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[First published in The Reader for February 25, 1865. I reproduce this essay chiefly to give a place to the speculation concerning the solar spots which forms the latter portion of it.]

The hypothesis of M. Faye, described in your numbers for January 28 and February 4, respectively, is to a considerable extent coincident with one which I ventured to suggest in an article on "Recent Astronomy and the Nebular Hypothesis," published in the Westminster Review for July, 1858. In considering the possible causes of the immense differences of specific gravity among the planets, I was led to question the validity of the tacit assumption that each planet consists of solid or liquid matter from centre to surface. It seemed to me that any other internal structure which was mechanically stable, might be assumed with equal legitimacy. And the hypothesis of a solid or liquid shell, having its cavity filled with gaseous matter at high pressure and temperature [and of great density], was one which seemed worth considering.

Hence arose the inquiry—What structure will result from the process of nebular condensation? [Here followed a long speculation respecting the processes going on in a concentrating nebulous spheroid; the general outcome of which is implied in Note III of the foregoing essay. I do not reproduce it because, not having the guidance of Prof. Andrew's researches, I had concluded that the formation of a molten shell would occur universally, instead of occasionally, as is now argued in the note named. The essay then proceeded thus:—]

The process of condensation being in its essentials the same for all concentrating nebular spheroids, planetary or solar, it was argued that the Sun is still passing through that incandescent stage which all the planets have long ago passed through: his later aggregation, joined with the immensely greater ratio of his mass to his surface, involving comparative lateness of cooling. Supposing the sun to have reached the state of a molten shell, inclosing a gaseous nucleus, it was concluded that this molten shell, ever radiating its heat, but ever acquiring fresh heat by further integration of the Sun's mass, must be constantly kept up to that temperature at which its substance evaporates.

[Here followed part of the paragraph quoted in the preceding essay on p. 155; and there succeeded, in subsequent editions, a paragraph aiming to show that the inferred structure of the Sun's interior was congruous with the low specific gravity of the Sun—a conclusion which, as indicated on p. 156, implies some very problematical assumptions respecting the natures of the unknown elements of the Sun. There then came this passage:—]

The conception of the Sun's constitution thus set forth, is like that of M. Faye in so far as the successive changes, the resulting structures, and the ultimate state, are concerned; but unlike it in so far as the Sun is supposed to have reached a later stage of concentration. As I gather from your abstract of M. Faye's paper [this referred to an article in The Reader], he considers the Sun to be at present a gaseous spheroid, having an envelope of metallic matters precipitated in the shape of luminous clouds, the local dispersions of which, caused by currents from within, appear to us as spots; and he looks forward to the future formation of a liquid film as an event that will soon be followed by extinction. Whereas the above hypothesis is that the liquid film already exists beneath the visible photosphere, and that extinction cannot result until, in the course of further aggregation, the gaseous nucleus has become so much reduced, and the shell so much thickened, that the escape of the heat generated is greatly retarded. … M. Faye's hypothesis appears to be espoused by him, partly because it affords an explanation of the spots, which are considered as openings in the photosphere, exposing the comparatively non-luminous gases filling the interior. But if these interior gases are non-luminous from the absence of precipitated matter, must they not for the same reason be transparent? And if transparent, will not the light from the remote side of the photosphere seen through them, be nearly as bright as that of the side next to us? By as much as the intensely-heated gases of the interior are disabled by the dissociation of their molecules from giving off luminiferous undulations, by so much must they be disabled from absorbing the light transmitted through them. And if their great light-transmitting power is exactly complementary to their small light-emitting power, there seems no reason why the interior of the Sun, disclosed to us by openings in the photosphere, should not appear as bright as its exterior.

Take, on the other hand, the supposition that a more advanced state of concentration has been reached. A shell of molten metallic matter enclosing a gaseous nucleus still higher in temperature than itself, will be continually kept at the highest temperature consistent with its state of liquid aggregation. Unless we assume that simple radiation suffices to give off all the heat generated by progressing integration, we must conclude that the mass will be raised to that temperature at which part of its heat is absorbed in vaporizing its superficial parts. The atmosphere of metallic gases hence resulting, cannot continue to accumulate without reaching a height above the Sun's surface, at which the cooling due to radiation and rarefaction will cause condensation into cloud—cannot, indeed, cease accumulating until the precipitation from the upper limit of the atmosphere balances the evaporation from its lower limit. This upper limit of the atmosphere of metallic gases, whence precipitation is perpetually taking place, will form the visible photosphere—partly giving off light of its own, partly letting through the more brilliant light of the incandescent mass below. This conclusion harmonizes with the appearances. Sir John Herschel, advocating though he does an antagonist hypothesis, gives a description of the Sun's surface which agrees completely with the processes here supposed. He says:—

"There is nothing which represents so faithfully this appearance as the slow subsidence of some flocculent chemical precipitates in a transparent fluid, when viewed perpendicularly from above: so faithfully, indeed, that it is hardly possible not to be impressed with the idea of a luminous medium intermixed, but not confounded, with a transparent and non-luminous atmosphere, either floating as clouds in our air, or pervading it in vast sheets and columns like flame, or the streamers of our northern lights".—Treatise on Astronomy, p. 208.

