PROBLEMS OF ASTROPHYSICS 451 



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 outbound 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 1 for 

 the sun-spot zones. 



(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. 



(3) A small proportion of the inner, and a large proportion of the 

 outer, coronal light are solar rays reflected and diffracted by the coro- 

 nal particles. 



Arrhenius has recently shown that Abbot's observation 2 of an ap- 

 parent temperature of the corona nearly equal to that of his observing 

 room is in harmony with the spectrographic evidence of an inner 

 corona composed of incandescent particles. Arrhenius 3 finds that one 

 minute dust particle to each 11 cubic meters of space in the coronal 

 region observed by Abbot, raised to the temperature of 4620 absolute 

 required by Stefan's law, would give a corona of the observed bright- 

 ness, and of the observed temperature. The bolometric strip meas- 

 ured the resultant temperature of the few highly heated particles 

 and the cold background of space upon which the particles are seen 

 in projection. 



Arrhenius further estimates that a corona composed of incandescent 

 dust particles need not have a total mass greater than 25,000,000 tons, 

 to radiate the quantity of light yielded by the brightest corona 

 observed. This is approximately that of a cube of granite only 200 

 meters on each side; a remarkably small mass for a volume whose 

 linear dimensions are millions of kilometers. 



This resume of solar theory necessarily overlooks many unsettled 

 points of great significance. Most important of all, perhaps, is that 

 of the solar constant: does it vary, and in accordance with what law? 

 Why is there a sun-spot period, and why are the large spots grouped 

 within limited zones? Why does the form of the corona vary in a 



1 Astrophysical Journal, xi, 231. 



2 Astrophysical Journal, xn, 71-75. 



3 Lick Observatory Bulletin, no. 58. 



