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by the air renders them entirely invisible to vision, even with the telescope, except when, during total eclipses of the sun, the glare is cut off by the intervention of the moon. They may then be seen, even with the naked eye, rising up as if from the black disk of the moon.

The prominences seem to be of two forms, the eruptive and the cloud-like. The first rise from the sun like immense sheets of flame; the latter seem to be at rest above it, like clouds floating in the air. But there is no air around the sun for these objects to float in, and we cannot certainly say what supports them. Very likely, however, it is a repulsive force of the sun's rays, which will be mentioned in a later chapter.

Spectrum analysis shows that these prominences are composed mostly of hydrogen gas, mixed with the vapours of calcium and magnesium. It is to the hydrogen that they owe their red colour. Continued study of the prominences shows them to be connected with a thin layer of gases which surrounds and rests upon the photosphere. This layer is called the chromosphere, from its deep red colour, similar to that of the prominences. As in the case of the latter, most of its light seems to be that of hydrogen; but it contains many other substances in seemingly varying proportions.

The last appendage of the sun to be considered is the corona. This is seen only during total eclipses as a soft effulgence surrounding the sun, and extending from it in long rays, sometimes exceeding the diameter of the sun in length. Its exact nature is still in doubt. It will be described in the chapter on eclipses.