PROGRESS IN ELECTRICITY 



and will attract silk thread, paper, etc., though ren- 

 dered electrical likewise. Amber, on the contrary, 

 will attract electric glass and other substances of the 

 same class, and will repel gum-sack, copal, silk thread, 

 etc. Two silk ribbons rendered electrical will repel I 

 each other; two woollen threads will do the like; but a ' 

 woollen thread and a silken thread will mutually at- 

 tract each other. This principle very naturally ex- 

 plains why the ends of threads of silk or wool recede 

 from each other, in the form of pencil or broom, when 

 they have acquired an electric quality. From this 

 principle one may with the same ease deduce the ex- 

 planation of a great number of other phenomena ; and 

 it is probable that this truth will lead us to the further 

 discovery of many other things. 



'" In order to know immediately to which of the two 

 classes of electrics belongs any body whatsoever, one 

 need only render electric a silk thread, which is known 

 to be of the resinuous electricity, and see whether that 

 body, rendered electrical, attracts or repels it. If it 

 attracts it, it is certainly of the kind of electricity 

 which I call vitreous; if, on the contrary, it repels it, it 

 is of the same kind of electricity with the silk that is, 

 of the resinous. I have likewise observed that com- 

 municated electricity retains the same properties; for 

 if a ball of ivory or wood be set on a glass stand, and 

 this ball be rendered electric by the tube, it will repel 

 such substances as the tube repels; but if it be ren- 

 dered electric by applying a cylinder of gum-sack near 

 it, it will produce quite contrary effects namely, pre- 

 cisely the same as gum-sack would produce. In order 

 to succeed in these experiments, it is requisite that the 



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