[Illustration: FIG. 1.--The electric eel. There are several species inhabiting the water, and which have the power of producing electric discharges by certain portions of their organism. The best known of these are the _Torpedo_, the _Gymnotus_, and the _Silurus_, found in the Nile and the Tiger. The Electric Ray, of which there are three species inhabiting the Mediterranean and Atlantic is provided with an electric organ on the back of its head, as shown in the illustration. This organ consists of laminæ composed of polygonal cells to the number of 800 or 1000, or more, supplied with four large bundles of nerve fibres; the under surface of the fish is -, the upper +. In the Surinam eel, the electric organ goes the whole length of the body along both sides. It is able to give a very severe shock, and is a formidable antagonist when it has attained its full length of 5 or 6 feet.] =Frictional Electricity= is that produced by the friction of one substance against another. =Resinous Electricity.=--The kind of electricity produced upon a resinous substances such as sealing wax, resin, shellac, rubber or amber when rubbed with wool or fur. Resinous electricity is _negative electricity_. =Vitreous Electricity.=--A term applied to the positive electricity developed in a glass rod by rubbing it with silk. This electric charge will attract to itself bits of pith or paper which have been repelled from a rod of sealing wax or other resinous substance which had been rubbed with wool or fur. CHAPTER II STATIC ELECTRICITY Static electricity may be defined simply as _electricity at rest_; the term properly applies to an isolated charge of electricity produced by friction. The presence of static electricity manifests itself by _attraction_ or _repulsion_. =Electrical Attraction and Repulsion.=--When a glass rod, or a stick of sealing wax or shellac is held in the hand and rubbed with a piece of flannel or cat skin, the parts will be found to have the property of attracting bodies, such as pieces of silk, wool, feathers, gold leaf, etc.; they are then said to be _electrified_. In order to ascertain whether bodies are electrified or not, instruments called _electroscopes_ are used. There are two opposite kinds of electrification: 1. Positive; 2. Negative. Franklin called the electricity excited upon glass by rubbing it with silk _positive_ electricity, and that produced on resinous bodies by friction with wool or fur, _negative_ electricity. The electricity developed on a body by friction depends on the rubber as well as the body rubbed. Thus glass becomes negatively electrified when rubbed with catskin, but positively electrified when rubbed with silk. [Illustration: FIGS. 2 and 3.--Pith ball pendulum or electroscope; the figures illustrate also electrical attraction and repulsion.]