70 CHEMICAL PHYSICS. 



tables, water, moist earth and stones. Non-conductors are : Shellac, 

 rubber, resins, sulphur, wax, glass, silk, wool, porcelain, dry paper 

 and dry air. 



Duality of electricity. For a further study of electrical phe- 

 nomena the simple instrument known as the electric pendulum or 

 pith -ball electroscope may be used. It consists of a pith-ball sus- 

 pended by a silk fibre from an insulated support. When an electri- 

 fied glass rod is brought near the ball, the latter is attracted ; but 

 as soon as it touches the rod the attraction is changed to repulsion, 

 which lasts as long as the ball retains the electricity it has acquired 

 by contact. The same phenomena may be shown by employing any 

 other electrified body, for example a piece of sealing-wax, sulphur, 

 etc., in place of the glass rod. If while the pith-ball exhibits re- 

 pulsion for the glass electrified resin is brought near the ball, it is 

 attracted by the resin ; and when it is repelled by the resin it is 

 attracted by the glass. These phenomena clearly show that the 

 electricity developed on the resin is not of the same kind as that 

 developed on the glass. They exhibit opposite forces toward any 

 third electrified body, each attracting what the other repels. They 

 have accordingly received names which indicate opposition. The 

 electricity which glass acquires when rubbed with silk is called vitre- 

 ous or positive, and that which resin acquires by friction with flannel, 

 resinous or negative electricity. 



On repeating the experiment with other substances, it is found 

 that all electrified bodies behave like either glass or resin. An elec- 

 trified body is spoken of as being charged with electricity ; the 

 charge may be either positive or negative. Experiments show 

 that, whenever electricity of one kind is developed, whether by 

 friction or other means, an equal quantity of the opposite kind is 

 simultaneously developed. Thus, in rubbing glass and silk the glass 

 is charged with positive, the silk with negative electricity. If a 

 conductor receives two charges of electricity of equal quantity but 

 opposite kind, it exhibits no trace of electricity, the two charges 

 having neutralized one another. 



The kind of electricity which a body obtains by friction with 

 another body evidently depends on the nature of the bodies. For 

 instance, if glass be rubbed with cat's skin the glass becomes charged 

 with negative, but if rubbed with silk it becomes charged with posi- 

 tive electricity. In the following " potential series " any one of the 

 bodies named becomes positively electrified when rubbed with one 



