SKCT. XXVni. ATMOSPHERIC ELECTRICITY. 281 



night,/ Atmospheric electricity arises partly from an 

 evolution of the electric fluid during the evaporation 

 that is so abundant at the surface of the earth, though 

 not under all circumstances. M. Pouillet has recently 

 come to the conclusion, that simple evaporation never 

 produces electricity, unless accompanied by chemical 

 action, but that electricity is always disengaged when 

 the water holds a salt or some other substance in solu- 

 tion, f He found when water contains lime, chalk, or 

 any solid alkali, that the vapor arising from it is nega- 

 tively electric ; and when the body held in solution is 

 either gas, acid, or some of the salts, that the vapor 

 given out is positively electric. / The ocean must there- 

 fore afford a great supply of"positive electricity to the 

 atmosphere ; but as M. Becquerel has shown that elec- 

 tricity of one kind or other is developed, whenever the 

 molecules of bodies are deranged from their natural 

 positions of equilibrium by any cause whatever, the 

 chemical changes on the surface of the globe must occa- 

 sion many variations in the electrical state of the atmos- 

 phere. 



Clouds probably owe their existence, or at least their 

 form, to electricity, for according to some authors they 

 consist of hollow vesicles of vapor coated with it. As 

 the electricity is either entirely positive or negative, the 

 vesicles repel each other, which prevents them from 

 uniting and falling down in rain. The friction of the 

 surfaces of two gjrata of air moving in different direc- 

 tions, probably developes electricity; and if the strata 

 be of different temperatures, a portion of the vapor they 

 always contain will be deposited ; the electricity evolved 

 will be 'taken up by the vapor, and cause it to assume 

 the vesicular state constituting a cloud. A vast deal of 

 electricity may be accumulated in this manner, which 

 may be either positive or negative. When two clouds, 

 charged with opposite kinds, approach within a certain 

 distance, the thickness of the coating of electricity in- 

 creases on the two sides of the clouds that are nearest 

 to one another; and when the accumulation becomes 

 so great as to overcome the coercive pressure of the 

 atmosphere, a discharge takes place, which occasions a 

 flash of lightning. The actual quantity of electricity in 



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