Page:Popular Science Monthly Volume 1.djvu/161

Rh the universe, which has been built around him and for him, and with reference to him as the supreme object. This led to the belief in the inhabitability of worlds; for, if stars and planets were not for man to live on, what can they be good for! And, hence, solar theories were so framed that it might be possible to conceive of the sun as the dwelling-place of man. The nucleus or body of the sun was assumed to be opaque and solid, or, at all events, that it had a solid shell corresponding to the earth's crust. Surrounding the spherical nucleus at a certain distance above it, there was supposed to be a first atmosphere which may be compared to the earth's atmosphere when the latter is occupied by a continuous layer of opaque, reflecting clouds. Above this first layer, and more or less distant from it, there was held to be a second atmosphere which is luminous, and answers to the photosphere, or the visible periphery of the solar orb.

On this view a solar spot results from a rent in the atmospheric layers, by which the dark nucleus becomes visible, forming the umbra. The rupture of the two atmospheres, it was supposed, might be caused by volcanic action, or by "gaseous matter formed from time to time at the surface of the dark nucleus, the high temperature of which causes its deflagration." Again it was said: "It may happen that the opening is wider in the cloudy atmosphere than in the luminous envelope or photosphere, in which case the dark nucleus alone would be seen, and we should have a spot without a penumbra. Or the rupture of the first gray envelope becoming closed before that of the photosphere, would have for effect to shut out the view of the dark globe, and we should have a penumbra without a nucleus."

The later tendency is to abandon the notion of a dark nucleus. Indeed, the explanations of the spots now most in favor recall that of Galileo: he suggested a floating scum, while all the late physicists hold that the spots are due to the agency of precipitated clouds.

Kirchhoff, whose honor it is to have first applied spectrum analysis to the study of the sun, and discovered its chemical elements, maintains that the visible portion of the sun, the surface which constitutes the photosphere, is a solid or liquid sphere in a state of incandescence. Its temperature is very high, and it is surrounded by a dense atmosphere formed of the elements which constitute the incandescent globe itself, whose extremely high temperature maintains them in a state of vapor or gas. The lines of the solar spectrum, instead of being bright and variously colored, are dark, which proves that the light has passed through a medium of absorption. Kirchhoff's view is, that the light emanates from the solid or liquid photosphere, and is filtered of its colors or has its lines reversed in passing through the sun's atmosphere.

He explains the spots by supposing that, from some unknown cause, certain parts of the sun's surface undergo a temporary cooling, by which clouds are condensed above that intercept the rays and then