Electroluminescence 259 



De Mairan held that he had a right to demand an explanation of 

 these difficulties from those who believed electricity to be the cause 

 of the aurora and asked " Can the sun produce the effects of elec- 

 tricity here on earth? " 



De Mairan was pleased that the hypotheses of Eusebia Sguario 

 (1738) , of R. J. Boscovich (1738) and C. Noceti (1747) in a poem 

 differed little from his own, but spent much time in refuting that 

 of Leonard Euler (1746) , who believed the aurora was made up of 

 subtile parts of the earth's atmosphere rather than the sun's atmos- 

 phere, for he could not conceive of the sun's atmosphere extending 

 to the earth. The aurora was much like the tails of comets, made 

 up of " opaque particles [from the earth] receiving the light of the 

 sun and perhaps emitting their own light, as they may vibrate in 

 the new medium and may be ignited." 



THE AURORA AND THE ELECTRIC LIGHT 



De Mairan and others had overlooked the most important de- 

 velopment of his century. Francis Hauksbee's (1709) observations 

 of the " electric light " in evacuated tubes and the general use of 

 electrical machines had given a great impetus to the study of elec- 

 tricity. Just as electric discharges in air resemble thunder and light- 

 ning, so under certain conditions the discharge in vacuo greatly 

 resembles an aurora. Halley called attention to the similarity, and 

 another early comparison was made by G. M. Bose (1744) , after 

 observing the light which " flowed and turned and wandered and 

 flashed " in evacuated glass vessels as a result of electric discharges. 



It is not surprising to find that Benjamin Franklin (1706-1790) , 

 independently, had definite views on the aurora, nor that he re- 

 garded the display as an example of electric fire which is diffuse 

 rather than concentrated as in a lightning flash. In a long letter (V) , 

 addressed to Dr. John Mitchel, F. R. S. London, dated Philadelphia, 

 April 29, 1749, Franklin stated his " Observations and suppositions 

 toward forming a new Hypothesis for explaining the several Phe- 

 nomena of Thunder-Gusts." Among other things he wrote: 



When the air, with its vapours raised from the ocean between the 

 tropics, comes to descend in the polar regions, and to be in contact with 

 the vapours arising there, the electrical fire they brought begins to be 

 communicated, and is seen in clear nights, being first visible where 'tis 

 first in motion, that is, where the contact begins, or in the most northern 

 part; from thence the streams of light seem to shoot southerly, even up 

 to the zenith of northern countries. But though the light seems to shoot 

 from the north southerly, the progress of the fire is really from the south 

 northerly, its motion beginning in the north, being the reason that 'tis 

 there seen first. . . . This is supposed to account for the Aurora borealis. 



