200. _Eruptive Prominences._--The _eruptive_ prominences ordinarily consist of brilliant spikes or jets, which change very rapidly in form and brightness. As a rule, their altitude is not more than twenty thousand or thirty thousand miles; but occasionally they rise far higher than even the largest of the quiescent protuberances. Their spectrum is very complicated, especially near their base, and often filled with bright lines. The most conspicuous lines are those of sodium, magnesium, barium, iron, and titanium: hence Secchi calls them _metallic_ prominences. [Illustration: Fig. 220.] They usually appear in the immediate vicinity of a spot, never very near the solar poles. They change with such rapidity, that the motion can almost be seen with the eye. Sometimes, in the course of fifteen or twenty minutes, a mass of these flames, fifty thousand miles high, will undergo a total transformation; and in some instances their complete development or disappearance takes no longer time. Sometimes they consist of pointed rays, diverging in all directions, as represented in Fig. 220. "Sometimes they look like flames, sometimes like sheaves of grain, sometimes like whirling water-spouts capped with a great cloud; occasionally they present most exactly the appearance of jets of liquid fire, rising and falling in graceful parabolas; frequently they carry on their edges spirals like the volutes of an Ionic column; and continually they detach filaments, which rise to a great elevation, gradually expanding and growing fainter as they ascend, until the eye loses them." [Illustration: Fig. 221.] 201. _Change of Form in Prominences._--Fig. 221 represents a prominence as seen by Professor Young, Sept. 7, 1871. It was an immense quiescent cloud, a hundred thousand miles long and fifty-four thousand miles high. At _a_ there was a brilliant lump, somewhat in the form of a thunder-head. On returning to the spectroscope less than half an hour afterwards, he found that the cloud had been literally blown into shreds by some inconceivable uprush from beneath. The prominence then presented the form shown in Fig. 222. The _débris_ of the cloud had already attained a height of a hundred thousand miles. While he was watching them for the next ten minutes, they rose, with a motion almost perceptible to the eye, till the uppermost reached an altitude of two hundred thousand miles. As the filaments rose, they gradually faded away like a dissolving cloud. [Illustration: Fig. 222.] Meanwhile the little thunder-head had grown and developed into what appeared to be a mass of rolling and ever-changing flame. Figs. 223 and 224 give the appearance of this portion of the prominence at intervals of fifteen minutes. Other similar eruptions have been observed. [Illustration: Fig. 223.] [Illustration: Fig. 224.] V. THE CORONA.