XI. _Of medical Galvanism._ 154 XII. _The identity of Galvanism with electricity considered._ 157 EPITOME OF ELECTRICITY. DIVISION I. CHAP. I. _Explanation of Terms; with some general Remarks._ If a glass tube be rubbed in the dark with a dry hand or piece of buckskin, upon applying the knuckle to it a luminous stream or spark will appear, passing from the glass to the knuckle, attended with a crackling noise: this luminous spark or stream is called electricity.[8] It is produced by the friction of several other substances, and was first observed on amber.—Hence its name, from ηλεκτρον the greek term for amber. It is a fluid extremely subtle, abounding in all nature, and is one of her principal agents; which, though generally imperceptible, sometimes becomes the object of our sight and other senses. A glass tube, having been rubbed and producing the appearances above described, is said to be _excited_. The hand or buckskin, by which this is effected, is called the _rubber_. _Electrics_ are all substances which can be made to produce the same appearances; the most perfect are glass, amber, sealing-wax, sulphur, gum lac, rosin &c. These are also called _non-conductors_, from their inability to conduct the electric fluid. _Conductors_ or _non-electrics_ are those bodies which cannot be excited, but have the power of transmitting electricity; such are metals, water, the bodies of animals, an imperfect vacuum, heat &c. But strictly speaking, there are no _perfect_ conductors or non-conductors. A body is said to be in its _natural state_, when it is in the same state, with respect to electricity, as the mass of the earth. When a body has more of the electric fluid than its natural quantity, it is said to be _electrified positively_, when less, _negatively_; but neither of these cases can occur in a conductor, unless the communication between it and the earth be cut off by the _intervention of an electric or non-conductor_. When this happens, the conductor is said to be _insulated_.