Researches in Electricity 117 



theory, — or as he modestly termed it, a conjecture — pro- 

 posed in his twelfth letter was based upon the fact, which 

 he had in the meantime established, that the charge of 

 clouds was usually negative. When a body, according 

 to Franklin, contains a certain amount of the electric 

 fluid it is neutral or unelectrified. An excess produces 

 the phenomena of positive electrification and deficiency 

 that of negative electrification. Water in its ordinary 

 condition is neutral but if it be converted into vapor 

 without loss of the electric fluid it is capable of con- 

 taining a greater quantity on account of the increase of 

 volume. Clouds formed by the evaporation of unelec- 

 trified water will therefore show a negative charge. 

 Such a cloud coming within striking distance of the 

 earth will receive additional electricity in the form of 

 a flash and will impart the fluid received to other clouds 

 in the neighborhood until equilibrium is established. 

 To account for the occasional positive charge of thunder 

 clouds Franklin imagined that a cloud having had its 

 deficiency of electricity supplied from the earth might 

 be compressed from the action of the wind, " so that part 

 of what it had absorbed was forced out and formed an 

 electric atmosphere around it in its denser state." 



That speculations upon so diflicult a subject as the 

 origin of atmospheric electricity should afford no final 

 theory, even at the hands of a Franklin, was inevitable. 

 The necessary experimental basis for such a result did 



