CHROMOSPHERE AND SOLAR PROMINENCES. 399 



the largest of the clouds of the preceding class. Their spectrum is 

 very complicated, especially near their base, and often filled with 

 bright lines, those of sodium, magnesium, barium, iron, and titanium, 

 being especially conspicuous, while calcium, chromium, manganese, 

 and probably sulphur, are by no means rare, and for this reason Sec- 

 chi calls them metallic prominences. 



They usually appear in the immediate neighborhood of a spot, 

 never occurring very near the solar poles. Their form and appearance 

 change with great rapidity, so that the motion can almost be seen 

 with the eye — an interval of fifteen or twenty minutes being often 

 sufficient to transform, quite beyond recognition, a mass of these 

 flames 50,000 miles high, and sometimes embracing the whole period 

 of their complete development or disappearance. Sometimes they 

 consist of pointed rays, diverging in all directions, like hedgehog 

 spines. 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 snirals 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. Our figures present some of the more common and typi- 

 cal forms, and illustrate their rapidity of change, but there is no end 

 to the number of curious and interesting appearances which they ex- 

 hibit under varying circumstances. 



The velocity of the motions often exceeds 100 miles a second, and 

 sometimes, though very rarely, reaches 200 miles. That we have to 

 do with actual motions, and not with mere change of place of a lumi- 

 nous form, is rendered certain by the fact that the lines of the spectrum 

 are often displaced and distorted in a manner to indicate that some of 

 the cloud-masses are moving either toward or from the earth (and, of 

 course, tangential to the solar surface) with similar swiftness. 



When we come to inquire what forces impart such a velocity, the 

 subject becomes difficult. If we could admit that the surface of the 

 sun is solid, or even liquid, as Zollner thinks, then it would be easy 

 to understand the phenomena as eruptions, analogous to those of vol- 

 canoes on the earth, though on the solar scale. But it is next to 

 certain that the sun is mainly gaseous, and that its luminous surface 

 or photosphere is a sheet of incandescent clouds, like those of the 

 earth, except that water-droplets are replaced by droplets of the 

 metals ; and it is difficult to see how such a shell could exert sufficient 

 confining power upon the imprisoned gases to explain such tremendous 

 velocity in the ejected matter. 



Possibly the difficulty may be met by taking account of the enor- 

 mous amount of condensation which must be going on within the pho- 

 tosphere. To supply the heat which the sun throws off (enough to melt 



