262 History of Luminescence 



(1788) , in which he attributed the aurora to hydrogen, generated 

 by putrefaction of animal and vegetable substances in the tropics, 

 and from volcanoes. The gas was presumably kindled by electricity, 

 as Kirwin indicated that the northern lights, highest of all meteors, 

 were regarded by meteorologists as electrical, following Dr. Frank- 

 lin's conjecture. 



About the same time another gas theory was advanced by Antoine 

 Libes (1752-1832) , who suggested that the aurora was due to vapors 

 of nitric acid which had been formed by electrical discharges through 

 nitrogen and oxygen, when illuminated by the sun. He was led to 

 this theory by observing the play of reddish colors in a flask of nitric 

 oxide in sunlight in 1790. 



In the late 1700's and early 1800's auroral theories returned to 

 the view of J. H. Vallerius (1708) , that the aurora was a reflection 

 of the sun below the horizon. The reflecting particles might be ice 

 crystals or clouds, a theory favored by Julius Ludwig Ideler (1809- 

 1842) , a meteorologist, Privatdocent at the University of Berlin and 

 son of Christian Ludwig Ideler (1766-1846) , the Royal Astronomer 

 and Professor at the same university. J. L. Ideler took this view in 

 his book Ueber den Ursprung des Feuerkugeln und des Nordlicht 

 (Berlin, 1822). He held that cloud-like precipitates became at- 

 tracted to the magnetic poles and arranged themselves in a stable 

 series of forms which are recognized on the earth as northern lights 

 by reflected light. 



However, more modern views were to prevail, well expressed by 

 Thomas Young in A Course of Lectures in Natural Philosophy and 

 the Mechanical Arts (1807: 716) : 



It is doubtful whether the light of the aurora borealis may not be of an 

 electrical nature; the phenomenon is certainly connected with the gen- 

 eral cause of magnetism; the primary beams of light are supposed to be 

 at an elevation of at least 50 to 100 miles above the earth, and every- 

 where in a direction parallel to that of the dipping needle; but perhaps, 

 although the substance is magnetical, the illumination, which renders 

 it visible, may still be derived from passage of electricity, at too great a 

 distance to be discovered by any other test. 



This statement represents the general opinion throughout the 

 nineteenth century up to the time when discovery of new types of 

 particles again raised questions as to the exact mechanism of light 

 emission. A good example of the electro-magnetic theory of the 

 aurora, with a model of the various effects observed, is to be found 

 in the papers of A. De la Rive (1862, 1872). As indicated in a 

 later section, De la Rive was apparently the first (1849) to observe 



