320 ATMOSPHEKIC ELECTRICITY. [SECT, xxvni. 



the day than in the night. The intensity increases for two 

 or three hours from the time of sunrise, comes to a maximum 

 between seven and eight, then decreases towards the middle 

 of the day, arrives at its minimum between one and two, and 

 again augments as the sun declines, till about the time of 

 sunset, after which it diminishes, and continues feeble during 

 the 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 conclu- 

 sion, 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 sub- 

 stance in solution. He found, when water contains lime, 

 chalk, or any solid alkali, that the vapour arising from it is 

 negatively electric ; and, when the body held in solution is 

 either gas, acid, or some of the salts, that the vapour given 

 out is positively electric. The ocean must therefore afford 

 a great supply of positive electricity to the atmosphere; but, 

 as M. Becquerel has shown that 'electricity 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 occasion many variations in the electrical state of 

 the atmosphere. 



Clouds probably owe their existence, or at least their form, 

 to electricity, for according to some authors they consist of 

 hollow vesicles of vapour 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 strata of air moving 

 in different directions probably developes electricity ; and, if 

 the strata be of different temperatures, a portion of the vapour 

 they always contain will be deposited ; the electricity evolved 

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

 vesicular state constituting a cloud. A vast deal of electricity 



