To this conclusion Arago came:
"If I were asked," said the astronomer, "is the sun inhabited? I must reply that I do not know. But, if I were asked whether the sun can be inhabited by beings of organization similar to that of dwellers upon our globe, I should not hesitate to reply in the affirmative."
At the present day Arago would hesitate, for science has made a great advance in the question of the physical constitution of the sun. The new method invented by MM. Kirchhoff and Bunsen, and known as analysis of the luminous spectrum, being applied to the solar rays, has given rise to an entirely new conception of the nature of the sun. We have returned to the opinion of the physicists of the middle ages, who regarded the sun as a globe of fire, a sort of gigantic torch.
It would be impossible to enter into the details of the optical experiments which rendered accurate analysis of the solar rays possible, and enabled us to deduce a new theory of the constitution of the sun from their properties. We shall confine ourselves to explaining this theory, as it evolves itself from the experiments of M. Kirchhoff.
According to the German philosopher, the sun is not, as it has hitherto been supposed, a cold, dark, and solid body, surrounded by a burning atmosphere; it is a globe, a sphere, probably liquid, which burns throughout its whole mass, and in all its parts. This incandescent globe is surrounded by a very heavy atmosphere, formed of the vapours which proceed from the incandescent globe, and which are themselves kept burning in consequence of the high temperature of all those masses of fire.
How are the spots on the sun to be explained according to this theory? M. Kirchhoff admits that, owing to unknown causes, a cooling process may take place in the vaporous atmosphere which surrounds the body of the sun. This cooling process would form at certain points condensations of vapour analogous to the condensation of the vapour of water, which on our globe produces clouds and rain. These agglomerations of condensed vapours would form a species of cloud in the atmosphere of the sun, and those clouds, which would intercept the light of the solar disc from us, would produce the effect of a spot on this disc. The cloud, once formed, cools portions of the neighbouring vapours, and, by provoking a partial condensation, gives rise to the penumbra which surround the umbra. Thus, according to M. Kirchhoff, the solar spots are clouds suspended in the sun's atmosphere. Galileo had previously propounded an analogous hypothesis. Without abandoning M. Kirchhoff's theory we may mention another explanation of the spots. A German physicist considers the spots, not as clouds in the sun's atmosphere, but as partial solidifications of the liquid matter which forms the body of the sun; a kind of scoria, analogous to those which may be observed in crucibles containing matters in a state of fusion, and which come from particles of metal not yet melted, or which are beginning to solidify. The penumbra of the spots would be the half-molten, and consequently, half-transparent pollicule which always surrounds the edges of metallic scoria with a semi-liquid ring.
M. Faye, a French astronomer, has propounded a theory, which somewhat modifies that of M. Kirchhoff. He thinks that the nucleus of the sun is neither solid nor liquid, but entirely gaseous. The solar spot, he, like M. Kirchhoff, takes to be an opening made accidentally in the sun's atmosphere by the condensation of vapours on certain points of that atmosphere. According to M. Faye, the spots are due to vertical currents of vapour ascending and descending, and the interception of the light of the sun's atmosphere by the predominant intensity of the ascending current.[7]
The new theory, the result of the optical experiments of the German physicists, appears to explain all the facts which have been observed, and it has therefore been generally accepted. Some divergences exist on questions of detail, but astronomers are nowadays almost unanimous in regarding the sun as a great body, incandescent in all its parts, as a globe in a state of fusion, surrounded by a burning atmosphere, or, as M. Faye states it, a simple agglomeration of incandescent gases.
