371. =Attraction and Repulsion in Electricity.=--One of the most common effects of electricity is attraction. If we rub a tube or rod of glass with woolen or silk it will attract light articles, such as cotton, feathers, lint, etc., so that they will adhere to it. But repulsion is also an effect of electricity under certain circumstances. In order that the explanation of these two opposite effects may be clear to you, I will detail some of the experiments which exhibit both. Suppose that we have a pith ball, A, Fig. 257, suspended by a silk thread, B, from a stand, C. I must premise that silk does not readily let electricity pass over it, or is a non-conductor, and therefore any electricity communicated to the pith ball will remain there unless something be brought in contact with it or very near it. If now you rub a glass tube, thus exciting electricity upon it, and then bring it near the ball, it will attract the ball to it, and then in a moment repel it, so that it will stand off from the tube and retreat from it if you follow the ball with the tube. Why is this? It is supposed that there is a subtile fluid on the electrified glass, some of which passes to the ball as it touches the glass, so that the ball and the glass are in a similar condition. But the particles of the fluid repel each other; and this is the reason that the ball is repelled from the glass as soon as it becomes charged with a part of the electricity of the glass. For the same reason if two pith balls hanging from a standard become electrified from a glass tube or rod they will repel each other, for they are in the same electrical condition. 372. =Vitreous and Resinous Electricity.=--Suppose now that you rub a rod of sealing-wax with woolen or silk and hold it near a pith ball which has been electrified from glass. It will attract the ball. The reason is that an electricity is excited on the sealing-wax of a different kind from that which is excited on the glass. The former is called _resinous_, and the latter _vitreous_ electricity. They are supposed to be two fluids, which have a strong attraction for each other, while, on the other hand, the particles of either fluid are repellent to each other. It is this attraction between the two fluids which causes in the case just stated the sealing-wax to attract the ball to itself. We can illustrate this attraction in another way. Take two pith balls and electrify them, the one from glass and the other from sealing-wax. Brought near together they will attract each other, because they have two unlike electricities. This, you see, is just the reverse of the effect produced in an experiment cited at the conclusion of § 371, in which the electricities were alike in the two pith balls. Again, if you bring the rubbed sealing-wax near to the ball electrified from glass, the ball will be attracted, and the same effect will follow if you bring the rubbed glass near to the ball electrified from sealing-wax.