256 History of Luminescence 



Or, if you will this Meteor (or one very like it) is thus generated, 

 viz. when a great quantitie of hot and drie Exhalation (which indeed 

 may fitly be called a drie cloud) is set on fire in the midst, and because 

 the cloud is not so compact that it should suddenly rend, as when 

 thunder is caused, the fire breaks out at the edges of it, kindling the 

 thin Exhalations which shoot out in great number like to fiery spears 

 or darts, the streaming or flashing being so much the whiter by how 

 much the Exhalation is the thinner. Such like coruscations as these we 

 use to see many nights in the North and North-east parts of the skie. 



One of the first eighteenth-century theories to account for an 

 aurora was presented by Harald Vallerius (1646-1716) , professor of 

 mathematics at the University of Upsala, in a small tract Exercitium 

 Philosophicum de Chasmatibus (Upsala, 1708) . He listed eighteen 

 characteristics of the aurora of 1707, which he explained as due to 

 the reflection of the rays of the sun (below the horizon) by thin, 

 light, six-sided ice plates, floating in the upper atmosphere, a view 

 also supported (1724) by J. C. Spidberg (1684-1762), the Bishop 

 of Christiansand in Norway. 



The paper of Vallerius was little known in Europe, but the 

 auroras of 1716 and 1717 were seen over a vast area, and gave rise 

 to many publications and much speculation in all countries. ^^ One 

 of the most extensive series of observations was by William Whiston 

 (1667-1752) , a divine who followed Newton as Lucasian Professor 

 of Mathematics at Cambridge in 1703, and was expelled in 1710. 

 His booklet (1716) of seventy-eight pages was entitled, An Account 

 of a Surprising Meteor Seen in the Air, March the sixth, 1715-16 

 at Night. 



Whiston recognized the similarity of the aurora to other lumines- 

 cences and in fact referred to it as " an aerial phosphorus." The 

 phenomenon was believed to result from 



no other than such Exhalations are arose out of the Earth some Time 

 before; and which at this time got together, as Vapours do before they 

 compose dense Clouds for Rain, or Matter for great Storms ... as for 

 the Nature of these Exhalations, it seems plainly to be like that of the 

 Ingredients of Gunpowder, Nitre and Sulphur, especially the latter: 

 Which as they are proper in Fermentation to cause Light, and if that 

 Fermentation be very violent, Thunder and Lightning also . . . properly 

 exhibiting such a thin Light in the Northern cold Countries, as they do 

 Thunder and Lightning when they are more ripened in the Southern hot 

 Ones. 



^''Christian Wolf (1679-1754), professor of physics at Halle, explained (1716) the 

 appearance as " a thunder and lightning storm [Gewitter] which has not attained full 

 strength," a pronouncement which has been repeated and over-emphasized because 

 of the great authority of Wolf. 



