--is one of like meaning.

Speaking generally (and of course differences of distance negative

anything beyond average statements), the spiral nebulae are smaller than

the irregular nebulae, and more resolvable; at the same time that they

are not so small as the regular nebulae, and not so resolvable. This is

as, according to the hypothesis, it should be. The degree of

condensation causing spiral movement, is a degree of condensation also

implying masses of flocculi that are larger, and therefore more visible,

than those existing in an earlier stage. Moreover, the forms of these

spiral nebulae are quite in harmony with the explanation given. The

curves of luminous matter which they exhibit, are _not_ such as would be

described by discrete masses starting from a state of rest, and moving

through a resisting medium to a common centre of gravity; but they _are_

such as would be described by masses having their movements modified by

the rotation of the medium.

In the centre of a spiral nebula is seen a mass both more luminous and

more resolvable than the rest. Assume that, in process of time, all the

spiral streaks of luminous matter which converge to this centre are

drawn into it, as they must be; assume further, that the flocculi, or

other discrete portions constituting these luminous streaks, aggregate

into larger masses at the same time that they approach the central

group, and that the masses forming this central group also aggregate

into larger masses; and there will finally result a cluster of such

larger masses, which will be resolvable with comparative ease. And, as

the coalescence and concentration go on, the constituent masses will

gradually become fewer, larger, brighter, and more densely collected

around the common centre of gravity. See now how completely this

inference agrees with observation. "The circular form is that which most

commonly characterises resolvable nebulae," writes Arago. Resolvable

nebulae, says Sir John Herschel, "are almost universally round or oval."

Moreover, the centre of each group habitually displays a closer

clustering of the constituent masses than the outer parts; and it is

shown that, under the law of gravitation, which we now know extends to

the stars, this distribution is _not_ one of equilibrium, but implies

progressing concentration. While, just as we inferred that, according to

circumstances, the extent to which aggregation has been carried must

vary; so we find that, in fact, there are regular nebulae of all degrees

of resolvability, from those consisting of innumerable minute masses, to

those in which their numbers are smaller and the sizes greater, and to

those in which there are a few large bodies worthy to be called stars.

On the one hand, then, we see that the notion, of late years

uncritically received, that the nebulae are extremely remote galaxies of

stars like those which make up our own Milky Way, is totally

irreconcilable with the facts--involves us in sundry absurdities. On the

other hand, we see that the hypothesis of nebular condensation

harmonizes with the most recent results of stellar astronomy: nay

more--that it supplies us with an explanation of various appearances

which in its absence would be incomprehensible.

* * * * *

Descending now to the Solar System, let us consider first a class of

phenomena in some sort transitional--those offered by comets. In them,

or at least in those most numerous of them which lie far out of the

plane of the Solar System, and are not to be counted among its members,

we have, still existing, a kind of matter like that out of which,

according to the Nebular Hypothesis, the Solar System was evolved.

Hence, for the explanation of them, we must go back to the time when the

substances forming the sun and planets were yet unconcentrated.

When diffused matter, precipitated from a rarer medium, is aggregating,

there are certain to be here and there produced small flocculi, which

long remain detached; as do, for instance, minute shreds of cloud in a

summer sky. In a concentrating nebula these will, in the majority of

cases, eventually coalesce with the larger flocculi near to them. But it

is tolerably evident that some of those formed at the outermost parts of

the nebula, will _not_ coalesce with the larger internal masses, but

will slowly follow without overtaking them. The relatively greater

resistance of the medium necessitates this. As a single feather falling

to the ground will be rapidly left behind by a pillow-full of feathers;

so, in their progress to the common centre of gravity, will the

outermost shreds of vapour be left behind by the great masses of vapour

internally situated. But we are not dependent merely on reasoning for

this belief. Observation shows us that the less concentrated external

parts of nebulae, _are_ left behind by the more concentrated internal

parts. Examined through high powers, all nebulae, even when they have

assumed regular forms, are seen to be surrounded by luminous streaks, of

which the directions show that they are being drawn into the general

mass. Still higher powers bring into view still smaller, fainter, and

more widely-dispersed streaks. And it cannot be doubted that the minute

fragments which no telescopic aid makes visible, are yet more numerous

and widely dispersed. Thus far, then, inference and observation are at

one.

Granting that the great majority of these outlying portions of nebulous

matter will be drawn into the central mass long before it reaches a

definite form, the presumption is that some of the very small,

far-removed portions will not be so; but that before they arrive near

it, the central mass will have contracted into a comparatively moderate

bulk. What now will be the characters of these late-arriving portions?

In the first place, they will have either extremely eccentric orbits or

non-elliptic paths. Left behind at a time when they were moving towards

the centre of gravity in slightly-deflected lines, and therefore having

but very small angular velocities, they will approach the central mass

in greatly elongated curves; and rushing round it, will go off again

into space. That is, they will behave just as we see the majority of

comets do; the orbits of which are either so eccentric as to be

indistinguishable from parabolas, or else are not orbits at all, but are

paths which are distinctly either parabolic or hyperbolic.

In the second place, they will come from all parts of the heavens. Our

supposition implies that they were left behind at a time when the

nebulous mass was of irregular shape, and had not acquired a definite

rotation; and as the separation of them would not be from any one

surface of the nebulous mass more than another, the conclusion must be

that they will come to the central body from various directions in

space. This, too, is exactly what happens. Unlike planets, whose orbits

approximate to one plane, comets have orbits that show no relation to

one another; but cut the plane of the ecliptic at all angles, and have

axes inclined to it at all angles.

In the third place, these remotest flocculi of nebulous matter will, at

the outset, be deflected from their direct courses to the common centre

of gravity, not all on one side, but each on such side as its form, or

its original proper motion, determines. And being left behind before the

rotation of the nebula is set up, they will severally retain their

different individual motions. Hence, following the concentrated mass,

they will eventually go round it on all sides; and as often from right

to left as from left to right. Here again the inference perfectly

corresponds with the facts. While all the planets go round the sun from

west to east, comets as often go round the sun from east to west as from

west to east. Of 262 comets recorded since 1680, 130 are direct, and 132

are retrograde. This equality is what the law of probabilities would

indicate.

