AN ADDRESS ON ASTROPHYSICS. 301 



growth of brilliant faculae and flocculi in the regions of uprush sur- 

 rounding the spot, — a theory first suggested by Secchi ? 



The visible spots are not the sole evidences of circulation. The 

 surface is covered with a network of interstices, or vents between 

 clouds, which probably exercise all the functions of the visible spots, 

 but on a smaller scale. 



There is no reason to question the truth of Young's discovery that 

 the Fraunhofer lines originate in the absorption of a reversing layer — 

 a thin stratum of uncondensed vapors lying immediately over and 

 between the photospheric clouds. 



The chromospheric stratum, several thousand miles in thickness, 

 includes and extends far above the reversing layers, and contains the 

 lighter gases, such as hydrogen and helium, and the vapors of calcium, 

 sodium, magnesium and other elements which do not condense under 

 existing temperatures. 



The prominences have in general the same composition as the 

 chromosphere. In some the lighter gases, and in others the heavier 

 metallic vapors, predominate. They are portions of the chromosphere 

 projected beyond its usual level by the more violent ascending currents, 

 or perhaps by eruptions of a volcanic character; and these forces are 

 almost certainly augmented by the pressure of the sun's radiation. It 

 is difficult to account for the quiescent, cloud-like prominences in 

 regions far above the chromosphere on any supposition other than that 

 they are in equilibrium under the opposing influences of gravity and 

 radiation pressure. 



The nature of the forces which control the general and detailed 

 coronal forms is but little understood. Motion within the corona has 

 never been directly observed. Yet we can not question that the com- 

 ponent particles are driven outward from the sun, and that many of 

 them probably fall back into the sun, either singly or after combining 

 to form larger masses. It is suggested that out-bound particles may 

 be started on their way by the violent solar circulation, continued on 

 their journey by radiation pressure, and arranged in the characteristic 

 streamers under the influence of magnetic forces. 



The light received from the corona is of three kinds : 



1. A small quantity of bright-line radiations from a gas overlying 

 the chromosphere. This gas is unknown to terrestrial chemistry, and 

 astronomers provisionally call it coronium. It is distributed very 

 irregularly over the solar sphere, and shows a decided preference for 

 the sun-spot zone. 



2. The bright-line radiations from coronium are almost a negligible 

 quantity, in comparison with those from the same regions which form 

 a strictly continuous spectrum, and which seem to be due to the 

 incandescence of minute particles heated by the intense thermal radia- 

 tions from the sun. 