If the constitution of the Sun be that which is above inferred, it does not seem difficult to conceive still more specifically the production of these appearances. Everywhere throughout the atmosphere of metallic vapours which clothes the solar surface, there must be ascending and descending currents. The magnitude of these currents must obviously depend on the depth of this atmosphere. If it is shallow, the currents must be small; but if many thousands of miles deep, the currents may be wide enough to render visible to us the places at which they severally impinge on the limit of the atmosphere, and the places whence the descending currents commence. The top of an ascending current will be a space over which the thickness of condensed cloud is the least, and through which the greatest amount of light from beneath penetrates. The clouds perpetually formed at the top of such a current, will be perpetually thrust aside by the uncondensed gases from below them; and, growing while they are thrust aside, will collect in the spaces between the ascending currents, where there will result the greatest degree of opacity. Hence the mottled appearance—hence the "pores," or dark interspaces, separating the light-giving spots.[25]

Of the more special appearances which the photosphere presents, let us take first the faculæ. These are ascribed to waves in the photosphere; and the way in which such waves might produce an excess of light has been variously explained in conformity with various hypotheses. What would result from them in a photosphere constituted and conditioned as above supposed? Traversing a canopy of cloud, here thicker and there thinner, a wave would cause a disturbance very unlikely to leave the thin and thick parts without any change in their average permeability to light. There would probably be, at some parts of the wave, extensions in the areas of the light-transmitting clouds, resulting in the passage of more rays from below. Another phenomenon, less common but more striking, appears also to be in harmony with the hypothesis. I refer to those bright spots, of a brilliancy greater than that of the photosphere, which are sometimes observed. In the course of a physical process so vast and so active as that here supposed to be going on in the Sun, we may expect that concurrent causes will occasionally produce ascending currents much hotter than usual, or more voluminous, or both. One of these, on reaching the stratum of luminous and illuminated cloud forming the photosphere, will burst through it, dispersing and dissolving it, and ascending to a greater height before it begins itself to condense: meanwhile allowing to be seen, through its transparent mass, the incandescent molten shell of the sun's body.

[The foregoing passages, to most of which I do not commit myself as more than possibilities, I republish chiefly as introductory to the following speculation, which, since it was propounded in 1865, has met with some acceptance.]

"But what of the spots commonly so called?" it will be asked. In the essay on the Nebular hypothesis, above quoted from, it was suggested that refraction of the light passing through the depressed centres of cyclones in this atmosphere of metallic gases, might possibly be the cause; but this, though defensible as a "true cause," appeared on further consideration to be an inadequate cause. Keeping the question in mind, however, and still taking as a postulate the conclusion of Sir John Herschel, that the spots are in some way produced by cyclones, I was led, in the course of the year following the publication of the essay, to an hypothesis which seemed more satisfactory. This, which I named at the time to Prof. Tyndall, had a point in common with the one afterward published by Prof. Kirchhoff, in so far as it supposed cloud to be the cause of darkness; but differed in so far as it assigned the cause of such cloud. More pressing matters prevented me from developing the idea for some time; and, afterwards, I was deterred from including it in the revised edition of the essay, by its inconsistency with the "willow-leaf" doctrine, at that time dominant. The reasoning was as follows:—The central region of a cyclone must be a region of rarefaction, and, consequently, a region of refrigeration. In an atmosphere of metallic gases rising from a molten surface, and presently reaching a limit at which condensation takes place, the molecular state, especially toward its upper part, must be such that a moderate diminution of density, and fall of temperature, will cause precipitation. That is to say, the rarefied interior of a solar cyclone will be filled with cloud: condensation, instead of taking place only at the level of the photosphere, will here extend to a great depth below it, and over a wide area. What will be the characters of a cloud thus occupying the interior of a cyclone? It will have a rotatory motion; and this it has been seen to have. Being funnel-shaped, as analogy warrants us in assuming, its central parts will be much deeper than its peripheral parts, and therefore more opaque. This, too, corresponds with observation. Mr. Dawes has discovered that in the middle of the spot there is a blacker spot: just where there would exist a funnel-shaped prolongation of the cyclonic cloud down toward the Sun's body, the darkness is greater than elsewhere. Moreover, there is furnished an adequate reason for the depression which one of these dark spaces exhibits. In a whirlwind, as in a whirlpool, the vortex will be below the general level, and all around, the surface of the medium will descend toward it. Hence a spot seen obliquely, as when carried toward the Sun's limb, will have its umbra more and more hidden, while its penumbra still remains visible. Nor are we without some interpretation of the penumbra. If, as is implied by what has been said, the so-called "willow-leaves," or "rice-grains," are the tops of the currents ascending from the Sun's body, what changes of appearance are they likely to undergo in the neighbourhood of a cyclone? For some distance round a cyclone there will be a drawing in of the superficial gases toward the vortex. All the luminous spaces of more transparent cloud forming the adjacent photosphere, will be changed in shape by these centripetal currents. They will be greatly elongated; and there will so be produced that "thatch"-like aspect which the penumbra presents.

