CHROMOSPHERE AND SOLAR PROMINENCES. 397 



Sometimes they appear to lie upon the limb of the sun like a bank 

 of clouds in the horizon ; probably because they are so far from the 

 edge of the disk that only their upper portions are in sight. When 

 seen in their full extent they are ordinarily connected to the under- 

 lying chromosphere by slender columns, which are usually smallest 

 at the base, and appear often to be made up of separate filaments 

 closely intertwined, and expanding upward. Sometimes the whole 

 under surface is fringed with down-hanging filaments, which remind 

 one of a summer shower falling from a heavy thunder- cloud. Some- 

 times they float entirely free from the chromosphere ; indeed, as a 

 general rule, the layer clouds are attended by detached cloudlets for 

 the most part horizontal in their arrangement. 



The figures give an idea of some of the general appearances of 

 this class of prominences, but their delicate, filmy beauty can be ade- 

 quately rendered only by a far more elaborate style of engraving. 



Their spectrum is usually very simple, consisting of the four lines 

 of hydrogen and the orange D 3 — hence the appellation hydrogenous. 

 Occasionally the sodium and magnesium lines also appear, and that 

 even near the summit of the clouds; and this phenomenon was so 

 much more frequently O'bserved in the clear atmosphere of Sherman as 

 to suggest that, if the power of our spectroscopes were sufficiently in- 

 creased, it would cease to be unusual. 



The genesis of this sort of prominence is problematical. They 

 have been commonly looked upon as the d&bris and relics of eruptions, 

 consisting of gases which have been ejected from beneath the solar 

 surface, and then abandoned to the action of the currents of the sun's 

 upper atmosphere. But near the poles of the sun distinctively erup- 

 tive prominences never appear, and there is no evidence of aerial cur- 

 rents which would transport to those regions matters ejected nearer 

 the sun's equator. Indeed, the whole appearance of these objects indi- 

 cates that they originate where we see them. Possibly, although in 

 the polar regions there are no violent eruptions, there yet may be a 

 quiet outpouring of heated hydrogen sufficient to account for their 

 production — an outrush issuing through the smaller pores of the solar 

 surface, which abound near the poles as well as elsewhere. 



But Secchi reports an observation (not yet, however, confirmed by 

 other spectroscopists, so far as we know) which, if correct, puts a very 

 different face upon the matter. He has seen isolated cloudlets form 

 and grow spontaneously without any perceptible connection with 

 the chromosphere or other masses of hydrogen, just as in our own 

 atmosphere clouds form from aqueous vapor, already present in the 

 air, but invisible until some local cooling or change of pressure causes 

 its condensation. Granting the correctness of the observation, these 

 prominences are, therefore, formed by some local heating or other 

 luminous excitement of hydrogen already present, and not by any 

 transportation and aggregation of materials from a distance. The 



