On the Connexion of the Physical Sciences

Bodies surrounded by non-conductors are said to be insulated, because when charged the electricity cannot escape. When that is not the case, the electricity is conveyed to the earth: consequently it is impossible to accumulate electricity in a conducting substance that is not insulated. There are a great many substances called non-electrics in which electricity is not sensibly developed by friction unless they be insulated, because it is carried off by their conducting power as soon as elicited. Metals, for example, which are said to be non-electrics can be excited, but being conductors they cannot retain this state if in communication with the earth. It is probable that no bodies exist which are either perfect non-electrics or perfect non-conductors. But it is evident that electrics must be non-conductors to a certain degree, otherwise they could not retain their electric state.