1845] WRITINGS OF JOSEPH HENRY. 241 



to one end of a wire, the different parts of the wire become 

 charged successivel3% as though a wave of electricity passed 

 along it. He then showed that the charge passed along the 

 surface of the wire, and not through its whole mass, as was 

 supposed from the analogy of galvanic conduction. Hence 

 he inferred that dynamical electricity obeys the same laws 

 as the statical. He then detailed some experiments upon 

 the passage of electricity through plates, and showed that 

 when a charge was transmitted across a plate the tension 

 was greatest at the edges, the electricity apparently exercising 

 a self-repelling action; while if the charge were passed 

 through two pieces of tinfoil, tliese slips attract each other. 



Professor Henry believes that it may be justly inferred 

 from these experiments, that the attraction is due to pon- 

 derable matter, while the repulsion is due to electricity ; 

 thus showing that electricity is a separate principle, and not 

 a mere property of matter. 



He next passed to the subject of the discharge of a jar. 

 It was necessary, in his experiments, to get rid of the free 

 electricity arising from the thickness of the glass, and it 

 occurred to him that this might be done b}' removing the 

 knob, and making the coating upon the inside of less area 

 than that upon the outside. With this arrangement, when 

 the discharge was made through a long wire, and a test jar 

 brought near it during discharge, a bright spark passed ; 

 but upon approaching the jar to a delicate electrometer it 

 gave no indications of free electricity. Reflecting upon this, 

 and upon an experiment of Professor Wheatstone's, he was 

 led to believe that the jar is discharged by two waves, a 

 negative and a positive one, starting simultaneously from 

 the two ends of the wire. To prove this he broke the wire, 

 and interposed a pane of glass dusted with red lead and sul- 

 phur ; two figures of positive and negative electricity were 

 produced. He made several other experiments tending to 

 prove this same fact. He showed how these experiments 

 serve to explain that of Dr. Priestly, where a spark was found 

 to pass between the ends of a long bent wire, the ends being 

 brought within a few inches of each other. 

 16 



