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 a.s.suming 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 ma.s.s 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 pa.s.sing 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. Ura.n.u.s. 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. Ura.n.u.s is as dense as Jupiter, which it ought not to be. Ura.n.u.s 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 ma.s.ses 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 quant.i.ties of matter; for, other things equal, the mutual gravitation of atoms will make a large ma.s.s denser than a small one. 3. Differences between the structures: the ma.s.ses being either solid or liquid throughout, or having central cavities filled with elastic aeriform substance. Of these three conceivable causes, that commonly a.s.signed 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 ma.s.s, which is 390,000 times that of the Earth, the matter he is made of can, in such case, have no a.n.a.logy 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 sh.e.l.l 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 ma.s.s; 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 s.p.a.ce, 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 ma.s.s. 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 pa.s.s 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 s.p.a.ce 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 acc.u.mulation at the poles; the spheroid will be ever becoming more protuberant about the poles than the conditions of mechanical equilibrium permit. If, however, the ma.s.s 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 ma.s.s 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 s.p.a.ce. This rarefied and cooled ma.s.s 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 acc.u.mulation 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 ma.s.s 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 const.i.tution of a concentrating spheroid of gaseous matter, where will the gaseous matter begin to condense into liquid? The usual a.s.sumption has been, that in a nebulous ma.s.s approaching towards the planetary form, the liquefaction will first occur at the centre. We believe this a.s.sumption is inconsistent with established physical principles.

Observe first that it is contrary to a.n.a.logy. 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 a.n.a.logy. 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 ma.s.s, 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 ma.s.s 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 princ.i.p.al ma.s.s, 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 sh.e.l.l becomes very thick, and the internal cavity relatively small, the obstacle put to the escape of heat by this thick liquid sh.e.l.l, 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 un.o.bliterated.

"But what," it may be asked, "will become of this gaseous nucleus when exposed to the enormous gravitative pressure of a sh.e.l.l 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 a.s.sumed.

And now let us see how this hypothesis tallies with the facts. One inference from it must be, that large ma.s.ses will progress towards final consolidation more slowly than small ma.s.ses. 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 quant.i.ty 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 sh.e.l.ls, 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 Ura.n.u.s, 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 ma.s.s 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 ma.s.s, 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 ma.s.ses 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 a.s.signed, 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 ma.s.s, diameter, and rate of rotation, calculated the degrees of compression at the poles which his centrifugal force should produce, supposing his substance to be h.o.m.ogeneous; 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 sh.e.l.l 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 sh.e.l.l 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 ma.s.s, 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 ma.s.s, 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 const.i.tuted as is commonly a.s.sumed, no conceivable cause of it can be named. A solid ma.s.s may crack and fall to pieces, but it cannot violently explode. So, too, with a liquid ma.s.s covered by a crust. Though, if contained in an unyielding sh.e.l.l and artificially raised to a very high temperature, a liquid might so expand as to burst the sh.e.l.l 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 ma.s.s, 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 sh.e.l.l, 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 a.s.sumption, 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 sh.e.l.l is, as it must in that case be, greater than the current supposition implies; then there will be a larger quant.i.ty of matter contained in the equatorial protuberance, and an adequate cause for the precession. Manifestly there may be found some proportion between the central s.p.a.ce 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 a.s.sume 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 a.s.signed 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 a.s.sumed 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.

As far as observation can reach, the facts prove to be what theory requires. Various evidences conspire to show that, below a certain depth, the Earth is still molten. And that it was once wholly molten, is implied by the circ.u.mstance that the rate at which the temperature increases on descending below its surface, is such as would be found in a ma.s.s that had been cooling for an indefinite period. The Moon, too, shows us, by its corrugations and its conspicuous volcanoes, that in it there has been a process of refrigeration and contraction, like that which had gone on in the Earth. And in Venus, the existence of mountains similarly indicates an igneous reaction of the interior upon a solidifying crust.

On the common theory of creation, these phenomena are inexplicable. To what end the Earth should once have existed in a molten state, incapable of supporting life, it cannot say. To satisfy this supposition, the Earth should have been originally created in a state fit for the a.s.sumed purposes of creation; and similarly with the other planets. While, therefore, to the Nebular Hypothesis the evidence of original incandescence and still continued internal heat, furnish strong confirmation, they are, to the antagonist hypothesis, insurmountable difficulties.

But the argument from temperature does not end here. There remains to be noticed a more conspicuous and still more significant fact. If the Solar System was formed by the concentration of diffused matter, which evolved heat while gravitating into its present dense form; then there are certain obvious corollaries respecting the relative temperatures of the resulting bodies. Other things equal, the latest-formed ma.s.s will be the latest in cooling--will, for an almost infinite time, possess a greater heat than the earlier-formed ones. Other things equal, the largest ma.s.s will, because of its superior aggregative force, become hotter than the others, and radiate more intensely. Other things equal, the largest ma.s.s, notwithstanding the higher temperature it reaches, will, in consequence of its relatively small surface, be the slowest in losing its evolved heat. And hence, if there is one ma.s.s which was not only formed after the rest, but exceeds them enormously in size, it follows that this one will reach an intensity of incandescence much beyond that reached by the rest; and will continue in a state of intense incandescence long after the rest have cooled.

