LECTURE IX. 



EXPERIMENTAL PHILOSOPHY 



ELECTRICAL PHENOMENA AND GALVANISM. 



SOME of you will, I doubt not, be disposed to 

 remind me, that I have neglected to explain why 

 the electrical machine exhibited different effects 

 when the chain, which communicates with the 

 earth, was put over the prime conductor, from 

 those which take place in its ordinary mode of 

 operation, when the chain was connected with 

 the cushion. 



In a very early stage of the science, two kinds 

 of electricity were observed, or, according to Dr. 

 Franklin's theory, two different effects from the 

 same cause. A ball of rosin or sealing-wax, and 

 a globe of glass, when excited, will each of them 

 electrify ; but the electricity produced from each 

 will differ in some of its effects. Thus, if we 

 electrify two cork balls, suspended by silken 

 threads, with the same substance, either glass or 

 sealing-wax, they will mutually repel each other ; 

 but if one of them is electrified with glass, and 

 the other with sealing-wax, they will be mu- 

 tually attracted. From this circumstance it was 

 conjectured at first, that there were two kinds 

 of electricity ; that from glass was called the 



