224 ESSAYS BIOGRAPHICAL AND CHEMICAL 



equator, however, the light will fade, owing to the spread- 

 ing and weakening of these lines. 



An imperfect attempt has been made to imitate the 

 auroral phenomena, but it nevertheless shows in some 

 degree the appearance of the northern lights. A globe, 

 containing krypton at very low pressure, is suspended 

 between the poles of a powerful electro- magnet ; 1 a ring, 

 consisting of five or six coils of covered wire, is laid on 

 the top of the globe, and by help of an induction-coil and 

 a Leiden jar, strong induction discharges are made to 

 pass through the coil. Each discharge is accompanied by 

 a circular discharge in the interior of the globe. The 

 effect is that of a ring or halo near the upper side of the 

 jar. On 'making' the electro-magnet, the ring sends 

 out streamers, exactly similar in appearance to auroral 

 streamers, and like them they have a rotary motion and 

 flicker, shortening and lengthening, just as natural 

 streamers do. If it were possible to place a magnetic 

 model of the earth inside such a globe, I doubt not that 

 the streamers would follow directions similar to those in 

 the figure, and that the imitation would still more closely 

 resemble the reality. The light evolved from pure 

 krypton, under the influence of such discharges, is of a 

 whitish steel-blue colour with occasional green and lilac 

 flickers, and it also recalls the appearance of the natural 

 aurora. But, as already remarked, it is more than 

 probable that the spectrum of the aurora, seen at different 

 times and in different altitudes, may show not only the 

 spectrum of krypton, but also those of the other atmo- 

 spheric gases. 



One more remark before concluding. The temperature 

 of the upper atmosphere is undoubtedly very low ; but at 

 such altitudes even xenon, the least volatile of the atmo- 

 spheric gases, possesses so high a vapour pressure that it 



1 The electro'inagnet belonging to a small 1 h.p. dynamo was used. 



