We will only add, that if the probabilit_Illustrations of Universal Progress: A Series of Discussions

then music must take rank as the highest of the fine arts--as the one

which, more than any other, ministers to human welfare. And thus, even

leaving out of view the immediate gratifications it is hourly giving, we

cannot too much applaud that progress of musical culture which is becoming

one of the characteristics of our age.

VI. THE NEBULAR HYPOTHESIS.

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

unfavorable 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 unmistakeably 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 from 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 positive 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. The man who gave it

its general shape, by promulgating the doctrine 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 still 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, as far as we know,

have all the evidences, even of one class, been fully stated; and still

less has there been an adequate statement of the several groups of

evidences in their _ensemble_. 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

resolvability is solely a question of 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 an ordinary telescope, 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 those 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 just as they were. It is a perfectly 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 we do not grant these conclusions. Receiving them though we have, 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 hypotheses. 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 next to 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."

Now, in this somewhat confused sentence there is expressed a more or less

decided belief, that the distances of the nebulae from our galaxy of stars

as much transcend the distances of our stars from each other, as these

interstellar distances transcend the dimensions of our planetary system.

Just as the diameter of the Earth's orbit, is an inappreciable point when

compared with the distance of our Sun from Sirius; so is the distance of

our Sun from Sirius, an inappreciable 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

surrounded by a swarm of bees, extending, as they sometimes do, so high in

the air as to be individually 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. Astounding 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 a million times the distance of the Earth from the Sun; and,

according to the hypothesis, the distance of a nebula is something like 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."[I] And since it is

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

follows that our distance from the remotest parts of it is about 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 a million times the distance of Sirius? The implication is,

that a star rendered invisible by distance becomes visible if taken two

thousand five 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?

[I] Cosmos. (Seventh Edition.) Vol. i. pp. 79, 80.

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 totally 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 alternative.

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 [alpha]

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 a great

proportion 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 can be given of this 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 'cluster 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

semi-diameter 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 centre. 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 magnitude, nearly three hundred nebulae, and globular and

other clusters _of all degrees of resolvability_, and smaller

scattered stars of every inferior magnitude, from the tenth to such

as by their magnitude 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," pp. 614, 615.

Now, we think this supplies a _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 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 positions can be entertained. What, then,

is the conclusion that remains? This, only:--that the nebulae are not

further off 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.

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 reconcileable

with the Nebular Hypothesis.

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

diameter, say as great as the distance from the Sun to Sirius,[J] what are

the successive changes that will take place in it? Mutual gravitation will

approximate its atoms; but their approximation will be opposed by atomic

repulsion, 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. Eventually,

this slow movement of the atoms towards their common centre of gravity,

will bring about phenomena of another order.

[J] Any objection made to the extreme tenuity this involves, is met

by the calculation of Newton, who proved that were a spherical

inch of air removed four thousand miles from the Earth, it would

expand into a sphere more than filling the orbit of Saturn.

Arguing from the known laws of atomic combination, it will happen that 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 (the heat and pressure both increasing as the aggregation

progresses); some of them will suddenly enter into chemical union. Whether

the binary atoms so 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

combination of some species will finally take place. When it does take

place, it will be accompanied by a great and sudden disengagement of heat;

and until this excess of heat has escaped, the newly-formed binary atoms

will remain uniformly diffused, or, as it were, dissolved in the

pre-existing nebulous medium.

But now mark what must by-and-by happen. When radiation has adequately

lowered the temperature, these binary atoms will precipitate; and having

precipitated, they will not remain uniformly diffused, but will aggregate

into _flocculi_: just as water, when precipitated from air, collects into

clouds. This _a priori_ conclusion is confirmed by the observation of those

still extant portions of nebulous matter which constitute comets; for,

"that the luminous part of a comet is something in the nature of a smoke,

fog, or cloud, suspended in a transparent atmosphere, is evident," says Sir

John Herschel.

Concluding, then, 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 will be the

mechanical results. We shall find that they will be quite different from

those occurring in the original homogeneous mass; and also quite different

from those which would occur among discrete masses dispersed through empty

space. Bodies dispersed through empty space, would move in straight lines

towards their common centre of gravity. So, too, would bodies dispersed

through a resisting medium, provided they were spherical, or of forms

presenting symmetrical faces to their lines of movement. But _irregular_

bodies dispersed through a resisting medium, will _not_ move in straight

lines towards their common centre of gravity. A mass which presents an

irregular face to its line of movement through a resisting medium, must

necessarily be deflected from its original course, by the unequal reactions

of the medium on its different sides. Hence each _flocculus_, as by analogy

we term one of these precipitated masses of gas or vapour, will acquire a

movement, not towards the common centre of gravity, but towards one or

other side of it; and this oblique movement, accelerated as well as changed

in direction by the increasing centripetal force, but retarded by the

resisting medium, will result in a spiral, ending in the common centre of

gravity. Observe, however, that this conclusion, valid as far as it goes,

by no means proves a common spiral movement of all the flocculi; for as

they must not only be varied in their forms, but disposed in all varieties

of position, their respective movements will be deflected, not towards one

side of the common centre of gravity, but towards various sides. How then

can there result a spiral movement common to them all? Very simply. Each

flocculus, in describing its spiral course, must give motion to the rarer

medium through which it is moving.

