Essays: Scientific, Political, & Speculative (Vol. 1-3). Spencer Herbert

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СКАЧАТЬ 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 СКАЧАТЬ