[_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 faculae. 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 developpee par M.
Kirchoff, est fondee jusqu'a un certain point; l'interieur des taches,
si ce sont des lacunes dans la photosphere, doit etre froid
relativement.... Il est donc impossible qu'elles proviennent d'eruptions
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 striae: 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.]