Now, the probabilities are infinity to one against all the respective

motions thus impressed on this rarer medium, exactly balancing each other.

And if they do not balance each other, the inevitable result must be a

rotation of the whole mass of the rarer 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.

Before comparing these conclusions with the facts, let us pursue the

reasoning a little further, and observe the subordinate actions, and the

endless modifications which will result from them. The respective flocculi

must not only be drawn towards their common centre of gravity, but also

towards neighbouring flocculi. Hence the whole assemblage of flocculi will

break up into subordinate 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. Now, 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 each other. 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 the 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."

Now this coexistence of largeness, irresolvability, irregularity, and

indefiniteness of outline, 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 to "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 an average statement), 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 more or

less 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 bodies

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 (both which are

necessary assumptions); and there will finally result a more or less

globular group 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 characterizes 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 elsewhere; and it is shown that,

under the law of gravitation, which we 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 discrete masses, 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 irreconcileable 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 comets we

have now existing a kind of matter like that out of which, according to the

Nebular Hypothesis, the Solar System was evolved. For the explanation of

them, we must hence 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,

either in consequence of local currents or the conflicting attractions of

adjacent masses, remain detached; as do, for instance, minute shreds of

cloud in a summer sky. In a concentrating nebula these will, in the great

majority of cases, eventually coalesce with the larger flocculi near to

them. But it is tolerably evident that some of the remotest of these small

flocculi, 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 extremely eccentric orbits. 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

ellipses; and rushing round it will go off again into space. That is, they

will behave just as we see comets do; whose orbits are usually so eccentric

as to be indistinguishable from parabolas.

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 rotary motion;

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 each other; but cut the plane of the

ecliptic at all angles.

In the third place, applying the reasoning already used, these remotest

flocculi of nebulous matter will, at the outset, be deflected from their

straight courses to the common centre of gravity, not all on one side, but

each on such side as its form 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 concentrating 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. Out of

210 comets known in 1855, 104 are direct, and 106 are retrograde. This

equality is what the law of probabilities would indicate.

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

accords with the hypothesis. 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. We find that they do so.

Comets consist of an extremely rare medium, which, as shown by the

description already quoted from Sir John Herschel, has characters like

those we concluded would belong to partially-condensed nebulous matter.

Yet another very significant fact is seen in the distribution of comets.

Though they come from all parts of the heavens, they by no means come in

equal abundance from all parts of the heavens; but are far more numerous

about the poles of the ecliptic than about its plane. Speaking generally,

comets having orbit-planes that are highly inclined to the ecliptic, are

comets having orbits of which the major axes are highly inclined to the

ecliptic--comets that come from high latitudes. This is not a necessary

connexion; for the planes of the orbits _might_ be highly inclined to the

ecliptic while the major axes were inclined to it very little. But in the

absence of any habitually-observed relation of this kind, it may safely be

concluded that, _on the average_, highly-inclined cometary orbits are

cometary orbits with highly-inclined major axes; and that thus, a

predominance of cometary orbits cutting the plane of the ecliptic at great

angles, implies a predominance of cometary orbits having major axes that

cut the ecliptic at great angles. Now the predominance of highly inclined

cometary orbits, may be gathered from the following table, compiled by M.

Arago, to which we have added a column giving the results up to a date two

years later.

-------------------------------------------------------------------------

| |in 1831. |in 1853. |in 1855. |

-------------------------------------------------------------------------

| From 0 to 10 | 9 | 19 | 19 |

| " 10 " 20 | 13 | 18 | 19 |

| " 20 " 30 | 10 | 13 | 14 |

| " 30 " 40 | 17 | 22 | 22 |

| " 40 " 50 | 14 | 35 | 36 |

| " 50 " 60 | 23 | 27 | 29 |

| " 60 " 70 | 17 | 23 | 25 |

| " 70 " 80 | 19 | 26 | 27 |

| " 80 " 90 | 15 | 18 | 19 |

-------------------------------------------------------------------------

| Total | 137 | 201 | 210 |

-------------------------------------------------------------------------

At first sight this table seems not to warrant our statement. Assuming the

alleged general relation between the inclinations of cometary orbits, and

the directions in space from which the comets come, the table may be

thought to show that the frequency of comets increases as we progress from

the plane of the ecliptic up to 45 deg., and then decreases up to 90 deg. But this

apparent diminution arises from the fact that the successive zones of space

rapidly diminish in their areas on approaching the poles. If we allow for

this, we shall find that the excess of comets continues to increase up to

the highest angles of inclination. In the table below, which, for

convenience, is arranged in inverted order, we have taken as standards of

comparison the area of the zone round the pole, and the number of comets it

contains; and having ascertained the areas of the other zones, and the

numbers of comets they should contain were comets equally distributed, we

have shown how great becomes the deficiency in descending from the poles of

the ecliptic to its plane.