[The explanation of the solar spots above suggested, which was originally propounded in opposition to that of M. Faye, was eventually adopted by him in place of his own. In the Comptes Rendus for 1867, Vol. LXIV., p. 404, he refers to the article in the Reader, partly reproduced above, and speaks of me as having been replied to in a previous note. Again in the Comptes Rendus for 1872, Vol. LXXV., p. 1664, he recognizes the inadequacy of his hypothesis, saying:—"Il est certain que l'objection de M. Spencer, reproduit et développée par M. Kirchoff, est fondée jusqu'à un certain point; l'intérieur des taches, si ce sont des lacunes dans la photosphère, doit être froid relativement. … Il est donc impossible qu'elles proviennent d'éruptions ascendantes." He then proceeds to set forth the hypothesis that the spots are caused by the precipitation of vapour in the interiors of cyclones. But though, as above shown, he refers to the objection made in the foregoing essay to his original hypothesis, and recognizes its cogency, he does not say that the hypothesis which he thereupon substitutes is also to be found in the foregoing essay. Nor does he intimate this in the elaborate paper on the subject read before the French Association for the Advancement of Science, and published in the Revue Scientifique for the 24th March 1883. The result is that the hypothesis is now currently ascribed to him.[26]

About four months before I had to revise this essay on "The Constitution of the Sun," while staying near Pewsey, in Wiltshire, I was fortunate enough to witness a phenomenon which furnished, by analogy, a verification of the above hypothesis, and served more especially to elucidate one of the traits of solar spots, otherwise difficult to understand. It was at the close of August, when there had been a spell of very hot weather. A slight current of air from the West, moving along the line of the valley, had persisted through the day, which, up to 5 o'clock, had been cloudless, and, with the exception now to be named, remained cloudless. The exception was furnished by a strange-looking cloud almost directly overhead. Its central part was comparatively dense and structureless. Its peripheral part, or to speak strictly, the two-thirds of it which were nearest and most clearly visible, consisted of converging streaks of comparatively thin cloud. Possibly the third part on the remoter side was similarly constituted; but this I could not see. It did not occur to me at the time to think about its cause, though, had the question been raised, I should doubtless have concluded that as the sky still remained cloudless everywhere else, this precipitated mass of vapour must have resulted from a local eddy. In the space of perhaps half-an-hour, the gentle breeze had carried this cloud some miles to the East; and now its nature became obvious. That central part which, seen from underneath, seemed simply a dense, confused part, apparently no nearer than the rest, now, seen sideways, was obviously much lower than the rest and rudely funnel-shaped—nipple-shaped one might say; while the wide thin portion of cloud above it was disk-shaped: the converging streaks of cloud being now, in perspective, merged together. It thus became manifest that the cloud was produced by a feeble whirlwind, perhaps a quarter to half-a-mile in diameter. Further, the appearances made it clear that this feeble whirlwind was limited to the lower stratum of air: the stratum of air above it was not implicated in the cyclonic action. And then, lastly, there was the striking fact that the upper stratum, though not involved in the whirl, was, by its proximity to a region of diminished pressure, slightly rarified; and that its precipitated vapour was, by the draught set up towards the vortex below, drawn into converging streaks. Here, then, was an action analogous to that which, as above suggested, happens around a sun-spot, where the masses of illuminated vapour constituting the photosphere are drawn towards the vortex of the cyclone, and simultaneously elongated into striæ: so forming the penumbra. At the same time there was furnished an answer to the chief objection to the cyclonic theory of solar spots. For if, as here seen, a cyclone in a lower stratum may fail to communicate a vortical motion to the stratum above it, we may comprehend how, in a solar cyclone, the photosphere commonly fails to give any indication of the revolving currents below, and is only occasionally so entangled in these currents as itself to display a vortical motion.

Let me add that apart from the elucidations furnished by the phenomenon above described, the probabilities are greatly in favour of the cyclonic origin of the solar spots. That some of them exhibit clear marks of vortical motion is undeniable; and if this is so, the question arises—What is the degree of likelihood that there are two causes for spots? Considering that they have so many characters in common, it is extremely improbable that their common characters are in some cases the concomitants of vortical motion and in other cases the concomitants of a different kind of action. Recognizing this great improbability, even in the absence of a reconciliation between the apparently conflicting traits, it is, I think, clear that when, in the way above shown, we are enabled to understand how it happens that the vortical motion, not ordinarily implicating the photosphere, may consequently be in most cases unapparent, the reasons for accepting the cyclonic theory become almost conclusive.