Then, in the fourth place, the physical constitution of comets accords

with the hypothesis.[15] The ability of nebulous matter to concentrate

into a concrete form, depends on its mass. To bring its ultimate atoms

into that proximity requisite for chemical union--requisite, that is,

for the production of denser matter--their repulsion must be overcome.

The only force antagonistic to their repulsion, is their mutual

gravitation. That their mutual gravitation may generate a pressure and

temperature of sufficient intensity, there must be an enormous

accumulation of them; and even then the approximation can slowly go on

only as fast as the evolved heat escapes. But where the quantity of

atoms is small, and therefore the force of mutual gravitation small,

there will be nothing to coerce the atoms into union. Whence we infer

that these detached fragments of nebulous matter will continue in

their original state. Non-periodic comets seem to do so.

We have already seen that this view of the origin of comets harmonizes

with the characters of their orbits; but the evidence hence derived is

much stronger than was indicated. The great majority of cometary orbits

are classed as parabolic; and it is ordinarily inferred that they are

visitors from remote space, and will never return. But are they rightly

classed as parabolic? Observations on a comet moving in an extremely

eccentric ellipse, which are possible only when it is comparatively near

perihelion, must fail to distinguish its orbit from a parabola.

Evidently, then, it is not safe to class it as a parabola because of

inability to detect the elements of an ellipse. But if extreme

eccentricity of an orbit necessitates such inability, it seems quite

possible that comets have no other orbits than elliptic ones. Though

five or six are said to be hyperbolic, yet, as I learn from one who has

paid special attention to comets, "no such orbit has, I believe, been

computed for a well-observed comet." Hence the probability that all the

orbits are ellipses is overwhelming. Ellipses and hyperbolas have

countless varieties of forms, but there is only one form of parabola;

or, to speak literally, all parabolas are similar, while there are

infinitely numerous dissimilar ellipses and dissimilar hyperbolas.

Consequently, anything coming to the Sun from a great distance must have

one exact amount of proper motion to produce a parabola: all other

amounts would give hyperbolas or ellipses. And if there are no

hyperbolic orbits, then it is infinity to one that all the orbits are

elliptical. This is just what they would be if comets had the genesis

above supposed.

* * * * *

And now, leaving these erratic bodies, let us turn to the more familiar

and important members of the Solar System. It was the remarkable harmony

among their movements which first made Laplace conceive that the Sun,

planets, and satellites had resulted from a common genetic process. As

Sir William Herschel, by his observations on the nebulae, was led to the

conclusion that stars resulted from the aggregation of diffused matter;

so Laplace, by his observations on the structure of the Solar System,

was led to the conclusion that only by the rotation of aggregating

matter were its peculiarities to be explained. In his _Exposition du

Systeme du Monde_, he enumerates as the leading evidences:--1. The

movements of the planets in the same direction and in orbits approaching

to the same plane; 2. The movements of the satellites in the same

direction as those of the planets; 3. The movements of rotation of these

various bodies and of the sun in the same direction as the orbital

motions, and mostly in planes little different; 4. The small

eccentricities of the orbits of the planets and satellites, as

contrasted with the great eccentricities of the cometary orbits. And the

probability that these harmonious movements had a common cause, he

calculates as two hundred thousand billions to one.

This immense preponderance of probability does not point to a common

cause under the form ordinarily conceived--an Invisible Power working

after the method of "a Great Artificer;" but to an Invisible Power

working after the method of evolution. For though the supporters of the

common hypothesis may argue that it was necessary for the sake of

stability that the planets should go round the Sun in the same direction

and nearly in one plane, they cannot thus account for the direction of

the axial motions.[16] The mechanical equilibrium would not have been

interfered with, had the Sun been without any rotatory movement; or had

he revolved on his axis in a direction opposite to that in which the

planets go round him; or in a direction at right angles to the average

plane of their orbits. With equal safety the motion of the Moon round

the Earth might have been the reverse of the Earth's motion round its

axis; or the motions of Jupiter's satellites might similarly have been

at variance with his axial motion; or those of Saturn's satellites with

his. As, however, none of these alternatives have been followed, the

uniformity must be considered, in this case as in all others, evidence

of subordination to some general law--implies what we call natural

causation, as distinguished from arbitrary arrangement.

Hence the hypothesis of evolution would be the only probable one, even

in the absence of any clue to the particular mode of evolution. But when

we have, propounded by a mathematician of the highest authority, a

theory of this evolution based on established mechanical principles,

which accounts for these various peculiarities, as well as for many

minor ones, the conclusion that the Solar System _was_ evolved becomes

almost irresistible.

The general nature of Laplace's theory scarcely needs stating. Books of

popular astronomy have familiarized most readers with his

conceptions;--namely, that the matter now condensed into the Solar

System, once formed a vast rotating spheroid of extreme rarity extending

beyond the orbit of the outermost planet; that as this spheroid

contracted, its rate of rotation necessarily increased; that by

augmenting centrifugal force its equatorial zone was from time to time

prevented from following any further the concentrating mass, and so

remained behind as a revolving ring; that each of the revolving rings

thus periodically detached, eventually became ruptured at its weakest

point, and, contracting on itself, gradually aggregated into a rotating

mass; that this, like the parent mass, increased in rapidity of rotation

as it decreased in size, and, where the centrifugal force was

sufficient, similarly left behind rings, which finally collapsed into

rotating spheroids; and that thus, out of these primary and secondary

rings, there arose planets and their satellites, while from the central

mass there resulted the Sun. Moreover, it is tolerably well known that

this _a priori_ reasoning harmonizes with the results of experiment. Dr.