-------------------------------------------------------------------------

-------------------------------------------------------------------------

| 90 and 80 | 1 | 19 | 19| 0 | 11.5 |

| 80 " 70 | 2.98| 56.6| 27| 29.6| 5.5 |

| 70 " 60 | 4.85| 92 | 25| 67 | 3.12|

| 60 " 50 | 6.6 | 125 | 29| 96 | 2.66|

| 50 " 40 | 8.13| 154 | 36| 118 | 2.68|

| 40 " 30 | 9.42| 179 | 22| 157 | 1.4 |

| 30 " 20 | 10.42| 198 | 14| 184 | 0.8 |

| 20 " 10 | 11.1 | 210 | 19| 191 | 1.04|

| 10 " 0 | 11.5 | 218 | 19| 199 | 1 |

-------------------------------------------------------------------------

In strictness, the calculation should be made with reference, not to the

plane of the ecliptic, but to the plane of the sun's equator; and this

might or might not render the progression more regular. Probably, too, the

progression would be made somewhat different were the calculation based, as

it should be, not on the inclinations of orbit-planes, but on the

inclinations of major axes. But even as it is, the result is sufficiently

significant: since, though the conclusion that comets are 11.5 times more

abundant about the poles of the ecliptic than about its plane, can be but a

rough approximation to the truth, yet no correction of it is likely very

much to change this strong contrast.

What, then, is the meaning of this fact? It has several meanings. It

negatives the supposition, favoured by Laplace among others, that comets

are bodies that were wandering in space, or have come from other systems;

for the probabilities are infinity to one against the orbits of such

wandering bodies showing any definite relation to the plane of the Solar

System. For the like reason, it negatives the hypothesis of Lagrange,

otherwise objectionable, that comets have resulted from planetary

catastrophes analogous to that which is supposed to have produced the

asteroids. It clearly shows that, instead of comets being _accidental_

members of the Solar System, they are _necessary_ members of it--have as

distinct a structural relation to it as the planets themselves. That comets

are abundant round the axis of the Solar System, and grow rarer as we

approach its plane, implies that the genesis of comets has followed some

_law_--a law in some way concerned with the genesis of the Solar System.

If we ask for any so-called final cause of this arrangement, none can be

assigned: until a probable use for comets has been shown, no reason can be

given why they should be thus distributed. But when we consider the

question as one of physical science, we see that comets are antithetical to

planets, not only in their great rarity, in their motions as indifferently

direct or retrograde, in their eccentric orbits, and in the varied

directions of those orbits; but we see the antithesis further marked in

this, that while planets have some relation to the plane of nebular

rotation, comets have some relation to the axis of nebular rotation.[K] And

without attempting to explain the nature of this relation, the mere fact

that such a relation exists, indicates that comets have resulted from a

process of evolution--points to a past time when the matter now forming the

Solar System extended to those distant regions of space which comets visit.

[K] It is alike remarkable and suggestive, that a parallel relation

exists between the distribution of nebulae and the axis of our

galaxy. Just as comets are abundant around the poles of our Solar

System, and rare in the neighbourhood of its plane: so are nebulae

abundant around the poles of our sidereal system, and rare in the

neighbourhood of its plane.

See, then, how differently this class of phenomena bears on the

antagonistic hypotheses. To the hypothesis commonly received, comets are

stumbling-blocks: why there should be hundreds (or probably thousands) of

extremely rare aeriform masses rushing to and fro round the sun, it cannot

say; any more than it can explain their physical constitutions, their

various and eccentric movements, or their distribution. The hypothesis of

evolution, on the other hand, not only allows of the general answer, that

they are minor results of the genetic process; but also furnishes us with

something like explanations of their several peculiarities.

* * * * *

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

subsisting 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 of evolution:--1.

The movements of the planets in the same direction and almost in the same

plane; 2. The movements of the satellites in the same direction as those of

the planets; 3. The movement of rotation of these various bodies and of the

sun in the same direction as the orbitual motions, and in planes little

different; 4. The small eccentricity of the orbits of the planets and

satellites, as contrasted with the great eccentricity of the cometary

orbits. And the probability that these harmonious movements had a common

cause, he calculates as two hundred thousand billions to one.

Observe that 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. The mechanical equilibrium would not have been at all 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 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 motion of Jupiter's satellites

might similarly have been at variance with his axial motion; or that 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 whose authority is second to none, a

definite theory of this evolution based on established mechanical laws,

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 Neptune; 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 threw off 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 we have explained, 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.[L]

[L] It is true that, as expressed by him, these propositions of

Laplace are not all beyond dispute. An astronomer of the highest

authority, who has favoured me with some criticisms on this

essay, alleges that instead of a nebulous ring rupturing at one

point, and collapsing into a single mass, "all probability would

be in favour of its breaking up into many masses." This

alternative result certainly seems to be more likely. But

granting that a nebulous ring would break up into many masses, it

may still be contended that, since the chances are infinity to

one against these being of equal sizes and equidistant, they

could not remain evenly distributed round their orbit: this

annular chain of gaseous masses would break up into groups of

masses; these groups would eventually aggregate into larger

groups; and the final result would be the formation of a single

mass. I have put the question to an astronomer scarcely second in

authority to the one above referred to, and he agrees that this

would probably be the process.

But now let us inquire whether, besides these most conspicuous

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 saw must

eventually arise, and such as we at present see indicated in the spiral

nebulae, is the gradual approach toward motion in one plane--the plane of

greatest momentum. But this plane can only 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 long

time their more or less independent directions; seeing that neither by

friction nor by the central forces will they be so much restrained. 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.

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 will, we believe, show that the anomaly

is by no means an insoluble one; 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 relatively insignificant, even if it exists:

and others in which, though existing to a considerable extent, it will not

suffice to determine the direction of rotation.

