Lesson xx i v.] NORTHBR-N LIGHTS.. 153 



acid, and nitric acid, which give birlh to ruddy 

 vapours, whose red colour will vary according to 

 ihe quantity and proportion of those different sub- 

 stances, which are generated. These vapours are 

 carried towards the south, where the air is most 

 dilated, so that they approach more and more to- 

 wards the spectator; and it is probable their mo- 

 tion may be assisted by a north wind. Sometimes 

 they rise as if to the zenith of the spectator, and 

 then descend again towards the south; and a great 

 number of causes may carry the vapour towards 

 the different points of the heavens, whence origi- 

 nate the different motions taken by the Aurora 

 Borealis, or its several parts. Lastly, the slight 

 detonations which are sometimes heard, depend 

 upon the small quantity of hydrogenous gas, 

 which is found in the upper regions of the atmos- 

 phere, and .which combines with the oxygen to 

 form water. 



These principles, at the same time that they ac- 

 count in. M. L'bcs's estimation, for all thephaeno- 

 mena accompanying the Aurora Borealis, explain 

 also why it is so common towards the poles^ and 

 so rare in the temperate regions; while thunder, 

 which is frequent in the torrid zone, is scarcely eve.* 1 

 heard in the polar. regions. The disengagement 

 of hydrogenous gas is considerable near the equa- 

 tor, and very little towards the poles: and svhea 

 we excite the electric spark in a mixture of hydro- 

 gen, oxygen, and azote, it combines in preference 

 the bases of the two former gases;, the electric 

 spark ought, therefore, to occasion thunder solely 

 H .> in 