Plateau has shown that when a mass of fluid is, as far may be, protected

from the action of external forces, it will, if made to rotate with

adequate velocity, form detached rings; and that these rings will break

up into spheroids which turn on their axes in the same direction with

the central mass. Thus, given the original nebula, which, acquiring a

vortical motion in the way indicated, has at length concentrated into a

vast spheroid of aeriform matter moving round its axis--given this, and

mechanical principles explain the rest. The genesis of a Solar System

displaying movements like those observed, may be predicted; and the

reasoning on which the prediction is based is countenanced by

experiment.[17]

But now let us inquire whether, besides these most conspicuous

structural and dynamic peculiarities of the Solar System, sundry minor

ones are not similarly explicable.

* * * * *

Take first the relation between the planes of the planetary orbits and

the plane of the Sun's equator. If, when the nebulous spheroid extended

beyond the orbit of Neptune, all parts of it had been revolving exactly

in the same plane, or rather in parallel planes--if all its parts had

had one axis; then the planes of the successive rings would have been

coincident with each other and with that of the Sun's rotation. But it

needs only to go back to the earlier stages of concentration, to see

that there could exist no such complete uniformity of motion. The

flocculi, already described as precipitated from an irregular and

widely-diffused nebula, and as starting from all points to their common

centre of gravity, must move not in one plane but in innumerable planes,

cutting each other at all angles. The gradual establishment of a

vortical motion such as we at present see indicated in the spiral

nebulae, is the gradual approach towards motion in one plane. But this

plane can but slowly become decided. Flocculi not moving in this plane,

but entering into the aggregation at various inclinations, will tend to

perform their revolutions round its centre in their own planes; and only

in course of time will their motions be partly destroyed by conflicting

ones, and partly resolved into the general motion. Especially will the

outermost portions of the rotating mass retain for a long time their

more or less independent directions. Hence the probabilities are, that

the planes of the rings first detached will differ considerably from the

average plane of the mass; while the planes of those detached latest

will differ from it less.

Here, again, inference to a considerable extent agrees with observation.

Though the progression is irregular, yet, on the average, the

inclinations decrease on approaching the Sun; and this is all we can

expect. For as the portions of the nebulous spheroid must have arrived

with miscellaneous inclinations, its strata must have had planes of

rotation diverging from the average plane in degrees not always

proportionate to their distances from the centre.

* * * * *

Consider next the movements of the planets on their axes. Laplace

alleged as one among other evidences of a common genetic cause, that the

planets rotate in a direction the same as that in which they go round

the Sun, and on axes approximately perpendicular to their orbits. Since

he wrote, an exception to this general rule has been discovered in the

case of Uranus, and another still more recently in the case of

Neptune--judging, at least, from the motions of their respective

satellites. This anomaly has been thought to throw considerable doubt on

his speculation; and at first sight it does so. But a little reflection

shows that the anomaly is not inexplicable, and that Laplace simply went

too far in putting down as a certain result of nebular genesis, what is,

in some instances, only a probable result. The cause he pointed out as

determining the direction of rotation, is the greater absolute velocity

of the outer part of the detached ring. But there are conditions under

which this difference of velocity may be too insignificant, even if it

exists. If a mass of nebulous matter approaching spirally to the central

spheroid, and eventually joining it tangentially, is made up of parts

having the same absolute velocities; then, after joining the equatorial

periphery of the spheroid and being made to rotate with it, the angular

velocity of its outer parts will be smaller than the angular velocity of

its inner parts. Hence, if, when the angular velocities of the outer and

inner parts of a detached ring are the same, there results a tendency to

rotation in the same direction with the orbital motion, it may be

inferred that when the outer parts of the ring have a smaller angular

velocity than the inner parts, a tendency to retrograde rotation will be

the consequence.

Again, the sectional form of the ring is a circumstance of moment; and

this form must have differed more or less in every case. To make this

clear, some illustration will be necessary. Suppose we take an orange,

and, assuming the marks of the stalk and the calyx to represent the

poles, cut off round the line of the equator a strip of peel. This strip

of peel, if placed on the table with its ends meeting, will make a ring

shaped like the hoop of a barrel--a ring of which the thickness in the

line of its diameter is very small, but of which the width in a

direction perpendicular to its diameter is considerable. Suppose, now,

that in place of an orange, which is a spheroid of very slight

oblateness, we take a spheroid of very great oblateness, shaped somewhat

like a lens of small convexity. If from the edge or equator of this

lens-shaped spheroid, a ring of moderate size were cut off, it would be

unlike the previous ring in this respect, that its greatest thickness

would be in the line of its diameter, and not in a line at right angles

to its diameter: it would be a ring shaped somewhat like a quoit, only

far more slender. That is to say, according to the oblateness of a

rotating spheroid, the detached ring may be either a hoop-shaped ring or

a quoit-shaped ring.

One further implication must be noted. In a much-flattened or

lens-shaped spheroid, the form of the ring will vary with its bulk. A

very slender ring, taking off just the equatorial surface, will be

hoop-shaped; while a tolerably massive ring, trenching appreciably on

the diameter of the spheroid, will be quoit-shaped. Thus, then,

according to the oblateness of the spheroid and the bulkiness of the

detached ring, will the greatest thickness of that ring be in the

direction of its plane, or in a direction perpendicular to its plane.

But this circumstance must greatly affect the rotation of the resulting

planet. In a decidedly hoop-shaped nebulous ring, the differences of

velocity between the inner and outer surfaces will be small; and such a

ring, aggregating into a mass of which the greatest diameter is at right

angles to the plane of the orbit, will almost certainly give to this

mass a predominant tendency to rotate in a direction at right angles to

the plane of the orbit. Where the ring is but little hoop-shaped, and

the difference between the inner and outer velocities greater, as it

must be, the opposing tendencies--one to produce rotation in the plane

of the orbit, and the other, rotation perpendicular to it--will both be

influential; and an intermediate plane of rotation will be taken up.

While, if the nebulous ring is decidedly quoit-shaped, and therefore

aggregates into a mass whose greatest dimension lies in the plane of

the orbit, both tendencies will conspire to produce rotation in that

plane.