Note, in the first place, that in virtue of their origin, the different

strata of a concentrating nebulous spheroid, will be very unlikely to move

with equal angular velocities: only by friction continued for an indefinite

time will their angular velocities be made uniform; and especially will the

outermost strata, for reasons just now assigned, maintain for the longest

time their differences of movement. Hence, it is possible that in the rings

first detached the outer rims may not have greater absolute velocities; and

thus the resulting planets may have retrograde rotations. 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 whose thickness in the line of its diameter is very small,

but whose 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 fact 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

very small; and such a ring, aggregating into a mass whose 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 of the inner and outer velocities also

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 if the nebulous mass was at that time less oblate than

afterwards, which it must have been. Hence, a plane of rotation nearly

perpendicular to his orbit, and a direction of rotation having no reference

to his orbitual 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 considerably 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 seems to us that the respective rotatory motions are not at variance

with the hypothesis.

Not only the directions, but also the velocities of rotation are 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 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 that 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 that is broadest in the

direction of its plane will produce a mass rotating faster than one that 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 goes round

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

the conditions were less favourable to rapid rotation, takes ten hours and

a half. While Mars, Earth, Venus, and Mercury, whose rings must have been

slender, take more than double the 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 same directions that 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 the respects just named, and

in the order of the 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.

Moreover, 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 analogies 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; what is to be said of Mars, which, placed half as far again from

the Sun as we are, has yet no moon? Or again, 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 actually

enables us to predict, by a not very complex calculation, where satellites

will be abundant and where they will be absent. The reasoning is as

follows.

In a rotating nebulous spheroid that 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 strongly opposed by this outward impetus of its particles,

according as its 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 centrifugal force produced by 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 proportions 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.

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

1 1 1 1 1 1 1

--- --- --- --- --- --- ---

362 282 289 326 14 6.2 9

1 4 8 4 (or 6

Satellite. Satellites. Satellites according

and three to

rings. Herschel.)

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

see that Mercury and Mars, in which the centrifugal force is relatively

less, have no moons. Jupiter, in which it is far greater, has four moons.

Uranus, in which it is greater still, has certainly four, and probably more

than four. 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 imperfect conformity with observation is that of Venus. Here

it appears that the centrifugal force is relatively a very little greater

than in the Earth; and according to the hypothesis, Venus ought, therefore,

to have a satellite. Of this seeming anomaly there are two explanations.

Not a few astronomers have asserted that Venus _has_ a satellite. Cassini,

Short, Montaigne of Limoges, Roedkier, and Montbarron, professed to have

seen it; and Lambert calculated its elements. Granting, however, that they

were mistaken, there is still the fact that the diameter of Venus is

variously estimated; and that a very small change in the data would make

the fraction less instead of greater than that of the Earth. But admitting

the discrepancy, we think that this correspondence, even as it now stands,

is one of the strongest confirmations of the Nebular Hypothesis.[M]

[M] Since this essay was published, the data of the above

calculations have been changed by the discovery that the Sun's

distance is three millions of miles less than was supposed. Hence

results a diminution in his estimated mass, and in the masses of

the planets (except the Earth and Moon). No revised estimate of

the masses having yet been published, the table is re-printed in

its original form. The diminution of the masses to the alleged

extent of about one-tenth, does not essentially alter the

relations above pointed out.

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 matter 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.[N] 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.

[N] "Mecanique Celeste," p. 346.

But besides the existence of these rings, and their movements in the

required manner, there is a highly suggestive circumstance which Laplace

has not remarked--namely, the place of their occurrence. If the Solar

System was produced after the manner popularly supposed, then there is no

reason why the rings of Saturn should not have encircled him at a

comparatively great distance. Or, instead of being given to Saturn, who in

their absence would still have had eight satellites, such rings might have

been given to Mars, by way of compensation for a moon. Or they might have

been given to Uranus, who, for purposes of illumination, has far greater

need of them. On the common hypothesis, we repeat, no reason can be

assigned for their existence in the place where we find them. But on the

hypothesis of evolution, the arrangement, so far from offering a

difficulty, offers another confirmation. These rings are found where alone

they could have been produced--close to the body of a planet whose

centrifugal force bears a great proportion to his gravitative force. That

permanent rings should exist at any great distance from a planet's body,

is, on the Nebular Hypothesis, manifestly impossible. Rings detached early

in the process of concentration, and therefore consisting of gaseous matter

having extremely little power of cohesion, can have no ability to resist

the disrupting forces due to imperfect balance; and must, therefore,

collapse into satellites. A liquid ring is the only one admitting of

permanence. But a liquid ring can be produced only when the aggregation is

approaching its extreme--only when gaseous matter is passing into liquid,

and the mass is about to assume the planetary form. And even then it cannot

be produced save 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

detachment 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 liquid rings be formed. Thus the Nebular

Hypothesis shows us why such appendages surround Saturn, but exist nowhere

else.

And then, let us not forget the fact, discovered within these few years,

that Saturn possesses a _nebulous_ ring, through which his body is seen as

through a thick veil. In a position where alone such a thing seems

preservable--suspended, as it were, between the denser rings and the

planet--there still continues one of these annular masses of diffused

matter from which satellites and planets are believed to have originated.

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, is 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, will 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.