Such a ma.s.s we have in the Sun. It is a corollary from the Nebular Hypothesis, that the matter forming the Sun a.s.sumed its present concrete form, at a period much more recent than that at which the planets became definite bodies. The quant.i.ty of matter contained in the Sun is nearly five million times that contained in the smallest planet, and above a thousand times that contained in the largest. And while, from the enormous gravitative force of the atoms, the evolution of heat has been intense, the facilities of radiation have been relatively small. Hence the still-continued high temperature. Just that condition of the central body which is a necessary inference from the Nebular Hypothesis, we find actually existing in the Sun.

It may be well to consider a little more closely, what is the probable condition of the Sun's surface. Round the globe of incandescent molten substances, thus conceived to form the visible body of the Sun, there is known to exist a voluminous atmosphere: the inferior brilliancy of the Sun's border, and the appearances during a total eclipse, alike show this.[S] What now must be the const.i.tution of this atmosphere? At a temperature approaching a thousand times that of molten iron, which is the calculated temperature of the solar surface, very many, if not all, of the substances we know as solid, would become gaseous; and though the Sun's enormous attractive force must be a powerful check on this tendency to a.s.sume the form of vapour, yet it cannot be questioned that if the body of the Sun consists of molten substances, some of them must be constantly undergoing evaporation. That the dense gases thus continually being generated will form the entire ma.s.s of the solar atmosphere, is not probable. If anything is to be inferred, either from the Nebular Hypothesis, or from the a.n.a.logies supplied by the planets, it must be concluded that the outermost part of the solar atmosphere consists of what are called permanent gases--gases that are not condensible into fluid even at low temperatures. If we consider what must have been the state of things here, when the surface of the Earth was molten, we shall see that round the still molten surface of the Sun, there probably exists a stratum of dense aeriform matter, made up of sublimed metals and metallic compounds, and above this a stratum of comparatively rare medium a.n.a.logous to air. What now will happen with these two strata? Did they both consist of permanent gases, they could not remain separate: according to a well-known law, they would eventually form a h.o.m.ogeneous mixture. But this will by no means happen when the lower stratum consists of matters that are gaseous only at excessively high temperatures. Given off from a molten surface, ascending, expanding, and cooling, these will presently reach a limit of elevation above which they cannot exist as vapour, but must condense and precipitate.

Meanwhile the upper stratum, habitually charged with its quantum of these denser matters, as our air with its quantum of water, and ready to deposit them on any depression of temperature, must be habitually unable to take up any more of the lower stratum; and therefore this lower stratum will remain quite distinct from it.

[S] See Herschel's "Outlines of Astronomy."

Since the foregoing paragraph was originally published, in 1858, the proposition it enunciates as a corollary from the Nebular Hypothesis, has been in great part verified. The marvellous disclosures made by spectrum-a.n.a.lysis, have proved beyond the possibility of doubt, that the solar atmosphere contains, in a gaseous state, the metals, iron, calcium, magnesium, sodium, chromium, and nickel, along with small quant.i.ties of barium, copper, and zinc. That there exist in the solar atmosphere other metals like those which we have on the Earth, is probable; and that it contains elements which are unknown to us, is very possible.

Be this as it may, however, the proposition that the Sun's atmosphere consists largely of metallic vapours, must take rank as an established truth; and that the incandescent body of the Sun consists of molten metals, follows almost of necessity. That an _a priori_ inference which probably seemed to many readers wildly speculative, should be thus conclusively justified by observations, made without reference to any theory, is a striking fact; and it gives yet further support to the hypothesis from which this _a priori_ conclusion was drawn. It may be well to add that Kirchhoff, to whom we owe this discovery respecting the const.i.tution of the solar atmosphere, himself remarks in his memoir of 1861, that the facts disclosed are in harmony with the Nebular Hypothesis.

And here let us not omit to note also, the significant bearing which Kirchhoff's results have on the doctrine contended for in a foregoing section. Leaving out the barium, copper, and zinc, of which the quant.i.ties are inferred to be small, the metals existing as vapours in the Sun's atmosphere, and by consequence as molten in his incandescent body, have an average specific gravity of 425. But the average specific gravity of the Sun is about 1. How is this discrepancy to be explained? To say that the Sun consists almost wholly of the three lighter metals named, would be quite unwarranted by the evidence: the results of spectrum-a.n.a.lysis would just as much warrant the a.s.sertion that the Sun consists almost wholly of the three heavier. Three metals (two of them heavy) having been already left out of the estimate because their quant.i.ties appear to be small, the only legitimate a.s.sumption on which to base an estimate of specific gravity, is that the rest are present in something like equal amounts. Is it then that the lighter metals exist in larger proportions in the molten ma.s.s, though not in the atmosphere? This is very unlikely: the known habitudes of matter rather imply that the reverse is the case. Is it then that under the conditions of temperature and gravitation existing in the Sun, the state of liquid aggregation is wholly unlike that existing here?

This is a very strong a.s.sumption: it is one for which our terrestrial experiences afford no adequate warrant; and if such unlikeness exists, it is very improbable that it should produce so immense a contrast in specific gravity as that of 4 to 1. The more legitimate conclusion is that the Sun's body is not made up of molten matter all through; but that it consists of a molten sh.e.l.l with a gaseous nucleus. And this we have seen to be a corollary from the Nebular Hypothesis.