On referring to the facts, we find them, as far as can be judged, in

harmony with this view. Considering the enormous circumference of

Uranus's orbit, and his comparatively small mass, we may conclude that

the ring from which he resulted was a comparatively slender, and

therefore a hoop-shaped one: especially as the nebulous mass must have

been at that time less oblate than afterwards. Hence, a plane of

rotation nearly perpendicular to his orbit, and a direction of rotation

having no reference to his orbital movement. Saturn has a mass seven

times as great, and an orbit of less than half the diameter; whence it

follows that his genetic ring, having less than half the circumference,

and less than half the vertical thickness (the spheroid being then

certainly _as_ oblate, and indeed _more_ oblate), must have had a much

greater width--must have been less hoop-shaped, and more approaching to

the quoit-shaped: notwithstanding difference of density, it must have

been at least two or three times as broad in the line of its plane.

Consequently, Saturn has a rotatory movement in the same direction as

the movement of translation, and in a plane differing from it by thirty

degrees only. In the case of Jupiter, again, whose mass is three and a

half times that of Saturn, and whose orbit is little more than half the

size, the genetic ring must, for the like reasons, have been still

broader--decidedly quoit-shaped, we may say; and there hence resulted a

planet whose plane of rotation differs from that of his orbit by

scarcely more than three degrees. Once more, considering the comparative

insignificance of Mars, Earth, Venus, and Mercury, it follows that, the

diminishing circumferences of the rings not sufficing to account for the

smallness of the resulting masses, the rings must have been slender

ones--must have again approximated to the hoop-shaped; and thus it

happens that the planes of rotation again diverge more or less widely

from those of the orbits. Taking into account the increasing oblateness

of the original spheroid in the successive stages of its concentration,

and the different proportions of the detached rings, it may fairly be

held that the respective rotatory motions are not at variance with the

hypothesis but contrariwise tend to confirm it.

Not only the directions, but also the velocities of rotation seem thus

explicable. It might naturally be supposed that the large planets would

revolve on their axes more slowly than the small ones: our terrestrial

experiences of big and little bodies incline us to expect this. It is a

corollary from the Nebular Hypothesis, however, more especially when

interpreted as above, that while large planets will rotate rapidly,

small ones will rotate slowly; and we find that in fact they do so.

Other things equal, a concentrating nebulous mass which is diffused

through a wide space, and whose outer parts have, therefore, to travel

from great distances to the common centre of gravity, will acquire a

high axial velocity in course of its aggregation; and conversely with a

small mass. Still more marked will be the difference where the form of

the genetic ring conspires to increase the rate of rotation. Other

things equal, a genetic ring which is broadest in the direction of its

plane will produce a mass rotating faster than one which is broadest at

right angles to its plane; and if the ring is absolutely as well as

relatively broad, the rotation will be very rapid. These conditions

were, as we saw, fulfilled in the case of Jupiter; and Jupiter turns

round his axis in less than ten hours. Saturn, in whose case, as above

explained, the conditions were less favourable to rapid rotation, takes

nearly ten hours and a half. While Mars, Earth, Venus, and Mercury,

whose rings must have been slender, take more than double that time: the

smallest taking the longest.

* * * * *

From the planets let us now pass to the satellites. Here, beyond the

conspicuous facts commonly adverted to, that they go round their

primaries in the directions in which these turn on their axes, in planes

diverging but little from their equators, and in orbits nearly circular,

there are several significant traits which must not be passed over.

One of them is that each set of satellites repeats in miniature the

relations of the planets to the Sun, both in certain respects above

named and in the order of their sizes. On progressing from the outside

of the Solar System to its centre, we see that there are four large

external planets, and four internal ones which are comparatively small.

A like contrast holds between the outer and inner satellites in every

case. Among the four satellites of Jupiter, the parallel is maintained

as well as the comparative smallness of the number allows: the two outer

ones are the largest, and the two inner ones the smallest. According to

the most recent observations made by Mr. Lassell, the like is true of

the four satellites of Uranus. In the case of Saturn, who has eight

secondary planets revolving round him, the likeness is still more close

in arrangement as in number: the three outer satellites are large, the

inner ones small; and the contrasts of size are here much greater

between the largest, which is nearly as big as Mars, and the smallest,

which is with difficulty discovered even by the best telescopes. But the

analogy does not end here. Just as with the planets, there is at first a

general increase of size on travelling inwards from Neptune and Uranus,

which do not differ very widely, to Saturn, which is much larger, and to

Jupiter, which is the largest; so of the eight satellites of Saturn, the

largest is not the outermost, but the outermost save two; so of

Jupiter's four secondaries, the largest is the most remote but one. Now

these parallelisms are inexplicable by the theory of final causes. For

purposes of lighting, if this be the presumed object of these attendant

bodies, it would have been far better had the larger been the nearer: at

present, their remoteness renders them of less service than the

smallest. To the Nebular Hypothesis, however, these analogies give

further support. They show the action of a common physical cause. They

imply a _law_ of genesis, holding in the secondary systems as in the

primary system.

Still more instructive shall we find the distribution of the

satellites--their absence in some instances, and their presence in other

instances, in smaller or greater numbers. The argument from design fails

to account for this distribution. Supposing it be granted that planets

nearer the Sun than ourselves, have no need of moons (though,

considering that their nights are as dark, and, relatively to their

brilliant days, even darker than ours, the need seems quite as

great)--supposing this to be granted; how are we to explain the fact

that Uranus has but half as many moons as Saturn, though he is at double

the distance? While, however, the current presumption is untenable, the

Nebular Hypothesis furnishes us with an explanation. It enables us to

predict where satellites will be abundant and where they will be absent.

The reasoning is as follows.