0.14 0.24 0.14 0.24 0.95 1.00 0.92 1.12 0.25

Two seemingly insurmountable objections are presented by this series. The

first is, that the progression is but a broken one. Neptune is as dense as

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

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 not much above that of water. Instead

of being far denser than the nearest planet, it is not one-fourth as dense.

And a parallel relation holds between Jupiter and his smallest

satellite.[O]

[O] The impending revision of the estimated masses of the planets,

entailed by the discovery that the Sun's distance is less than

was supposed, will alter these specific gravities. It will make

most of the contrasts still stronger.

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. On the

contrary, we believe that the facts admit of an interpretation quite

consistent with the hypothesis of Laplace.

There are three possible causes of unlike specific gravities in the members

of our Solar System:--1. Differences between the kinds of matter or matters

composing them. 2. Differences between the quantities of matter; for, other

things equal, the mutual gravitation of atoms will make a large mass denser

than a small one. 3. Differences between the structures: the masses being

either solid or liquid throughout, or having central cavities filled with

elastic aeriform substance. Of these three conceivable causes, that

commonly assigned is the first, more or less modified by the second. The

extremely low specific gravity of Saturn, which but little exceeds that of

cork (and, on this hypothesis, must at his surface be considerably less

than that of cork) is supposed to arise from the intrinsic lightness of his

substance. That the Sun weighs not much more than an equal bulk of water,

is taken as evidence that the matter he consists of is but little heavier

than water; although, considering his enormous gravitative force, which at

his surface is twenty-eight times the gravitative force at the surface of

the Earth, and considering his enormous mass, which is 390,000 times that

of the Earth, the matter he is made of can, in such case, have no analogy

to the liquids or solids we know. However, spite of these difficulties, the

current hypothesis is, that the Sun and planets, inclusive of the Earth,

are either solid or liquid, or have solid crusts with liquid nuclei: their

unlike specific gravities resulting from unlikenesses of substance. And

indeed, at first sight, this would seem to be the only tenable supposition;

seeing that, unless prevented by some immense resisting force, gravitation

must obliterate any internal cavity by collapsing the surrounding liquid or

solid matter.

Nevertheless, that the Earth, in common with other members of the Solar

System, is solid, or else consists of a solid shell having a cavity

entirely filled with molten matter, is not an established fact: it is

nothing but a supposition. We must not let its familiarity and apparent

feasibility delude us into an uncritical acceptance of it. If we find an

alternative supposition which, physically considered, is equally possible,

we are bound to consider it. And if it not only avoids the difficulties

above pointed out, but many others hereafter to be mentioned, we must give

it the preference.

Before proceeding to consider what the Nebular Hypothesis indicates

respecting the internal structures of the Sun and planets, we may state

that our reasonings, though of a kind not admitting of direct verification,

are nothing more than deductions from the established principles of

physics. We have submitted them to an authority not inferior to any that

can be named; and while unprepared to commit himself to them, he yet sees

nothing to object.

Starting, then, with a rotating spheroid of aeriform matter, in the later

stages of its concentration, but before it has begun to take a liquid or

solid form, let us inquire what must be the actions going on in it. Mutual

gravitation continually aggregates its atoms into a smaller and denser

mass; and the aggregating force goes on increasing, as the common centre of

gravity is approached. An obstacle to concentration, however, exists in the

centrifugal force, which at this stage bears a far higher ratio to gravity

than afterwards, and in a gaseous spheroid must produce a very oblate form.

At the same time, the approximation of the atoms is resisted by a force

which, in being overcome, is evolved as heat. This heat must be greatest

where the atoms are subject to the highest pressure--namely, about the

central parts. And as fast as it escapes into space, further approximation

and further generation of heat must take place. But in a gaseous spheroid,

having internal parts hotter than its external parts, there must be some

circulation. The currents must set from the hottest region to the coolest

by some particular route; and from the coolest to the hottest by some other

route. In a very oblate spheroid, the coolest region must be that about the

equator: the surface there bearing so large a ratio to the mass. Hence

there will be currents from the centre to the equator, and others from the

equator to the centre. What will be the special courses of these currents?

Supposing an original state of rest, about to pass into motion in obedience

to the disturbing forces, the currents commencing at the centre will follow

the lines of most rapidly-decreasing density; seeing that the inertia will

be least in those lines. That is to say, there will be a current from the

centre towards each pole, along the axis of rotation; and the space thus

continually left vacant will be filled by the collapse of matter coming in

at right angles to the axis. The process cannot end here, however. If there

are constant currents from the centre towards the poles, there must be a

constant accumulation at the poles; the spheroid will be ever becoming more

protuberant about the poles than the conditions of mechanical equilibrium

permit. If, however, the mass at the poles is thus ever in excess, it must,

by the forces acting on it, be constantly moved over the outer surface of

the spheroid from the poles towards the equator: thus only can that form

which rotation necessitates be maintained. And a further result of this

transfer of matter from the centre, by way of the poles, to the equator,

must be the establishment of counter-currents from the equator in

diametrical lines, to the centre.

Mark now the changes of temperature that must occur in these currents. An

aeriform mass ascending from the centre towards either pole, will expand as

it approaches the surface, in consequence of the diminution of pressure.

