392 WELLS'S NATUKAL PHILOSOPHY. 



to negative, and back again in the space of a few min- 

 utes. 



The principal causes which are supposed to 



What is snp- , , . . . , 



posed to occa- producc Blectricity in the atmosphere are, 

 in the atmos- evaporation from the earth's surface, chemical 

 changes which take place upon the earth's 

 surface, and the expansion, condensation, and variation of 

 temperature of the atmosphere and of the moisture con- 

 tained in it. 



"When a substance is burning, positive electricity escapes from it into the 

 atmosphere, wliile the substance itself becomes negatively electrified. Thus 

 the air becomes the receptacle of a vast amount of positive electricity gener- 

 ated in this manner. 



^^ . , The atmosr)here is most hicjhly charged with 



When is the ^ . . ^ i 



atmosphere electricitv whcu hot wcathcr succeeds a series 



most highly - • 



charged with 01 wct davs, Or wet weather lollows a succes- 



electricity? . r ^ -t 



sion 01 dry days. 

 There is more electricity in the atmosphere during the 

 cold of winter than in the summer months. 

 • Lightning is accumulated electricity, generally dis- 

 charged from the clouds to the earth, hut sometimes 

 from the earth to the clouds. 



Who first es- 764. Tlic identity of lightning and electric- 

 identity'* *of ity was first established by Dr. Frankhn, at 

 efeSf^r' Philadelphia, in 1752. 



The manner in which this fact was demonstrated was as fol- 

 DeKribe Frank- jo^-g :_naving made a kite of a large silk handkerchief stretcli- 

 ment ed upou a frame, and placed upon it a pointed iron wire con- 



nected with the string, he raised it upon the approach of 

 ft thunder-storm. A key was attached to the lower end of the hempen 

 Btring holding the kite, and to this one end of a silk ribbon was tied, 

 the other end being fastened to a post. The kite was now insulated, 

 and the experimenter for a considerable time awaited the result with 

 great solicitude. Finally, indications of electricity began to appear on the 

 string; and on Frankhn presenting his knuckles to the key, he received 

 an electric spark. The rain beginning to descend, wet the string, increased 

 its conducting power, and vivid sparks in great abundance Hashed from 

 the key. Franklin afterward charged Leyden jars with lightning, and 

 made other experiments, similar to those usually performed with electrical 

 machines. 