In a rotating nebulous spheroid which is concentrating into a planet,

there are at work two antagonist mechanical tendencies--the centripetal

and the centrifugal. While the force of gravitation draws all the atoms

of the spheroid together, their tangential momentum is resolvable into

two parts, of which one resists gravitation. The ratio which this

centrifugal force bears to gravitation, varies, other things equal, as

the square of the velocity. Hence, the aggregation of a rotating

nebulous spheroid will be more or less hindered by this resisting force,

according as the rate of rotation is high or low: the opposition, in

equal spheroids, being four times as great when the rotation is twice as

rapid; nine times as great when it is three times as rapid; and so on.

Now the detachment of a ring from a planet-forming body of nebulous

matter, implies that at its equatorial zone the increasing centrifugal

force consequent on concentration has become so great as to balance

gravity. Whence it is tolerably obvious that the detachment of rings

will be most frequent from those masses in which the centrifugal

tendency bears the greatest ratio to the gravitative tendency. Though it

is not possible to calculate what ratio these two tendencies had to each

other in the genetic spheroid which produced each planet, it is possible

to calculate where each was the greatest and where the least. While it

is true that the ratio which centrifugal force now bears to gravity at

the equator of each planet, differs widely from that which it bore

during the earlier stages of concentration; and while it is true that

this change in the ratio, depending on the degree of contraction each

planet has undergone, has in no two cases been the same; yet we may

fairly conclude that where the ratio is still the greatest, it has been

the greatest from the beginning. The satellite-forming tendency which

each planet had, will be approximately indicated by the proportion now

existing in it between the aggregating power, and the power that has

opposed aggregation. On making the requisite calculations, a remarkable

harmony with this inference comes out. The following table shows what

fraction the centrifugal force is of the centripetal force in every

case; and the relation which that fraction bears to the number of

satellites.[18]

Mercury. 1/360

Venus. 1/253

Earth. 1/289 1 Satellite.

Mars. 1/127 2 Satellites.

Jupiter. 1/11.4 4 Satellites.

Saturn. 1/6.4 8 Satellites, and three rings.

Uranus. 1/10.9 4 Satellites.

Thus taking as our standard of comparison the Earth with its one moon,

we see that Mercury, in which the centrifugal force is relatively less,

has no moon. Mars, in which it is relatively much greater, has two

moons. Jupiter, in which it is far greater, has four moons. Uranus, in

which it is greater still, has certainly four, and more if Herschel was

right. Saturn, in which it is the greatest, being nearly one-sixth of

gravity, has, including his rings, eleven attendants. The only instance

in which there is nonconformity with observation, is that of Venus. Here

it appears that the centrifugal force is relatively greater than in the

Earth; and, according to the hypothesis, Venus ought to have a

satellite. Respecting this anomaly several remarks are to be made.

Without putting any faith in the alleged discovery of a satellite of

Venus (repeated at intervals by five different observers), it may yet be

contended that as the satellites of Mars eluded observation up to 1877,

a satellite of Venus may have eluded observation up to the present time.

Merely naming this as possible, but not probable, a consideration of

more weight is that the period of rotation of Venus is but indefinitely

fixed, and that a small diminution in the estimated angular velocity of

her equator would bring the result into congruity with the hypothesis.

Further, it may be remarked that not exact, but only general, congruity

is to be expected; since the process of condensation of each planet from

nebulous matter can scarcely be expected to have gone on with absolute

uniformity: the angular velocities of the superposed strata of nebulous

matter probably differed from one another in degrees unlike in each

case; and such differences would affect the satellite-forming tendency.

But without making much of these possible explanations of the

discrepancy, the correspondence between inference and fact which we find

in so many planets, may be held to afford strong support to the Nebular

Hypothesis.

Certain more special peculiarities of the satellites must be mentioned

as suggestive. One of them is the relation between the period of

revolution and that of rotation. No discoverable purpose is served by

making the Moon go round its axis in the same time that it goes round

the Earth: for our convenience, a more rapid axial motion would have

been equally good; and for any possible inhabitants of the Moon, much

better. Against the alternative supposition, that the equality occurred

by accident, the probabilities are, as Laplace says, infinity to one.

But to this arrangement, which is explicable neither as the result of

design nor of chance, the Nebular Hypothesis furnishes a clue. In his

_Exposition du Systeme du Monde_, Laplace shows, by reasoning too

detailed to be here repeated, that under the circumstances such a

relation of movements would be likely to establish itself.

Among Jupiter's satellites, which severally display these same

synchronous movements, there also exists a still more remarkable

relation. "If the mean angular velocity of the first satellite be added

to twice that of the third, the sum will be equal to three times that of

the second;" and "from this it results that the situations of any two of

them being given, that of the third can be found." Now here, as before,

no conceivable advantage results. Neither in this case can the connexion

have been accidental: the probabilities are infinity to one to the

contrary. But again, according to Laplace, the Nebular Hypothesis

supplies a solution. Are not these significant facts?

Most significant fact of all, however, is that presented by the rings of

Saturn. As Laplace remarks, they are, as it were, still extant witnesses

of the genetic process he propounded. Here we have, continuing

permanently, forms of aggregation like those through which each planet

and satellite once passed; and their movements are just what, in

conformity with the hypothesis, they should be. "La duree de la rotation

d'une planete doit donc etre, d'apres cette hypothese, plus petite que

la duree de la revolution du corps le plus voisin qui circule autour

d'elle," says Laplace. And he then points out that the time of Saturn's

rotation is to that of his rings as 427 to 438--an amount of difference

such as was to be expected.[19]

Respecting Saturn's rings it may be further remarked that the place of

their occurrence is not without significance.

Rings detached early in the process of concentration, consisting of

gaseous matter having extremely little power of cohesion, can have

little ability to resist the disruptive forces due to imperfect balance;

and, therefore, collapse into satellites. A ring of a denser kind,

whether solid, liquid, or composed of small discrete masses (as Saturn's

rings are now concluded to be), we can expect will be formed only near

the body of a planet when it has reached so late a stage of

concentration that its equatorial portions contain matters capable of

easy precipitation into liquid and, finally, solid forms. Even then it

can be produced only under special conditions. Gaining a

rapidly-increasing preponderance as the gravitative force does during

the closing stages of concentration, the centrifugal force cannot, in

ordinary cases, cause the leaving behind of rings when the mass has

become dense. Only where the centrifugal force has all along been very

great, and remains powerful to the last, as in Saturn, can we expect

dense rings to be formed.