But expansion, involving an absorption of heat, will entail a diminished

temperature; and the temperature will be further lowered by the greater

freedom of radiation into space. This rarefied and cooled mass must be

still more rarefied and cooled in its progress over the surface of the

spheroid to the equator. Continually thrust further from the pole by the

ceaseless accumulation there, it must acquire an ever-increasing rotatory

motion and an ever-increasing centrifugal force: whence must follow

expansion and absorption of heat. To the refrigeration thus caused must be

added that resulting from radiation, which, at each advance towards the

equator, will be less hindered. And when the mass we have thus followed

arrives at the equator, it will have reached its maximum rarity and maximum

coolness. Conversely, every portion of a current proceeding in a

diametrical direction from the equator to the centre, must progressively

rise in temperature; in virtue alike of the increasing pressure, the

gradual arrest of motion, and the diminished rate of radiation. Note,

lastly, that this circulation will go on, but slowly. As the matter

proceeding from the equator towards the centre must have its rotatory

motion destroyed, while that proceeding from the poles to the equator must

have rotatory motion given to it, it follows that an enormous amount of

inertia has to be overcome; and this must make the currents so slow as to

prevent them from producing anything like an equality of temperature.

Such being the constitution of a concentrating spheroid of gaseous matter,

where will the gaseous matter begin to condense into liquid? The usual

assumption has been, that in a nebulous mass approaching towards the

planetary form, the liquefaction will first occur at the centre. We believe

this assumption is inconsistent with established physical principles.

Observe first that it is contrary to analogy. That the matter of the Earth

was liquid before any of it became solid, is generally admitted. Where has

it first solidified? Not at the centre, but at the surface. Now the general

principles which apply to the condensation of liquid matter into solid,

apply also to the condensation of gaseous matter into liquid. Hence if the

once liquid substance of the Earth first solidified at the surface, the

implication is that its once aeriform substance first liquified at the

surface.

But we have no need to rest in analogy. On considering what must happen in

a rotating gaseous spheroid having currents moving as above described, we

shall see that external condensation is a corollary. A nebulous mass, when

it has arrived at this stage, will consist of an aeriform mixture of

various matters; the heavier and more condensible matters being contained

in the rarer or less condensible, in the same way that water is contained

in air. And the inference must be, that at a certain stage, some of these

denser matters will be precipitated in the shape of a cloud.[P]

[P] The reader will perhaps say that this process is the one

described as having taken place early in the history of nebular

evolution; and this is true. But the same actions will be

repeated in media of different densities.

Now, what are the laws of precipitation from gases? If a gas through which

some other substance is diffused in a gaseous state, expands in consequence

of the removal of pressure, it will, when the rarefaction and consequent

cooling reach a certain point, begin to let fall the suspended substance.

Conversely, if, a gas, saturated even with some substance, is subject to

increased pressure, and is allowed to retain the additional heat which that

pressure generates; so far from letting fall what it contains, it will gain

the power to take up more. See then, the inference respecting condensation

in a nebulous spheroid. The currents proceeding from the equator to the

centre, subject to increasing pressure, and acquiring the heat due both to

this increasing pressure and to arrested motion, will have no tendency to

deposit their suspended substances, but rather the reverse: a formation of

liquid matter at the centre of the mass will be impossible. Contrariwise,

the gaseous currents moving from the centre to the poles and thence to the

equator, expanding as they go, first from diminished pressure and

afterwards from increased centrifugal force; and losing heat, not only by

expansion, but by more rapid radiation; will have less and less power to

retain the matter diffused through them. The earliest precipitation will

take place in the region of extremest rarefaction; namely, about the

equator. An equatorial belt of cloud will be first formed, and widened into

a zone, will by-and-by begin to condense into liquid.[Q] Gradually this

liquid film will extend itself on each side the equator, and encroaching on

the two hemispheres, will eventually close over at the poles: thus

producing a thin hollow globe, or rather spheroid, filled with gaseous

matter. We do not mean that this condensation will take place at the very

outermost surface; for probably, round the denser gases forming the

principal mass, there will extend strata of gases too rare and too cool to

be entangled in these processes. It is the surface of this inner spheroid

of denser gases to which our reasoning points as the place of earliest

condensation.

[Q] The formation of Saturn's rings is thus rendered comprehensible.

The internal circulation we have described, continuing, as it must, after

the formation of this liquid film, there will still go on the radiation of

heat, and the progressive aggregation. The film will thicken at the expense

of the internal gaseous substances precipitated on it. As it thickens, as

the globe contracts, and as the gravitative force augments, the pressure

will increase; and the evolution and radiation of heat will go on more

rapidly. Eventually, however, when the liquid shell becomes very thick, and

the internal cavity relatively small, the obstacle put to the escape of

heat by this thick liquid shell, with its slowly-circulating currents, will

turn the scale: the temperature of the outer surface will begin to

diminish, and a solid crust will form while the internal cavity is yet

unobliterated.

