214 Mr. William Euggins on the Solar Corona, [Feb. 20, 



view of the corona would bring it within the charmed circle of inter- 

 action which seems to obtain among the phenomena of sun-sjiots 

 and terrestrial magnetic disturbances and auroras. 



Many questions remain unconsidered ; among others, whether the 

 light emitted by the gaseous part of the corona is due directly to the 

 sun's heat, or to electrical discharges taking place in it of the nature 

 of the aurora. Further, what becomes of the coronal matter on the 

 theory which has been suggested ? Is it permanently carried away 

 from the sun, as the matter of the tails of comets is lost to them? 

 Amoncy other considerations it may be mentioned that electric re- 

 pulsion can maintain its sway only so long as the repelled particle 

 remains in the same electrical state : if through electric discharges it 

 ceases to maintain the electrical potentiarl it possessed, the repulsion 

 has no more power over it, and gravity will be no longer mastered. 

 If, when this takes place, the particle is not moving away with a 

 velocity sufficiently great to carry it from the sun, the particle will 

 return to the sun. Of course, if the effect of any electric discharges 

 or other conditions has been to change the potential of the particle 

 from positive to negative, or the reverse, as the case may be, then the 

 repulsion would be changed into an attraction acting in the same 

 direction as gravity. In Mr. Wesley's drawings of the corona, 

 especially in those of the eclipse of 1871, the longer rays or streamers 

 appear n-t to end, but to be lost in increasing faintness and diffusion, 

 but certain of the shorter rays are seen to turn round and to descend 

 to the sun.* 



It is difficult for us living in dense air to conceive of the state of 

 attenuation probably present in the outer parts of the corona. Mr, 

 Johnstone Stoney has calculated that more than twenty figures are 

 needed to express the number of molecules in a cubic centimetre of 

 ordinary air, and Mr. Crookes shows us in his tubes that matter, 

 even wdien reduced to one-millionth part of the density of ordinary 

 air, can become luminous under electrical excitement. [A glass bulb 

 about 4 inches in diameter, kindly lent to me by Mr. Crookes, was 

 exhibited, in which a metal ball about half an inch in diameter 

 formed the negative pole. Under a suitable condition of the induc- 

 tion current, this ball was seen to be surrounded by a corona of 

 blueish-grey light which was sufficiently bright to be seen from all 

 parts of the theatre.] Yet it is probable that these tubes must be 

 looked upon as crowded cities of molecules as compared with the 

 sparse molecular population of the great coronal wastes. 



I forbear to speculate further, as we may expect more information 

 as to the state of things in the corona from the daily photographs 

 which will be shortly commenced at the Cape of Good Hoj^e by Mr. 

 Kay Woods under the direction of Dr. Gill. [W. H.] 



* For a history of opinion of the nature of the corona, see Papers by Prof. 

 Norton, Prof. Young, and Prof. Langky in the 'American Journal of Science'; 

 also 'The Sun/ by Prof. Young; and 'The Sun the liulcr of the Pliiuetary 

 Syateni,' and various essays by Mr. R. A. Proctor. 