We find, then, that besides those most conspicuous peculiarities of the

Solar System which first suggested the theory of its evolution, there

are many minor ones pointing in the same direction. Were there no other

evidence, these mechanical arrangements would, considered in their

totality, go far to establish the Nebular Hypothesis.

* * * * *

From the mechanical arrangements of the Solar System, turn we now to its

physical characters; and, first, let us consider the inferences

deducible from relative specific gravities.

The fact that, speaking generally, the denser planets are the nearer to

the Sun, has been by some considered as adding another to the many

indications of nebular origin. Legitimately assuming that the outermost

parts of a rotating nebulous spheroid, in its earlier stages of

concentration, must be comparatively rare; and that the increasing

density which the whole mass acquires as it contracts, must hold of the

outermost parts as well as the rest; it is argued that the rings

successively detached will be more and more dense, and will form planets

of higher and higher specific gravities. But passing over other

objections, this explanation is quite inadequate to account for the

facts. Using the Earth as a standard of comparison, the relative

densities run thus:--

Neptune. Uranus. Saturn. Jupiter. Mars. Earth. Venus. Mercury. Sun.

0.17 0.25 0.11 0.23 0.45 1.00 0.92 1.26 0.25

Two insurmountable objections are presented by this series. The first

is, that the progression is but a broken one. Neptune is denser than

Saturn, which, by the hypothesis, it ought not to be. Uranus is denser

than Jupiter, which it ought not to be. Uranus is denser than Saturn,

and the Earth is denser than Venus--facts which not only give no

countenance to, but directly contradict, the alleged explanation. The

second objection, still more manifestly fatal, is the low specific

gravity of the Sun. If, when the matter of the Sun filled the orbit of

Mercury, its state of aggregation was such that the detached ring formed

a planet having a specific gravity equal to that of iron; then the Sun

itself, now that it has concentrated, should have a specific gravity

much greater than that of iron; whereas its specific gravity is only

half as much again as that of water. Instead of being far denser than

the nearest planet, it is but one-fifth as dense.

While these anomalies render untenable the position that the relative

specific gravities of the planets are direct indications of nebular

condensation; it by no means follows that they negative it. Several

causes may be assigned for these unlikenesses:--1. Differences among the

planets in respect of the elementary substances composing them; or in

the proportions of such elementary substances, if they contain the same

kinds. 2. Differences among them in respect of the quantities of matter

they contain; for, other things equal, the mutual gravitation of

molecules will make a larger mass denser than a smaller. 3. Differences

of temperatures; for, other things equal, those having higher

temperatures will have lower specific gravities. 4. Differences of

physical states, as being gaseous, liquid, or solid; or, otherwise,

differences in the relative amounts of the solid, liquid, and gaseous

matter they contain.

It is quite possible, and we may indeed say probable, that all these

causes come into play, and that they take various shares in the

production of the several results. But difficulties stand in the way of

definite conclusions. Nevertheless, if we revert to the hypothesis of

nebular genesis, we are furnished with partial explanations if nothing

more.

In the cooling of celestial bodies several factors are concerned. The

first and simplest is the one illustrated at every fire-side by the

rapid blackening of little cinders which fall into the ashes, in

contrast with the long-continued redness of big lumps. This factor is

the relation between increase of surface and increase of content:

surfaces, in similar bodies, increasing as the squares of the dimensions

while contents increase as their cubes. Hence, on comparing the Earth

with Jupiter, whose diameter is about eleven times that of the Earth, it

results that while his surface is 125 times as great, his content is

1390 times as great. Now even (supposing we assume like temperatures and

like densities) if the only effect were that through a given area of

surface eleven times more matter had to be cooled in the one case than

in the other, there would be a vast difference between the times

occupied in concentration. But, in virtue of a second factor, the

difference would be much greater than that consequent on these

geometrical relations. The escape of heat from a cooling mass is

effected by conduction, or by convection, or by both. In a solid it is

wholly by conduction; in a liquid or gas the chief part is played by

convection--by circulating currents which continually transpose the

hotter and cooler parts. Now in fluid spheroids--gaseous, or liquid, or

mixed--increasing size entails an increasing obstacle to cooling,

consequent on the increasing distances to be travelled by the

circulating currents. Of course the relation is not a simple one: the

velocities of the currents will be unlike. It is manifest, however, that

in a sphere of eleven times the diameter, the transit of matter from

centre to surface and back from surface to centre, will take a much

longer time; even if its movement is unrestrained. But its movement is,

in such cases as we are considering, greatly restrained. In a rotating

spheroid there come into play retarding forces augmenting with the

velocity of rotation. In such a spheroid the respective portions of

matter (supposing them equal in their angular velocities round the axis,

which they will tend more and more to become as the density increases),

must vary in their absolute velocities according to their distances from

the axis; and each portion cannot have its distance from the axis

changed by circulating currents, which it must continually be, without

loss or gain in its quantity of motion: through the medium of fluid

friction, force must be expended, now in increasing its motion and now

in retarding its motion. Hence, when the larger spheroid has also a

higher velocity of rotation, the relative slowness of the circulating

currents, and the consequent retardation of cooling, must be much

greater than is implied by the extra distances to be travelled.

And now observe the correspondence between inference and fact. In the

first place, if we compare the group of the great planets, Jupiter,

Saturn, and Uranus, with the group of the small planets, Mars, Earth,

Venus, and Mercury, we see that low density goes along with great size

and great velocity of rotation, and that high density goes along with

small size and small velocity of rotation. In the second place, we are

shown this relation still more clearly if we compare the extreme

instances--Saturn and Mercury. The special contrast of these two, like

the general contrast of the groups, points to the truth that low

density, like the satellite-forming tendency, is associated with the

ratio borne by centrifugal force to gravity; for in the case of Saturn

with his many satellites and least density, centrifugal force at the

equator is nearly 1/6th of gravity, whereas in Mercury with no satellite

and greatest density centrifugal force is but 1/360th of gravity.