"But what," it may be asked, "will become of this gaseous nucleus when

exposed to the enormous gravitative pressure of a shell some thousands of

miles thick? How can aeriform matter withstand such a pressure?" Very

readily. It has been proved that even when the heat generated by

compression is allowed to escape, some gases remain uncondensible by any

force we can produce. An unsuccessful attempt lately made at Vienna to

liquify oxygen, clearly shows this enormous resistance. The steel piston

employed was literally shortened by the pressure used: and yet the gas

remained unliquified! If, then, the expansive force is thus immense when

the heat evolved is dissipated, what must it be when that heat is in great

measure detained; as in the case we are considering? Indeed, the

experiments of M. Cagniard de Latour have shown that gases may, under

pressure, acquire the density of liquids while retaining the aeriform

state; provided the temperature continues extremely high. In such a case,

every addition to the heat is an addition to the repulsive power of the

atoms: the increased pressure itself generates an increased ability to

resist; and this remains true to whatever extent the compression is

carried. Indeed, it is a corollary from the persistence of force, that if,

under increasing pressure, a gas retains all the heat evolved, its

resisting force is _absolutely unlimited_. Hence, the internal planetary

structure we have described, is as physically stable a one as that commonly

assumed.

And now let us see how this hypothesis tallies with the facts. One

inference from it must be, that large masses will progress towards final

consolidation more slowly than small masses. Though a large concentrating

spheroid will, from its superior aggregative force, generate heat more

rapidly than a small one; yet, having, relatively to its surface, a much

greater quantity of heat to get rid of, it will be longer than a small one

in going through the changes we have described. Consequently, at a time

when the smaller members of our Solar System have arrived at so advanced a

stage of aggregation as almost to have obliterated their central cavities,

and so reached high specific gravities; the larger members will still be at

that stage in which the central cavities bear great ratios to the

surrounding shells, and will therefore have low specific gravities. This

contrast is just what we find. The small planets Mercury, Venus, the Earth,

and Mars, differing from each other comparatively little in density as in

size, are about four times as dense as Jupiter and Uranus, and seven times

as dense as Saturn and Neptune--planets exceeding them in size as oranges

exceed peas; and they are four times as dense as the Sun, which in mass is

nearly 5,000,000 times greater than the smallest of them.

The obvious objection that this hypothesis does not explain the minor

differences, serves but to introduce a further confirmation. It may be

urged that Jupiter is of greater specific gravity than Saturn, though,

considering his superior mass, his specific gravity should be less; and

that still more anomalous is the case of the Sun, which, though containing

a thousand times the matter that Jupiter does, is nearly of the same

specific gravity. The solution of these difficulties lies in the modifying

effects of centrifugal force. Had the various masses to be compared been

all along in a state of rest, then the larger should have been uniformly

the less dense. But during the concentrating process they have been

rotating with various velocities. The consequent centrifugal force has in

each case been in antagonism with gravitation; and, according to its

amount, has hindered the concentration to a greater or less degree. The

efficient aggregative force has in each case been the excess of the

centripetal tendency over the centrifugal. Whence we may infer that

wherever this excess has been the least, the consolidation must have been

the most hindered, and the specific gravity will be the smallest. This,

too, we find to be the fact. Saturn, at whose equator the centrifugal force

is even now almost one-sixth of gravity, and who, by his numerous

satellites, shows us how strong an antagonist to concentration it was in

earlier stages of his evolution, is little more than half as dense as

Jupiter, whose concentration has been hindered by a centrifugal force

bearing a much smaller ratio to the centripetal.

On the other hand, the Sun, whose latter stages of aggregation have met

with comparatively little of this opposition, and whose atoms tend towards

their common centre with a force ten times as great as that which Jupiter's

atoms are subject to, has, notwithstanding his immense bulk, reached a

specific gravity as great as that of Jupiter; and he has done this partly

for the reason assigned, and partly because the process of consolidation

has been, and still is, actively going on, while that of Jupiter has long

since almost ceased.

Before pointing out further harmonies let us meet an objection. Laplace,

taking for data Jupiter's mass, diameter, and rate of rotation, calculated

the degrees of compression at the poles which his centrifugal force should

produce, supposing his substance to be homogeneous; and finding that the

calculated amount of oblateness was greater than the actual amount,

inferred that his substance must be denser towards the centre. The

inference seems unavoidable; is diametrically opposed to the hypothesis of

a shell of denser matter with a gaseous nucleus; and we confess that on

first meeting with this fact we were inclined to think it fatal. But there

is a consideration, apt to be overlooked, which completely disposes of it.

A compressed elastic medium tends ever with great energy to give a

spherical figure to the chamber in which it is confined. This truth is

alike mathematically demonstrable, and recognized in practice by every

engineer. In the case before us, the expansive power of the gaseous nucleus

is such as to balance the gravitation of the shell of the planet; and this

power perpetually strives to make the planet a perfect sphere. Thus the

tendency of the centrifugal force to produce oblateness, is opposed not

only by the force of gravity but by another force of great intensity; and

hence the degree of oblateness produced is relatively small.

This difficulty being as we think, satisfactorily met, we go on to name

some highly significant facts giving indirect support to our hypothesis.

And first with respect to the asteroids, or planetoids, as they are

otherwise called. Now that these have proved to be so numerous--now that it

has become probable that beyond some sixty already discovered there are

many more--the supposition of Olbers, that they are the fragments of an

exploded planet which once occupied the vacant region they fill, has gained

increased probability. The alternative supposition of Laplace, that they

are the products of a nebulous ring which separated into many fragments

instead of collapsing into a single mass, seems inconsistent with the

extremely various, and in some cases extremely great, inclinations of their

orbits; as well as with their similarly various and great eccentricities.

