32 THE POPULAR SCIENCE MONTHLY. 



* reous surface. Rub your sealing-wax with vulcanized India-rubber, 

 the electricity of glass will be found upon the resinous surface. 



We now use the term positive electricity to denote that developed 

 on glass by the friction of silk ; and negative electricity to denote that 

 developed on sealing-wax by the friction of flannel. These terms are 

 adopted purely for the s,ake of convenience. There is no reason in 

 Nature why the resinous electricity should not be called positive, and 

 the vitreous electricity negative. Once agreed, however, to apply the 

 terms as here fixed, we must adhere to this agreement throughout. 



Sec. 10. Fundamental Law of Electric Action. In all the expeii- 

 ments which we have hitherto made, one of the substances has been 

 electrified by friction, and the other not. But, once engaged in inqui- 

 ries of this description, questions incessantly occur to the mind, the 

 answering of which extends our knowledge, and suggests other ques- 

 tions. Suppose, instead of exciting only one of the bodies presented 

 to each otber, we were to excite both of them, what would occur ? 

 This is the question which was asked and answered by Du Fay, and 

 which we must answer for ourselves. 



Here your wire loop (Fig. 1), comes again into play. Place an 

 unrubbed gutta-percha tube, or a stick of sealing-wax, in the loop, 

 and be sure that it is unrubbed that no electricity adheres to it from 

 former experiments. If it fail to attract light bodies, it is unexcited ; 

 if it attract them, pass your hand over it several times, or, better still, 

 pass it over or through the flame of a spirit-lamp or candle. This 

 will remove every trace of electricity. Attract the unrubbed gutta- 

 percha tube by a rubbed one. 



Remove the unrubbed tube from the loop, and excite it with its 

 flannel rubber. One end of the tube is held in your hand, and is there- 

 fore unexcited. Return the tube to the loop, keeping your eye upon 

 the excited end. Bring a second rubbed tube near the excited end 

 of the suspended one : strong repulsion is the consequence. Drive the 

 suspended tube round and round by this force of repulsion. 



Bring a rubbed glass tube near the excited end of the gutta-percha 

 tube : strong attraction is the result. 



Repeat this expei'iment step by step with two glass tubes. Prove 

 that the rubbed glass tube attracts the unrubbed one. Remove the 

 unrubbed tube from the loop, excite it by its rubber, return it to the 

 loop, and establish the repulsion of glass by glass. Bring rubbed 

 gutta-percha or sealing-wax near the rubbed glass : strong attraction 

 is the consequence. 



These experiments lead us directly to the fundamental law of elec- 

 tric action, which is this: Bodies charged with the same electricity 

 repel each other, while bodies charged with opposite electricities 

 attract each other. Positive repels positive, and attracts negative. 

 Negative repels negative, and attracts positive. 



Devise experiments which shall still further illustrate this funda- 