There are, however, certain factors which, working in an opposite way,

qualify and complicate these effects. Other things equal, mutual

gravitation among the parts of a large mass will cause a greater

evolution of heat than is similarly caused in a small mass; and the

resulting difference of temperature will tend to produce more rapid

dissipation of heat. To this must be added the greater velocity of the

circulating currents which the intenser forces at work in larger

spheroids will produce--a contrast made still greater by the relatively

smaller retardation by friction to which the more voluminous currents

are exposed. In these causes, joined with causes previously indicated,

we may recognize a probable explanation of the otherwise anomalous fact

that the Sun, though having a thousand times the mass of Jupiter, has

yet reached as advanced a stage of concentration. For the force of

gravity in the Sun, which at his surface is some ten times that at the

surface of Jupiter, must expose his central parts to a pressure

relatively very intense; producing, during contraction, a relatively

rapid genesis of heat. And it is further to be remarked that, though the

circulating currents in the Sun have far greater distances to travel,

yet since his rotation is relatively so slow that the angular velocity

of his substance is but about one-sixtieth of that of Jupiter's

substance, the resulting obstacle to circulating currents is relatively

small, and the escape of heat far less retarded. Here, too, we may note

that in the co-operation of these factors, there seems a reason for the

greater concentration reached by Jupiter than by Saturn, though Saturn

is the elder as well as the smaller of the two; for at the same time

that the gravitative force in Jupiter is more than twice as great as in

Saturn, his velocity of rotation is very little greater, so that the

opposition of the centrifugal force to the centripetal is not much more

than half.

But now, not judging more than roughly of the effects of these several

factors, co-operating in various ways and degrees, some to aid

concentration and others to resist it, it is sufficiently manifest that,

other things equal, the larger nebulous spheroids, longer in losing

their heat, will more slowly reach high specific gravities; and that

where the contrasts in size are so immense as those between the greater

and the smaller planets, the smaller may have reached relatively high

specific gravities when the greater have reached but relatively low

ones. Further, it appears that such qualification of the process as

results from the more rapid genesis of heat in the larger masses, will

be countervailed where high velocity of rotation greatly impedes the

circulating currents. Thus interpreted then, the various specific

gravities of the planets may be held to furnish further evidences

supporting the Nebular Hypothesis.

* * * * *

Increase of density and escape of heat are correlated phenomena, and

hence in the foregoing section, treating of the respective densities of

the celestial bodies in connexion with nebular condensation, much has

been said and implied respecting the accompanying genesis and

dissipation of heat. Quite apart, however, from the foregoing arguments

and inferences, there is to be noted the fact that in the present

temperatures of the celestial bodies at large we find additional

supports to the hypothesis; and these, too, of the most substantial

character. For if, as is implied above, heat must inevitably be

generated by the aggregation of diffused matter, we ought to find in all

the heavenly bodies, either present high temperatures or marks of past

high temperatures. This we do, in the places and in the degrees which

the hypothesis requires.

Observations showing that as we descend below the Earth's surface there

is a progressive increase of heat, joined with the conspicuous evidence

furnished by volcanoes, necessitate the conclusion that the temperature

is very high at great depths. Whether, as some believe, the interior of

the Earth is still molten, or whether, as Sir William Thomson contends,

it must be solid; there is agreement in the inference that its heat is

intense. And it has been further shown that the rate at which the

temperature increases on descending below the surface, is such as would

be found in a mass which had been cooling for an indefinite period. The

Moon, too, shows us, by its corrugations and its conspicuous extinct

volcanoes, that in it there has been a process of refrigeration and

contraction, like that which has gone on in the Earth. There is no

teleological explanation of these facts. The frequent destructions of

life by earthquakes and volcanoes, imply, rather, that it would have

been better had the Earth been created with a low internal temperature.

But if we contemplate the facts in connexion with the Nebular

Hypothesis, we see that this still-continued high internal heat is one

of its corollaries. The Earth must have passed through the gaseous and

the molten conditions before it became solid, and must for an almost

infinite period by its internal heat continue to bear evidence of this

origin.

The group of giant planets furnishes remarkable evidence. The _a priori_

inference drawn above, that great size joined with relatively high ratio

of centrifugal force to gravity must greatly retard aggregation, and

must thus, by checking the genesis and dissipation of heat, make the

process of cooling a slow one, has of late years received verifications

from inferences drawn _a posteriori_; so that now the current conclusion

among astronomers is that in physical condition the great planets are in

stages midway between that of the Earth and that of the Sun. The fact

that the centre of Jupiter's disc is twice or thrice as bright as his

periphery, joined with the facts that he seems to radiate more light

than is accounted for by reflection of the Sun's rays, and that his

spectrum shows the "red-star line", are taken as evidences of

luminosity; while the immense and rapid perturbations in his atmosphere,

far greater than could be caused by heat received from the Sun, as well

as the formation of spots analogous to those of the Sun, which also,

like those of the Sun, show a higher rate of rotation near the equator

than further from it, are held to imply high internal temperature. Thus

in Jupiter, as also in Saturn, we find states which, not admitting of

any teleological explanations (for they manifestly exclude the

possibility of life), admit of explanations derived from the Nebular

Hypothesis.

