i6o SECTION PHYSICS. 



As regards the theory of polar light, the knowledge we have 

 acquired of the electric state of the earth proves that it is a conducting 

 body, charged with a small quantity of negative electricity, and 

 surrounded by the atmosphere, in general charged with positive 

 electricity. Though this latter might be produced by an influence 

 from the earth, it is still very probable that it proceeds from the- 

 process of evaporation, either directly by this phenomenon itself, or 

 by the friction of vapour against particles of air. The atmospheric 

 air possesses a very small conducting power for electricity when dry 

 and of ordinary pressure, but the conducting power increases con- 

 siderably as soon as the air becomes moist and rarefied. It has been 

 proved by experiments that the conducting power is highest at a 

 pressure between 5 mw and io ram , and goes then 10,000 times beyond 

 the conducting power at a pressure of 760""". If the rarefaction of 

 the air is carried further than 5 mm , the conducting power diminishes 

 again, but very slowly. It is known that in proportion to its elevation 

 over the surface of the earth the air becomes more and more rarefied 

 according to an irrefragable law, which finds its expression in the 

 formula given by Laplace, and that consequently, at a certain elevation 

 the earth is surrounded by a layer of air that has a pressure of only 

 5 ram ; the conducting power for electricity in this layer is sufficiently 

 great to allow of its being regarded as a conductor in comparison to 

 the air in lower regions, and even in the highest. The negative 

 electric earth is thus surrounded by a conductor for electricity con- 

 centric with it. All the positive electricity that attains the space of 

 rarefied air of about 5 mm , or, as it might be called, this conductor of 

 air, submits almost to the same laws as if it were in a real conductor, 

 and must thus set in a restricted manner according to the influence of 

 the electro-negative earth. Part of the electricity, conducted by the 

 vapours, remains on the clouds in the atmosphere and discharges in 

 form of lightning and thunder ; another part attains the space of 

 rarefied air or conductor of air, by the fact that the vapour itself, sub- 

 mitting to well-known physical laws, rises to this elevation, and also 

 because electricity, according to its nature, endeavours always to set 

 on something. 



The manner in which the electricity divides itself on the two 

 conductors depends on their reciprocal position to each other as well 



