34

THE SUN.

We ought, however, to mention an important distinctionbetween the constitution of such prominences as are connectedwith spots and of those which occur upon other parts of theSun ’s surface. The former are generally of an eruptive character,and contain, especially in their lower portions, various othermetallic vapours in addition to that of hydrogen. The latterare of a more quiescent and cloud-like nature, and consistalmost entirely of hydrogen and of one other element (namedhelium), whose 1 nature is at present unknown, hut which givesa bright line in its spectrum near to the yellow lines of sodium.They are probably formed either by the gentle (and not eruptive)emission of hydrogen from the regions beneath them, or bysome action which causes masses of gas floating in the solaratmosphere to change from a dark to a luminous condition.

We have made special reference to the connection betweenSun -spots and prominences, because we believe that an outburstof the latter is very frequently the origin of the former. Notthat a prominence actually breaks out in the exact locality ofa spot, but rather (as Professor Young has recently suggested)that the projection of prominence-matter around a certainlocality causes a diminution of pressure beneath it. This ismost likely followed by a falling-in of some of the clouds of thephotosphere, which may be supposed to consist of more or lesscooled and condensed matter previously carried up in a gaseousform from regions below. Into the cavity currents of rising gasand some of the smaller filaments of the photosphere would thenOy be drawn, occasionally with a vertical or whirlpool-motion, as^ well as a good deal of the heavier vapours of the neighbouringprominence-eruption. There is also, in all probability, a con-siderable indraft of matter, so far cooled as to be in a fine stateof division, which may be somewhat vaguely compared tosmoke.

In this way, although there are many difficulties leftunexplained, and no theory hitherto suggested is reallysatisfactory, we may perhaps obtain a tolerably true idea ofthe nature and origin of a Sun -spot. If so, its beginning-takes place deep down in the gaseous body of the Sun , wheresome disturbance commences, which probably soon raises a