The electricity upon the surface of glass is called positive electricity; that upon rubber, negative electricity. When silk is rubbed upon glass it receives a negative charge from the glass and confers a positive charge upon the silk. Wool or fur rubbed on wax or rubber receives a positive charge in exchange for a negative charge; “equal and opposite charges of electricity are always produced.” A piece of glass and a piece of silk attract one another; two pieces of silk or two pieces of glass or wax repel one another, because a body which is positively charged is attracted by one negatively charged and repelled by one negatively charged, and vice versa. A piece of glass rubbed by a piece of silk, under suitable conditions, attracts any other body with which it has not been in contact. The piece of silk will do likewise. In all these cases, the attraction or repulsion becomes weaker with increase of distance between the attracting and repelling bodies. A third body which has been in contact with a piece of glass or a piece of silk acquires to some extent the properties of the glass or silk with which the third body has been in contact. And, conversely, the glass or silk with which the third body has been in contact attracts or repels with less force than before. If a hand is drawn over the surface of an object after it has been charged with electricity, the electricity disappears. It has been conducted through the hand and the body to the earth. This phenomenon shows that the human body is a _conductor_ of electricity. But most metals are much better conductors. Moist air and damp wood are rather poor conductors, while dry air, dry wood, porcelain, glass, hard rubber and sealing-wax are _non-conductors_, or _insulators_. The term _dielectric_ is used in preference to _insulation_ when reference is made to the property of transmitting _induction_—a process quite distinct from ordinary transmission of an electric current. In _electrostatic induction_, a body electrostatically charged induces in a neighboring conductor a like charge in the parts farthest from the charged body, and an unlike charge in the nearer parts; the repelled like charge being removed by connecting any part of the conductor momentarily with the earth, while the bound unlike charge spreads over the whole surface of the conductor and remains there even when the inducing body is moved away, or its charge neutralized, if the conductor is properly insulated.