Vlll PREFACE. 



no one has explained what electricity is 

 whether a separate and distinct agent a modi- 

 fication of some other exceedingly refined and 

 more comprehensive principle or a mere 

 effect, condition, or property of ponderable 

 matter. The celebrated Faraday at one time 

 adopted the simple and rational theory of 

 Franklin, that it is a material fluid, definite 

 measures of which belong to each element of 

 ponderable matter. And yet he speaks of it 

 very often as if he considered it to be a com- 

 pound fluid. But when treating of its chemical 

 agency, he represents it as " a modification of 

 the exertion of chemical forces." 



Again ; when by following up the discove- 

 ries of (Erstedt, Davy, Arago, and Schweig- 

 ger, who found that electricity is capable of 

 producing all the phenomena of magnetic 

 action on a small scale, he succeeded in 

 obtaining an electric spark from a permanent 

 magnet ; he arrived at the conclusion that 

 electricity and magnetism are identical ; ex- 

 cept that in the latter " the axis of power" is 

 greater than in the former. Yet, as if not 

 satisfied with any of the foregoing hypotheses, 

 he suggests, at another time, that electricity 

 may be resolved into undulations of an aether ; 



