Electricity is the most subtle and the least understood of these forms. In its simplest form it appears as the result of friction between dissimilar substances. Thus if we rub a glass rod with a piece of silk, taking care that both are warm and dry, we find that the glass has acquired the property of attracting light bodies, such as little bits of paper, or balls of elder-pith. Other substances, such as sealing-wax and amber, have the same property. Pursuing our research further we find that this influence is not, like that of gravity, uniform and always acting in the same direction, but of two kinds, equal and opposite. If we touch the pith-ball by the excited glass rod, it will after contact be repelled; but if we bring the ball which has been excited by contact with the glass within the influence of a stick of sealing-wax which has been excited by rubbing it with warm dry flannel, the ball instead of being repelled is attracted. Conversely, if the pith-ball has been first touched by excited sealing-wax, it will afterwards be repelled by excited sealing-wax and attracted by excited glass. It is clear, therefore, that there are two opposite electricities, and that bodies charged with similar electricities repel, and with unlike electricities attract, one another. For convenience, one of these electricities, that developed in glass, is called positive, and the other negative; and it has been clearly proved that one cannot exist without the other, and that whenever one electricity is produced, just as much is produced of an opposite description. If positive electricity is produced in glass by rubbing it with silk, just as much negative electricity is produced upon the silk. Another primary fact is that some substances are able to carry away and diffuse or neutralise this peculiar influence called electricity, while others are unable to do so and retain it. The former are called conductors, the latter non-conductors. Thus, glass is an insulator or non-conductor, while metal is a conductor of electricity; and the reason why the substances rubbed together, as glass and silk, must be dry is that water, in all its forms, is a conductor which carries away the electricity as fast as it is produced.