KNOWLEDGE OF THE SUN ST. JOHN 185 



great that even at the hi^h temperature the density is estimated to 

 be approximately twenty-ei<]:ht times that of water, though tiie mean 

 density is only 1.4. The radiation pressure bears a very appreciable 

 ratio to the weight of the superincumbent material. 



The photosphere is a relatively thin gaseous shell, through which 

 radiation from tiie interior does not pass directly, its optical opacity 

 being due to its highly ionized state (Stewart). The temperature of 

 its surface is api)roximately 0,000° K, and the pressure there a 

 thousandtii of an atnjosphere. The total mass (»f the overlying gases 

 according to Russell is approximately 10'" tons. The mass of the 

 earth's atmosphere is three times as great, but on the sun the pressure 

 (mass X solar gravity) is distributed over an area twelve thousand 

 times greater. As the sun's atmosphere is approximately 10- times 

 as deep, its volume is 12X10^^ as large, hence its average density is 

 extremely small, about 2X10"^^ 



The reversing layer is a few hundred kilometers in thickness and 

 contains all the elements found in the sun's atmosphere. In it the 

 vast majority of the Fraunhofer lines originate. It is supported be- 

 low mainly by gas pressure but above increasingly by the pressure of 

 radiation. The reversing la3'er merges gradually into the chromo- 

 sphere, which reaches 12,000-14,000 kilometers in height, and is sup- 

 ported by radiation pressure. The eclipse spectra of the chromo- 

 sphere (pi. 3) show conspicuously the emission lines of the lighter 

 gases, hydrogen and helium, and of ionized elements such as Ca + 

 and Ti + . The pressure at the top of the Ca + atmosphere is of the 

 order of 10" atmosphere (Milne) and at the photosphere 10"^ atmo- 

 sphere (St. John and Babcock). 



Prominences are luminous vapors, mainly hydrogen and ionized 

 calcium, that often rise above the chromosphere to gi'cat heights, 

 reaching at times to more than half the solar radius. (PI. 0, fig. 1.) 

 Formerly these were observed only at times of total eclipse of the 

 sun, but now are photographed daily with the spectroheliograph. 

 Their composition is known but there remains much to be explained 

 as to the forces that originate and support them and control their 

 movements. Dark masses of hydrogen are often photographed on the 

 disk of the sun. (PI. 0, fig. 2.) They were long suspected to bo 

 prominences seen in projection on the disk. This relation to promi- 

 nences became clear through such fortunate observations as shown 

 in Plate 7, Figure 1, where a dark hydrogen cloud was caught being 

 carried off the disk by the sun's rotation and later in Plate 7, Figure 

 2. when completely off the disk and bright. The motions of bolh 

 dark and bright hydrogen flocculi are now visually studied by means 

 of the spectrohelio.scope recently developed by Hale. 



