PHYSICS, EIGHTEENTH CENT. ELECTRICITY. 321 



made thoroughly dry and warm. All bodies without exception could 

 be rendered electrical by merely bringing an excited glass tube near 

 them. Gray had discovered this to be the case with some bodies; 

 but Du Fay showed that the fact was general, provided the bodies 

 were supported upon a perfectly dry glass stand. Gray had supposed 

 that differently coloured bodies were attracted in different degrees. 

 Du Fay made an experiment which seemed to confirm this ; but with 

 much sagacity he traced the effect to the substance of the dyes, and 

 not to the colours themselves. Du Fay also describes the electric 

 spark as drawn from a person suspended by non-conducting lines. But 

 the most important of all Du Fay's writings are those wherein he esta- 

 blishes the fact that there are two kinds 

 of electricity, one of which he calls the 

 vitreous and the other the resinous elec- 

 tricity, and that when two bodies are 

 charged with electricity of the same kind, 

 they repel each other; but bodies charged 

 with different electricities attract each other. 

 The facts and the theory will be perfectly 

 clear to any person who will try the follow- 

 ing very simple experiments. Take a fibre 

 of white silk, attach it to the middle of a 

 straw, B, Fig. 156, and hang from some 

 fixed point of support. At each end of 

 the straw fix a little disc of thin paper a 

 quarter of an inch in diameter. The only 

 other apparatus required are : ist, a piece 

 of sealing wax or of amber, or an ebonite 

 paper-knife, penholder, or similar article; 

 2nd, a piece of glass tube, or a common 

 glass tumbler, very dry and warm ; 3rd, a 

 dry or warm silk handkerchief. The ex- 

 periments and their explanation by Du 



Fay's theory are simply these : Rub the glass briskly with the hand- 

 kerchief, and immediately present the glass to one of the paper discs : 

 it will first be attracted into contact with the glass, from which it will 

 receive a charge of vitreous electricity, and will then be strongly re- 

 pelled. If, however, the stick of sealing-wax be rubbed and presented 

 to the same disc which is repelled by the glass, it will be strongly 

 attracted, because the friction has charged the sealing-wax with resinous 

 electricity. On contact with the sealing-wax the disc becomes charged 

 with this kind of electricity, and is repelled ; but in this condition it 

 is strongly attracted by the excited glass. 



The principle propounded by Du Fay opened a wide field for the 

 progress of electrical knowledge, and it threw a light upon all the facts 

 which were known at his time, permitting those facts which had befor 



21 



FIG. 156. 