But the argument from temperature does not end here. There remains to be

noticed a more conspicuous and still more significant fact. If the Solar

System was produced by the concentration of diffused matter, which

evolved heat while gravitating into its present dense form; then there

is an obvious implication. Other things equal, the latest-formed mass

will be the latest in cooling--will, for an almost infinite time,

possess a greater heat than the earlier-formed ones. Other things equal,

the largest mass will, because of its superior aggregative force, become

hotter than the others, and radiate more intensely. Other things equal,

the largest mass, notwithstanding the higher temperature it reaches,

will, in consequence of its relatively small surface, be the slowest in

losing its evolved heat. And hence, if there is one mass which was not

only formed after the rest, but exceeds them enormously in size, it

follows that this one will reach an intensity of incandescence far

beyond that reached by the rest; and will continue in a state of intense

incandescence long after the rest have cooled. Such a mass we have in

the Sun. It is a corollary from the Nebular Hypothesis, that the matter

forming the Sun assumed its present integrated shape at a period much

more recent than that at which the planets became definite bodies. The

quantity of matter contained in the Sun is nearly five million times

that contained in the smallest planet, and above a thousand times that

contained in the largest. And while, from the enormous gravitative force

of his parts to their common centre, the evolution of heat has been

intense, the facilities of radiation have been relatively small. Hence

the still-continued high temperature. Just that condition of the central

body which is a necessary inference from the Nebular Hypothesis, we find

actually existing in the Sun.

[The paragraph which here follows, though it contains some questionable

propositions, I reproduce just as it stood when first published in 1858,

for reasons which will presently be apparent.]

It may be well to consider more closely, what is the probable condition

of the Sun's surface. Round the globe of incandescent molten substances,

thus conceived to form the visible body of the Sun [which in conformity

with the argument in a previous section, now transferred to the Addenda,

was inferred to be hollow and filled with gaseous matter at high

tension] there is known to exist a voluminous atmosphere: the inferior

brilliancy of the Sun's border, and the appearances during a total

eclipse, alike show this. What now must be the constitution of this

atmosphere? At a temperature approaching a thousand times that of molten

iron, which is the calculated temperature of the solar surface, very

many, if not all, of the substances we know as solid, would become

gaseous; and though the Sun's enormous attractive force must be a

powerful check on this tendency to assume the form of vapour, yet it

cannot be questioned that if the body of the Sun consists of molten

substances, some of them must be constantly undergoing evaporation. That

the dense gases thus continually being generated will form the entire

mass of the solar atmosphere, is not probable. If anything is to be

inferred, either from the Nebular Hypothesis, or from the analogies

supplied by the planets, it must be concluded that the outermost part of

the solar atmosphere consists of what are called permanent gases--gases

that are not condensible into fluid even at low temperatures. If we

consider what must have been the state of things here, when the surface

of the Earth was molten, we shall see that round the still molten

surface of the Sun, there probably exists a stratum of dense aeriform

matter, made up of sublimed metals and metallic compounds, and above

this a stratum of comparatively rare medium analogous to air. What now

will happen with these two strata? Did they both consist of permanent

gases, they could not remain separate: according to a well-known law,

they would eventually form a homogeneous mixture. But this will by no

means happen when the lower stratum consists of matters that are gaseous

only at excessively high temperatures. Given off from a molten surface,

ascending, expanding, and cooling, these will presently reach a limit of

elevation above which they cannot exist as vapour, but must condense and

precipitate. Meanwhile the upper stratum, habitually charged with its

quantum of these denser matters, as our air with its quantum of water,

and ready to deposit them on any depression of temperature, must be

habitually unable to take up any more of the lower stratum; and

therefore this lower stratum will remain quite distinct from it.[20]

* * * * *

Considered in their _ensemble_, the several groups of evidences assigned

amount almost to proof. We have seen that, when critically examined,

the speculations of late years current respecting the nature of the

nebulae, commit their promulgators to sundry absurdities; while, on the

other hand, we see that the various appearances these nebulae present,

are explicable as different stages in the precipitation and aggregation

of diffused matter. We find that the immense majority of comets (_i.e._

omitting the periodic ones), by their physical constitution, their

immensely-extended and variously-directed paths, the distribution of

those paths, and their manifest structural relation to the Solar System,

bear testimony to the past existence of that system in a nebulous form.

Not only do those obvious peculiarities in the motions of the planets

which first suggested the Nebular Hypothesis, supply proofs of it, but

on closer examination we discover, in the slightly-diverging

inclinations of their orbits, in their various rates of rotation, and

their differently-directed axes of rotation, that the planets yield us

yet further testimony; while the satellites, by sundry traits, and

especially by their occurrence in greater or less abundance where the

hypothesis implies greater or less abundance, confirm this testimony. By

tracing out the process of planetary condensation, we are led to

conclusions respecting the physical states of planets which explain

their anomalous specific gravities. Once more, it turns out that what is

inferable from the Nebular Hypothesis respecting the temperatures of

celestial bodies, is just what observation establishes; and that both

the absolute and the relative temperatures of the Sun and planets are

thus accounted for. When we contemplate these various evidences in their

totality--when we observe that, by the Nebular Hypothesis, the leading

phenomena of the Solar System, and the heavens in general, are

explicable; and when, on the other hand, we consider that the current

cosmogony is not only without a single fact to stand on, but is at

variance with all our positive knowledge of Nature, we see that the

proof becomes overwhelming.

It remains only to point out that while the genesis of the Solar System,

and of countless other systems like it, is thus rendered comprehensible,

the ultimate mystery continues as great as ever. The problem of

existence is not solved: it is simply removed further back. The Nebular

Hypothesis throws no light on the origin of diffused matter; and

diffused matter as much needs accounting for as concrete matter. The

genesis of an atom is not easier to conceive than the genesis of a

planet. Nay, indeed, so far from making the Universe less a mystery than

before, it makes it a greater mystery. Creation by manufacture is a much

lower thing than creation by evolution. A man can put together a

machine; but he cannot make a machine develop itself. That our

harmonious universe once existed potentially as formless diffused

matter, and has slowly grown into its present organized state, is a far

more astonishing fact than would have been its formation after the

artificial method vulgarly supposed. Those who hold it legitimate to

argue from phenomena to noumena, may rightly contend that the Nebular

Hypothesis implies a First Cause as much transcending "the mechanical

God of Paley," as this does the fetish of the savage.