For these the theory of Olbers completely accounts--indeed, it necessarily

involves them; while at the same time it affords us a feasible explanation

of meteors, and especially the periodic swarms of them, which would else be

inexplicable. The fact, inferred from the present derangement of their

orbits, that if the planetoids once formed parts of one mass, it must have

exploded myriads of years ago, is no difficulty, but rather the reverse.

Taking Olbers' supposition, then, as the most tenable one, let us ask how

such an explosion could have occurred. If planets are internally

constituted as is commonly assumed, no conceivable cause of it can be

named. A solid mass may crack and fall to pieces, but it cannot violently

explode. So, too, with a liquid mass covered by a crust. Though, if

contained in an unyielding shell and artificially raised to a very high

temperature, a liquid might so expand as to burst the shell and

simultaneously flash into vapour; yet, if contained in a yielding crust,

like that of a planet, it would not do so: it would crack the crust and

give off its expansive force gradually. But the planetary structure above

supposed, supplies us with all the requisite conditions to an explosion,

and an adequate cause for it. We have in the interior of the mass, a cavity

serving as a sufficient reservoir of force. We have this cavity filled with

gaseous matters of high tension. We have in the chemical affinities of

these matters a source of enormous expansive power--power capable of being

quite suddenly liberated. And we have in the increasing heat of the shell,

consequent on progressing concentration, a cause of such instantaneous

chemical change and the resulting explosion. The explanation thus supplied,

of an event which there can be little doubt has occurred, and which is not

otherwise accounted for, adds to the probability of the hypothesis.

One further evidence, and that not the least important, is deducible from

geology. From the known rate at which the temperature rises as we pierce

deeper into the substance of the Earth, it has been inferred that its solid

crust is some forty miles thick. And if this be its thickness, we have a

feasible explanation of volcanic phenomena, as well as of elevations and

subsidences. But proceeding on the current supposition that the Earth's

interior is wholly filled with molten matter, Prof. Hopkins has calculated

that to cause the observed amount of precession of the equinoxes, the

Earth's crust must be at least eight hundred miles thick. Here is an

immense discrepancy. However imperfect may be the data from which it is

calculated that the Earth is molten at forty miles deep, it seems very

unlikely that this conclusion differs from the truth so widely as forty

miles does from eight hundred. It seems scarcely conceivable that if the

crust is thus thick, it should by its contraction and corrugation, produce

mountain chains, as it has done during quite modern geologic epochs. It is

not easy on this supposition to explain elevations and subsidences of small

area. Neither do the phenomena of volcanoes appear comprehensible. Indeed

to account for these, Prof. Hopkins has been obliged to make the gratuitous

and extremely improbable assumption, that there are isolated lakes of

molten matter enclosed in this thick crust, and situated, as they must be,

not far from its outer surface.

But irreconcileable as appear the astronomical with the geological facts,

if we take for granted that the Earth consists wholly of solid and liquid

substances, they become at once reconcileable if we adopt the conclusion

that the Earth has a gaseous nucleus. If there is an internal cavity of

considerable diameter occupied only by aeriform matter--if the density of

the surrounding shell is, as it must in that case be, greater than the

current supposition implies; then there will be a larger quantity of matter

contained in the equatorial protuberance, and an adequate cause for the

precession. Manifestly there may be found some proportion between the

central space and its envelope, which will satisfy the mechanical

requirements, without involving a thicker crust than geological phenomena

indicate.[R]

[R] Since this was written, M. Poinsot has shown that the precession

would be the same whether the Earth were solid or hollow.

We conceive, then, that the hypothesis we have set forth, is in many

respects preferable to that ordinarily received. We can know nothing by

direct observation concerning the central parts either of our own planet or

any other: indirect methods are alone possible. The idea which has been

tacitly adopted, is just as speculative as that we have opposed to it; and

the only question is, which harmonizes best with established facts. Thus

compared, the advantage is greatly on the side of the new one. It disposes

of sundry anomalies, and explains things that seem else incomprehensible.

We are no longer obliged to assume such wide differences between the

substances of the various planets: we need not think of any of them as like

cork or water. We are shown how it happens that the larger planets have so

much lower specific gravities than the smaller, instead of having higher

ones, as might have been expected; and we are further shown why Saturn is

the lightest of all. That Mercury is relatively so much heavier than the

Sun; that Jupiter is specifically lighter than his smallest satellite; that

Saturn's rings have a density one and a half times as great as Saturn; are

no longer mysteries. A feasible cause is assigned for the catastrophe which

produced the asteroids. And some apparently incongruous peculiarities in

the Earth's structure are brought to an agreement. May we not say, then,

that being deducible from the Nebular Hypothesis, this alleged planetary

structure gives further indirect support to that hypothesis?

* * * * *

In considering the specific gravities of the heavenly bodies, we have been

obliged to speak of the heat evolved by them. But we have yet to point out

the fact that in their present conditions with respect to temperature, we

find additional materials for building up our argument; and these too of

the most substantial character.

Heat must inevitably be generated by the aggregation of diffused matter

into a concrete form; and throughout our reasonings we have assumed that

such generation of heat has been an accompaniment of nebular condensation.

If, then, the Nebular Hypothesis be true, we ought to find in all the

heavenly bodies, either present high temperature or marks of past high

temperature.

We will only add, that if the probability of these corollaries be admitted,